Astronomy fanatica
-
Astronomy Picture of the Day
2005 April 19
Orion in Infrared
Explanation: Do you recognize the constellation Orion? This striking but unfamiliar looking picture of the familiar Orion region of the sky was produced using survey data from the now-defunct InfraRed Astronomical Satellite (IRAS). The above image combines information recorded at three different invisible infrared wavelengths and covers about 30x24 degrees on the sky. Most of Orion's visually impressive stars don't stand out, but bright Betelgeuse does appear as a small bright purplish dot on the lower left. The bright region on the right contains the Great Nebula in Orion, while the bright region just above the image bottom is the Rosette Nebula. Surrounding these regions are a jumble of chaotic glowing gas and dark dust jettisoned by stars forming and exploding over millions of years.
Credit: IRAS, IPAC, JPL/Caltech, NASA -
Fermi paradox - Cont'd
The Drake equation
Those people who believe in the more optimistic assumptions used in the Drake equation proposed by Dr. Frank Drake and the even more optimistic assumptions given by Dr. Carl Sagan, add that intelligent life is also common in the Universe. Some state that by making what they feel are reasonable assumptions and arguments we can ascertain that if life is possible at all, then the universe is so vast that it should not only be possible, but almost certain that there are large numbers of extraterrestrial civilizations in the Universe. However those people who adhere to the premise of the Fermi paradox believe that, due to a lack of evidence to the contrary, in all probability, humans (as a technologically advanced species) are effectively alone in at least our part of the Milky Way. They further say that since we cannot yet determine the variables of the Drake Equation with any real confidence, we cannot determine the numbers of extraterrestrial civilizations based solely on this equation. We must therefore, they argue, rely on data, which is only now beginning to be collected in a significant manner. Only then can we even begin to presume what the values of each of the variables in the Drake equation are, they say.
Current data
Our solar system, if seen from a radio telescope within a few tens of light years away, would seem unusual for the huge amount of radio waves (that is by leakage of human radio transmissions) being emitted from what appears to be an otherwise unremarkable main sequence star. One can presume that similar output by a nearby star would be immediately characterized as unusual by us.
Radio and observational data have for several decades been collected and analyzed by such projects as Project Ozma, the Search for Extraterrestrial Intelligence (SETI), and the various projects searching for extrasolar planets. So far the SETI data show no known main sequence stars with unusually bright radio emissions; this would seem to indicate that we are the only radio-transmitting species in at least that portion of our part of the Galaxy that has been surveyed. In addition, the majority of the extrasolar planetary systems that have been discovered to date appear to be harsh environments for advanced life-forms.
Some people contend that these results probably have a significant amount of sampling error:
- Other species may not use the radio frequencies we are searching or may not leak significant amounts of radio frequency energy (we leak less radio energy than a few decades ago because of the use of cable and satellite transmission). More advanced civilizations might use point-to-point laser or microwave communication systems which are much more efficient.
- We can more easily find planetary systems with planetary orbits and configurations that are less stable than our own.
Still others argue that we are probably the only spacefaring species in at least our galaxy, because otherwise we would be awash in extraterrestrial radio transmissions and be overrun by early colonization efforts.
Issue #57 -
Astronomy Picture of the Day
2005 April 20
Barnard's Loop Around Orion
Explanation: Why is the belt of Orion surrounded by a bubble? Although glowing like an emission nebula, the origin of the bubble, known as Barnard's Loop, is currently unknown. Progenitor hypotheses include the winds from bright Orion stars and the supernovas of stars long gone. Barnard's Loop is too faint to be identified with the unaided eye. The nebula was discovered only in 1895 by E. E. Barnard on long duration film exposures. Orion's belt is seen as the three bright stars across the center of the image, the upper two noticeably blue. Just to the right of the lowest star in Orion's belt is a slight indentation in an emission nebula that, when seen at higher magnification, resolves into the Horsehead Nebula. To the right of the belt stars is the bright, famous, and photogenic Orion Nebula.
Credit & Copyright: W. H. Wang (IfA, U. Hawaii) -
Fermi paradox - Cont'd
The argument over the premise behind the Fermi paradox
E.T. phone home
Some of those who subscribe to the Fermi principle state that given enough time to develop, the radio transmissions of any sufficiently advanced civilization will begin to outshine their parent star in the radio part of the spectrum. Since the use of the electromagnetic spectrum for information transmission is relatively cheap and easy, one would expect any technological civilization to take advantage of at least a part of this spectrum during their development. Critics respond that even though we have been actively searching for extraterrestrial signals for almost 30 years with projects such as SETI and have been passively listening to radio static for nearly 100 years, we have yet to hear any confirmed alien broadcasts nor have we observed any main sequence stars with unusual electromagnetic radio signatures that might indicate a technological civilization.
Those that believe the galaxy has many technologically advanced civilizations counter that the extraterrestrials may simply be using a medium other than radio or they eventually chose to hide their transmissions for some unknown reason. Proponents of the Fermi principle say that this could very well be so, but only if there are very few such civilizations in both space and time and only if they very quickly abandon radio as a means of data transmission. Either way, they say, if there were many of these civilizations their transmissions would make a large impact on at least some part of the electromagnetic spectrum for at least a small part of their development. They further state that if there are as many advanced extraterrestrial civilizations as Drake and Sagan have estimated, then their presence would be made obvious by their transmissions. The fact that we have been able to receive and produce these transmissions for only a tiny fraction of our history may be limiting radio SETI in this regard.
The anthropic principle
Similar to the Rare Earth hypothesis is the anthropic principle, the idea that the universe is "fine-tuned" to fit life as we know it. The principle states that because life on Earth could not exist if any one of many parameters of the physical universe were slightly modified, it would seem that humans have a unique advantage over any other form of intelligent life. This makes it conceivable that they are the unique intelligent species.
Critics argue that the anthropic principle is essentially a tautology; life as we know it may not exist if things were different, but a different sort of life might exist in its place. Even more striking are several 2004 publications of Stephen Hawking, which assert that there is a 98% chance that a universe of a type as ours will come from a Big Bang.
Freeman Dyson's contribution
Popularized by Dr. Freeman Dyson, a Dyson Sphere is an opaque shell around a star. Such a shell would be created by advanced alien civilizations that wished to harness as much of the radiant energy of their sun as possible. The exact design of the Dyson sphere was not specified; it could consist of billions of independent solar collectors and space habitats or be a single unified structure, but in any case it would be made of solid matter and would intercept most of the star's emitted light to re-radiate as waste heat. A star surrounded by a Dyson sphere would thus emit a distinctive black body spectrum without the strong emission lines that incandescent stellar plasma exhibits, probably with its peak unusually far into the infrared for a star of its size. With this speculation, he advised astronomers to search the night sky for unusually colored stars, which, he postulated, could only signify highly advanced and intelligent life. No such stars have yet been found.
Some adherents to the Fermi principle state that it is highly unlikely that all advanced civilizations would not eventually take full advantage of the power source of their home star, and in doing so changing the electromagnetic signature of their sun.
Dr. Dyson also proposed a type of invention which he deemed likely to appear within the life-span of an intelligent civilization, the absence of which tends to support the Fermi principle. He said that he thought that it would soon be possible for us to create an explorer-device which drew power from its surroundings to propel itself through the universe in search of intelligent life forms. Moreover, it would be possible to create versions of this device which could create and launch vast numbers of copies of itself by the process of machine reproduction. Even allowing for the realities of vast distances between stars and the relativistic speed-limit, if intelligent life were common, stars in our own galaxy much older than our own would be within a range to have built and launched fleets of these automated exploration devices.
Extraterrestrial colonization
Adherents to the Fermi principle furthermore argue that, from what we know about life's ability to overcome scarcity and colonize new habitats on our own planet, we can reasonably assume that life elsewhere will follow similar principles. Given this, Fermi principle adherents state that any advanced civilization will almost certainly try to seek out new resources and colonize first their solar system, and then surrounding solar systems. Several writers have tried to estimate the amount of time it would take for such a civilization to colonize the entire galaxy. What they have determined is that it would take 5 to 50 million years to accomplish this feat (see link) — a relatively small amount of time on a geological scale, let alone a cosmological scale.
Issue #58 -
Astronomy Picture of the Day
2005 April 21
G21.5-0.9: A Supernova's Cosmic Shell
Explanation: The picture is lovely, but this pretty cosmic shell was produced by almost unbelievable violence - created when a star with nearly 20 times the mass of the sun blasted away its outer layers in a spectacular supernova explosion. As the expanding debris cloud swept through surrounding interstellar material, shock waves heated the gas causing the supernova remnant to glow in x-rays. In fact, it is possible that all supernova explosions create similar shells, some brighter than others. Cataloged as G21.5-0.9, this shell supernova remnant is relatively faint, requiring about 150 hours of x-ray data from the orbiting Chandra Observatory to create this false-color image. G21.5-0.9 is about 20,000 light-years distant in the constellation Scutum and measures about 30 light-years across. Based on the remnant's size, astronomers estimate that light from the original stellar explosion first reached Earth several thousand years ago.
Credit: Heather Matheson & Samar Safi-Harb (Univ. Manitoba), CXC, NASA -
Fermi paradox - Cont'd
Possible explanations
A variety of solutions have been proposed in response to the Fermi paradox, which can generally be grouped as follows.
They exist and have arrived - but most people have yet to see them
Those who attribute UFOs to alien spacecraft have a ready answer to the paradox: that it is not unreasonable to believe that a lifeform intelligent enough to travel to our planet would also be sufficently intelligent to exist here undetected. Alternately, we may have been detected at a distance and either a return message, or an alien emissary itself, is currently en route. The fact that they have not detected us sooner or we did not detect them first may be simple coincidence.
Many UFO researchers and watchers argue that society as a whole is unfairly biased against claims of alien abduction, sightings, and encounters, and as a result may not be fully receptive to claims of proof that aliens have already visited our planet. Others use complex conspiracy theories to allege that evidence of alien visits is being concealed from the public by political elites who seek to hide the true extent of contact between aliens and humans. Scenarios such as these have been depicted in popular culture for decades, with recent favourites being The X-files television series, and the eponymous Men in Black (movie), named for the government agents who suppress knowledge of alien contact.
They exist - but we have missed them
Some commentators (such as British science fiction author Stephen Baxter) have pointed out that humanity's ability to detect and comprehend intelligent extraterrestrial life has existed for only a very brief period—by the early 21st century, perhaps only a century at best—and that Homo sapiens itself is a recent species, given the apparent size and age of the universe.
According to this view, humanity has simply not been around sufficiently long enough to encounter alien life. For example, one million years ago—a relatively brief period in cosmological terms—there would have been no humans for alien emissaries to meet, as modern humans only appeared about 200,000 years ago. For each step back further in time, there would arguably have been increasingly fewer indications to such emissaries that intelligent life as we know it would develop on Earth. In a large and already ancient universe, a space-faring alien species may well have had many other more promising worlds to visit and revisit.
Even if alien emissaries visited in more recent times, they may have been misinterpreted by early human cultures as supernatural entities.
They exist - but do not communicate with us
Another series of views is that advanced life either deliberately or accidentally hides evidence of its existence from humanity, or is otherwise unable to communicate with us. They may not communicate for ethical reasons (eg. concern for less advanced beings; desire to encourage cultural diversity), or be deliberately incommunicado to avoid detection or destruction by more advanced or belligerent civilizations.
Another proposed idea is the so-called Zoo hypothesis which suggests that Earth is being monitored by advanced civilizations for study or for ethical purposes. This idea is similar to the Prime Directive of the fictional Star Trek television series. It is sometimes extrapolated from this idea that humanity needs to pass a certain ethical or technological boundary before we will be able to make contact with advanced alien life.
Another possible explanation is that advanced civilizations would construct multiple concentric Dyson Spheres around their stars, each one radiating less energy per area than the next smallest one, with the outermost sphere radiating at close to the background radiation. These would be essentially unobservable from any distance (see Matrioshka Brain). The above has the uniformity of motive flaw common to other Fermi solutions, that of assuming all alien civilizations throughout time will behave in the same way.
It has also been proposed that a fundamental information theoretical axiom might be behind the lack of recognized signals. Information theory states that a message which is compressed maximally is indistinguishable from white noise. The counterargument to this would be that even though as bandwidth becomes a bottleneck to communication, there ought still be some niche technologies which would or could not strive to maximum data compression. However, the radio telescope SETI searches to date all assume the simplest form of radio beacon, which is a pure sine wave with no modulation, other than that due to relative motion, presumably explicitly intended as a beacon signal, not as a communication medium.
Yet another idea is that all intelligent life inevitably destroys itself or evolves towards a technological singularity and quickly becomes unrecognizable to humanity in our present state. An intriguing idea related to this is that technological singularity may reflect a Universal inflection point in the development of information processing capability. The development of information processing, and the existence of information itself, appears to be a fundamental product woven into the fabric of the Universe, just like gravity or the strong nuclear force. Therefore, information processing capability may be acted upon by ongoing developments in the ever-expanding Universe, and something like a technological singularity may simply be the product of the triggering of one or several metastable states, just like the condensation of matter when a critical temperature had been reached in the early Universe. In regards to extraterrestrial civilizations, a look out into space with telescopes is simply a glimpse at a rear-view mirror. Deep space observation provides no direct observation about the present state of the Universe, especially in terms of where evolution and development (i.e. technological singularity) will take us in the future. In other words, the very first civilizations Universe-wide could all be developing at the same time (now) due to the aforementioned hypothetical metastable trigger in information processing capability. Due to the limited speed of light, there is no way to see what's happening, save here on the Earth.
Another hypothesis is that the whole period of modern human existence to date (about 200,000 years) is a very brief period on a cosmological scale, a position which changes little even if our species survives for many more hundreds of thousand of years. Even if intelligent life undergoes a continuous cycle of birth, extinction and rebirth across the universe, civilizations may simply be too far apart in either time or space to actually meet.
A more recent idea, called the fiber optic objection, observes that the use of broadcast technologies like radio for the transmission of information are fundamentally wasteful of energy and that advanced technological civilizations might not use them for that reason. Because broadcasts are radiated in all directions evenly, a large amount of power is needed for a transmitter to send messages any significant distance. Adherents of this concept observe that human technology is currently moving away from broadcast for long-distance communication and replacing it with wires, optical fibers, and focused electromagnetic technologies like aimed narrow-beam radio, microwave, or laser transmission. Most recent technologies that employ broadcasting, such as mobile phones and Wi-Fi networks, use very short-range transmitters to communicate with fixed stations that are themselves connected by wires or narrow beams. It is argued that this trend may make Earth itself nearly undetectable from space within a few decades, and that therefore most civilizations would only be detectable for a short period of time between the discovery of radio and the switch to more efficient technologies.
Issue #59 -
Astronomy Picture of the Day
2005 April 22
Albert Einstein's Miraculous Year
Explanation: In 1905 Albert Einstein had a miraculous year. One hundred years ago, he wrote four papers which revolutionized our understanding of the Universe. The papers outlined; the idea that light could behave as a quantized particle (a photon), an explanation of the thermal motion of atoms and molecules (at a time when atoms themselves were just theories), a theory reconciling motion and the constant speed of light (Special Relativity), and the idea of mass-energy equivalence (E=mc²). Virtually every facet of our modern exploration of the Universe is touched by his now century old insights, along with his later theory of gravity and space-time - General Relativity. In centennial celebration, consider this thoughtful view of a small telescope beside the Einstein Memorial on the grounds of the National Academy of Sciences in Washington DC, USA. The marble platform at the bronze statue's feet is embedded with a map showing the positions of the planets, sun, moon and stars on the memorial's dedication date, 100 years after Einstein's birth in 1879. Albert Einstein died 50 years ago, on April 18, 1955.
Image Credit & Copyright: Greg Piepol -
Fermi paradox - Cont'd
Possible explanations - Cont'd
They exist and communicate - but we are not listening or dismiss the evidence
Another series of views, which tend to be disregarded by contemporary science, consider that alien entities have been communicating with humans throughout history, but for any number of reasons we are unable to scientifically detect these attempts; or that the accounts of communication which have been reported are generally dismissed by scientists for sociological reasons.
For example, radio telescopes may simply not be facing in the direction from which signals are arriving, or listening on the appropriate frequencies, or using the appropriate demodulation. Alternatively, intelligent life may be using an esoteric, highly advanced or other non-conventional method of communication which we are unable (or perhaps unwilling) to detect or interpret.
These views do not necessarily assert that aliens are physically present on Earth. Rather, these views suggest that, for instance, if there is a communication medium that is more effective than electromagnetism over interstellar distances, e.g. a form of nonlocal 'psychic' communication, then physical travel would be superfluous and unnecessary, thereby offering a solution to Fermi's paradox.
For example, some have interpreted the accounts of mystics, shamans, schizophrenics, and channelers as evidence for a type of ongoing communication. In support of this view it has been posited that if the brain can somehow operate using quantum mechanical processes, as proposed by Roger Penrose and Stuart Hameroff, then in principle a type of nonlocal communication may be possible, which may then be enhanced or facilitated by traditional shamanic, meditation, or kundalini yoga techniques.
In addition, Terence McKenna and others who have experimented with Dimethyltryptamine (DMT) experiences have suggested that aliens (perhaps originating from other 'dimensions' or parallel universes), are communicating with humanity through unidentified processes which are activated by DMT. Proponents of this view believe that such experiences may have occurred, in the past as well as the present, amongst certain indiviudals such as tribal shamans, as DMT is endogenously synthesized in the brain's pineal gland, or can be ingested by shamans as the drug ayahuasca.
It is suggested that the ability to utilise DMT for this purpose of contacting alien intelligences may have conferred evolutionary advantages by way of natural selection (eg. by inspiring shamans to lead their tribes to undiscovered lands (such as by crossing the Bering Strait), or out of life-threatening situations (such as in the case of the shaman Black Elk).
Although classical communication in the context of information theory is not possible using quantum nonlocal correlations, supporters of this view believe that it may explain the 'garbled', associative, and inspirational nature of the 'messages' recorded in the world's religious and anthropological history.
The concept of biology as a basis for communication with alien intelligence has certain analogies with the field of neurotheology, which studies biology as a basis for spirituality and transcendental experiences.
They no longer exist - or we do not exist for long enough
Science fiction authors have proposed another possible explanation — that someone, or something, is destroying intelligent life in the universe as fast as it is created. This theme can be found in novels such as Frederik Pohl's Heechee novels, Fred Saberhagen's Berserker novels, Alastair Reynolds' Revelation Space novels, Greg Bear's novel The Forge of God, Arthur C. Clarke's Childhood's End, and Jack McDevitt's novel The Engines of God.
Another alternative is that they have disappeared following a technological singularity, or that they simply destroy themselves. Life on Earth, and intelligent life on Earth, evolved as a result of the competition for scarce resources. The evolutionary psychology that developed during this struggle has left its mark on our characters, and left human beings subject to involuntary, instinctual drives to consume resources and to breed. It seems likely that intelligent life on other planets evolved subject to similar constraints, and as such pessimism about their long term viability is a justifiable position.
Technological civilizations may usually or invariably destroy themselves (via nuclear war, biological warfare, grey goo, or in a Malthusian catastrophe after destroying their planet's ecosphere) before or shortly after developing radio or spaceflight technology. This general theme is explored in The Mote in God's Eye by Larry Niven and Jerry Pournelle, which has as its central premise a civilisation that overtaxes its resource base and cyclically self-destructs, but which tries to preserve its culture from one cycle to the next.
It would be anthropocentric to suggest that humanity is immune to any of these fates. Therefore another view is that we ourselves do not exist long enough to encounter alien life. Indeed, there are probabilistic arguments which suggest that our end may occur sooner rather than later. See Doomsday argument.
They never existed
Others argue that the prerequisites for the establishment of life, or at least complex life, are rare in the universe. For example, the Rare Earth hypothesis (see section above) implies that the existence of complex life on Earth required a series of seemingly improbable events to coincide, such as the existence of a large satellite body like the Moon, which through gravitational effects could stimulate the tides, which in turn are posited as being necessary for the evolution of complex life. Lending some support to this hypothesis is the leading scientific theory for the creation of the Moon, the Giant impact theory, which posits that the Moon was formed as the result of a singular occurrence, whereby a body of a certain mass struck Earth at just the right angle and velocity to carve off sufficient material, which would eventually enter stable orbit and form the Moon. Some consider that the improbability of the occurrence of such an event is magnified by the relative proximity of the massive planets to the inner solar system (too far and the impact rate greatly increases for the inner planets; too close and their orbits are greatly disrupted). The likelihood of complex life evolving as a result of the conjunction of all these events and effects is seen by supporters of the Rare Earth hypothesis as being so unlikely that the evolution of complex life elsewhere must be exceedingly rare.
Although it is possible that complex life may evolve through other mechanisms, the Rare Earth Hypothesis posits that the prerequisites of life as we know it seem to be rare. Bolstering this is the fact that in the extremely long history of life on the Earth only one sentient species has evolved that has the capability of space flight and developing technologies such as radio. So, even when conditions for complex life exist there does not appear (based on the only evidence we have) to be any inevitability that a sentient space-faring species will develop. However it cannot be conclusively proved or disproved, without a survey of the entire Cosmos, that the conditions which are believed responsible for the evolution of complex life on Earth have not occurred elsewhere.
It has also been proposed that even if the conditions for life are common, the probability of sentient life developing on more than one world is so vanishingly rare that in fact, such life has not yet developed outside our solar system. Further, it has been proposed that even if complex life is relatively common, such life may not be interested in the exploration of outer space or developing relevant technology such as radio.
Another possibility is that ice ages, comet or meteor impacts, supernovae, gamma ray bursts or other catastrophic planetary or galactic events are so common that they usually prevent life, let alone complex life, from evolving in the first place. Conversely, if evolution is sometimes aided or facilitated by such events, they may not occur at the right time or place, or at the right frequency.
An episode of the PBS television show 'Nova', entitled 'Death Star', discussed the hypothesis of physicist Arnon Dar regarding galactic sterilization due to gamma ray bursts (see link).
Issue #60 -
Astronomy Picture of the Day
2005 April 23
Eclipsed Moon in Infrared
Explanation: In September of 1996, the Midcourse Space Experiment (MSX) satellite had a spectacular view of a total lunar eclipse from Earth orbit. SPIRIT III, an on board infrared telescope, was used to repeatedly image the moon during the eclipse. Above is one of the images taken during the 70 minute totality, the Moon completely immersed in the Earth's shadow. Infrared light has wavelengths longer than visible light - humans can not see it but feel it as heat. So, the bright spots correspond to the warm areas on the lunar surface, and dark areas are cooler. The brightest spot below and left of center is the crater Tycho, while the dark region at the upper right is the Mare Crisium. Of course, this Sunday's lunar eclipse will not be a total, or even a partial one. Instead, the Moon will glide through the subtle outer portion of the Earth's shadow in a penumbral eclipse of the Moon.
Credit: DCATT Team, MSX Project -
Fermi paradox - Cont'd
Conclusion
Many views and hypotheses have been put forward to explain, solve, or otherwise address the paradox. The scientific method provides useful principles and tools to assess the validity of these ideas in their own right and their value relative to each other.
For example, by using Occam's Razor (which generally states that for a given phenomenon the explanation that has the fewest assumptions should be preferred over more complicated ones), adherents to the premise of the Fermi paradox claim that the simplest explanation would be that we are the only species in our part of the Cosmos that relies on radio technology for communication.
Issue #61 -
Astronomy Picture of the Day
2005 April 24
M16: Stars from Eagle's EGGs
Explanation: Newborn stars are forming in the Eagle Nebula. This image, taken with the Hubble Space Telescope in 1995, shows evaporating gaseous globules (EGGs) emerging from pillars of molecular hydrogen gas and dust. The giant pillars are light years in length and are so dense that interior gas contracts gravitationally to form stars. At each pillars' end, the intense radiation of bright young stars causes low density material to boil away, leaving stellar nurseries of dense EGGs exposed. The Eagle Nebula, associated with the open star cluster M16, lies about 7000 light years away.
Credit: J. Hester, P. Scowen (ASU), HST, NASA -
Arecibo Observatory
The Arecibo Observatory is located in Arecibo, Puerto Rico on the north coast of the island. It is operated by Cornell University under cooperative agreement with the National Science Foundation. The observatory works as the National Astronomy and Ionosphere Center (NAIC) although both names are officially used to refer to it.
The observatory's radio telescope is the largest single telescope ever to be constructed. It collects radio astronomy, terrestrial aeronomy, and planetary radar data for scientists around the world. Usage of the telescope is gained by submitting proposals to an independent board of referees who decide which show the most promise.
The radio telescope at Arecibo.
Organization: Cornell, NSF
Location: Arecibo, Puerto Rico
Wavelength regime: radio
Completion date: 1963
Webpage: www.naic.edu
Physical Characteristics
Telescope Style: spherical reflector
Diameter: 305 m
Collecting Area: ~73,000 m²
Mounting transit instrument: fixed primary with secondary (Gregorian reflectors) on tracks for pointing
Dome: none
Although it has been given many usages, the observatory's main purpose is to detail and observe stellar objects.
The telescope is visually distinctive and has been used in the filming of two notable movies: as the villain's antenna in the James Bond movie GoldenEye and as the real antenna in the movie Contact. The telescope received international recognition after it was used in 1999 to collect data for the SETI@home project.
General information
The Arecibo telescope is distinguished by its enormous size; the main collecting dish is 305 m in diameter, constructed inside the depression left by a karst sinkhole. The dish is the largest curved focusing dish on Earth, giving Arecibo the largest electromagnetic-wave gathering capacity. The Arecibo telescope's dish surface is made of 38,778 perforated aluminium panels, each measuring about 1 m by 2 m (3 ft by 6 ft), supported by a mesh of steel cables.
It is a spherical reflector (as opposed to a parabolic reflector). This form is due to the method used to aim the telescope; the telescope's dish is fixed in place, but the receiver at its focal point is repositioned to intercept signals reflected from different directions by the spherical dish surface. The receiver is located on a 900-ton platform which is suspended 150 m (450 ft) in the air above the dish by 18 cables running from three reinforced concrete towers, one of which is 110 m (365 ft) high and the other two of which are 80 m (265 ft) high (the tops of the three towers are at the same elevation). The platform has a 93 m long rotating bow-shaped track called the azimuth arm on which receiving antennae, secondary and tertiary reflectors are mounted. This allows the telescope to observe any region of the sky within a forty degree cone of visibility about the local zenith (between -1 and 38 degrees of declination). Puerto Rico's location near the equator allows Arecibo to view all of the planets in the solar system.
Design and architecture
The construction of the Arecibo telescope was initiated by Professor William E. Gordon of Cornell University, who originally intended to use it for the study of Earth's ionosphere. Originally, a fixed parabolic reflector was envisioned, pointing in a fixed direction with a 150 m (500 ft) tower to hold equipment at the focus. This design would have had a very limited use for other potential areas of research, such as planetary science and radio astronomy, which require the ability to point at different positions in the sky and to track those positions for an extended period as Earth rotates. Ward Low of ARPA pointed out this flaw, and put Gordon in touch with the Air Force Cambridge Research Laboratory (AFCRL) in Boston, Massachusetts where a group headed by Phil Blacksmith was working on spherical reflectors and another group was studying the propagation of radio waves in and through the upper atmosphere. Cornell University proposed the project to ARPA in the summer of 1958 and a contract was signed between the AFCRL and the University in November of 1959. Construction began in the summer of 1960, with the official opening taking place on November 1, 1963.
The telescope has undergone several significant upgrades over its lifespan. The first major upgrade was in 1974 when a high precision surface was added for the current reflector. In 1997 a ground screen was installed around the perimeter to shield from ground radiation and a more powerful transmitter was installed.
Discoveries
The Arecibo telescope has made many significant scientific discoveries. On 7 April 1964, shortly after its inauguration, Gordon H. Pettengill's team used it to determine that the rotation rate of Mercury was not 88 days, as previously thought, but only 59 days.
In August of 1989, the observatory directly imaged an asteroid for the first time in history: asteroid 4769 Castalia. The following year, Polish astronomer Aleksander Wolszczan made the discovery of pulsar PSR B1257+12, which later led him to discover its two orbiting planets. These were the first extra-solar planets ever discovered.
Usage
The telescope also had military intelligence uses, for example locating Soviet radar installations by detecting their signals bouncing back off of the Moon. Arecibo was also the source of data for the SETI@home distributed computing project put forward by the SETI Institute.
In 1974, an attempt was made to send a message to other worlds. A 1,679 bit message was transmitted from the radio telescope toward the globular cluster M13, about 25,000 light-years away. The pattern of 1s and 0s defined a 23 by 73 pixel bitmap image that included numbers, stick figures, chemical formulas, and a crude image of the telescope itself.
From March 3 to 7, 2001 the observatory was used to observe asteroid (29075) 1950 DA, considered to be the most Near-Earth object.
Arecibo Observatory in Popular Culture
Arecibo Observatory was featured in the James Bond movie GoldenEye as the area where Alec Trevelyan communicated with a Russian satellite to destroy London.
In the X-Files episode "Little Green Men", Fox Mulder was sent to the Arecibo Observatory by a US senator because there had been contact made with extraterrestrial life. The observatory was destined to be destroyed by a group of government agents because of the desire to prevent public knowledge of the discovery.
The Arecibo Observatory was also featured in the movie Contact.
Issue #62 -
Astronomy Picture of the Day
2005 April 25
The Fairy of Eagle Nebula
Explanation: The dust sculptures of the Eagle Nebula are evaporating. As powerful starlight whittles away these cool cosmic mountains, the statuesque pillars that remain might be imagined as mythical beasts. Pictured above is one of several striking dust pillars of the Eagle Nebula that might be described as a gigantic alien fairy. This fairy, however, is ten light years tall and spews radiation much hotter than common fire. The greater Eagle Nebula, M16, is actually a giant evaporating shell of gas and dust inside of which is a growing cavity filled with a spectacular stellar nursery currently forming an open cluster of stars. The above image in scientifically re-assigned colors was released as part of the fifteenth anniversary celebration of the launch of the Hubble Space Telescope.
Credit: The Hubble Heritage Team, (STScI/AURA), ESA, NASA -
Von Neumann probe
A von Neumann probe is a fictional or hypothetical self-replicating machine, specifically designed for space exploration or colonisation. It is named after John von Neumann, a Hungarian-born American mathematician and physicist who was the first scientist to rigorously study the concept of self-replicating machines, although von Neumann himself never actually proposed or discussed the use of replicating machines for this purpose during his lifetime.
The probe would be launched to a neighbouring star-system. Upon its arrival it would immediately seek out raw materials (asteroids, moons, gas giants, etc.) to create replicas of itself. Once it had created a sufficient number of replica probes, the original probe to enter the system would go about exploring the star-system.
If a self-replicating probe only finds evidence of primitive life (such as an unstable, savage civilization) they might simply lie dormant on the moon, silently waiting for it to evolve into a stable higher civilization. After waiting quietly for several millennia, they may be activated when the emerging civilization is advanced enough to set up a lunar colony. Physicist Paul Davies of the University of Adelaide has even raised the possibility of a self-replicating probe resting on our own Moon, left over from a previous visit in our system aeons ago.
Meanwhile the duplicate probes would make their way to nearby star-systems and repeat the process, creating an exponentially increasing fleet of probes. It has been theorized that using self-replicating probes, a galaxy could be explored in as little as half a million years, although it might be destroyed in the process. (see link)
A variant of the Von Neumann probe is the berserker probe, a self-replicating machine that has been deliberately (or inadvertantly) designed to sterilize the entire galaxy so that life cannot emerge, and/or destroy all existing life in its path as the probes spread out at an exponentially increasing rate across the galaxy. Another subtle variant exists in the form of the astrochicken, which are space-faring and self-replicating automata.
Popular culture
The monoliths in Arthur C. Clarke's book and Stanley Kubrick's film 2001: A Space Odyssey are self-replicating probes, though the artifacts in "The Sentinel", Clarke's original short story upon which 2001 was based, were not. Originally, the film began with a series of scientists explaining how probes like these would be the most efficient method of exploring outer space. Kubrick cut the opening segment from his film at the last minute, however, and these monoliths became almost mystical entities.
In the computer game Star Control II, the Slylandro Probe is an out-of-control self-replicating probe.
In Iain Banks' novel Excession, Hegemonising swarms are described as a form of Outside Context Problem. An example of an Aggressive Hegemonising Swarm Object is given as an uncontrolled self-replicating probe with the goal of turning all matter into copies of itself. After causing great damage, they are reprogrammed by The Culture and become Evangelical Hegemonising Swarm Objects.
Issue #63 -
Astronomy Picture of the Day
2005 April 26
A Martian Dust Devil Passes
Explanation: What goes there across the plains of Mars? A dust devil. For the first time, definitive movies of the famous spinning dust towers have been created from ground level. The robot rover Spirit has now imaged several dust devils from its hillside perch just within the past two months. Each image in the above sequence was taken about 20 seconds apart. Inspection of the digitally resized images show the passing dust devil raising Martian dust so thick that it casts a shadow. The new dust devil movies have been made possible by a new hybrid interaction system where the robot Spirit on Mars takes many images and humans on Earth inspect thumbnails and decide which full resolution images to send back.
Credit: Mars Exploration Rover Mission, JPL, NASA -
Interstellar travel
Interstellar space travel is unmanned or manned travel between stars, though the term usually denotes the latter. The concept of interstellar travel in starships is a staple in science fiction. There is a tremendous difference between interstellar travel and interplanetary travel, mainly due to the much larger distances involved.
As a practical goal interstellar travel has been debated fiercely by various scientists, science fiction authors, hobbyists and enthusiasts.
Many scientific papers have been published about related concepts. Given sufficient travel time and engineering work, unmanned interstellar travel seems possible. NASA has been engaging in research into these topics for several years, and has accumulated a number of theoretical approaches.
The difficulty of interstellar travel
Interstellar travel poses a number of difficulties. There are all the difficulties of interplanetary travel, including hard vacuum, radiation, micrometeoroids, and free fall. These difficulties seem tractable; robot missions have been sent to almost every planet in the Solar system, humans have been sent to the Moon, and manned missions to Mars have been planned for years. Interstellar travel is made immeasurably more difficult by the tremendous distance to even the nearest stars. Intergalactic travel is even more difficult.
Interstellar distances
Astronomical distances are sometimes measured in the amount of time it would take a beam of light to travel. Light in a vacuum travels in approximately 3×10^8 metres per second, which is denoted with the letter c, so a light second is approximately 3×10^8 metres.
The distance between Earth and its Moon is about one and a quarter light seconds. With current propulsion technologies, such a trip will typically take about three days for a spacecraft.
The distance from Earth to other planets in the solar system ranges from three light minutes to about five and a half light hours. Depending on the planet and its alignment to earth, for a typical unmanned spacecraft these trips will take from a few months to a little over a decade.
The nearest star to the Sun is the triple system Alpha Centauri. Light radiating from that star takes a bit more than four years to reach Earth. Currently, the fastest spacecraft built can achieve a velocity of about 30 km per second (relative to Earth). At that rate, the journey would take about 40,000 years. Additionally, at the current stage of space technology, the longest space missions that have been initiated are expected to have an operational lifetime of about 40 years before failure of key components is likely to happen. Significant engineering advances such as automated self-repair may be required to ensure survival.
In short, current spacecraft propulsion technology cannot send objects fast enough to reach the stars in a reasonable time.
Even theoretical interstellar travel is expected to be slow. Current theories of physics predict that it will be impossible or very difficult to travel faster than light, and that if it were possible, it would also be possible to build a time machine. This possibility seems unlikely, although science fiction author and scientist Robert L. Forward has written a novel, Timemaster, which describes how this could be done, assuming that one could find negative matter.
However, special relativity and general relativity offer the possibility of shortening the apparent travel time: with sufficiently advanced engines, a starship could make interstellar voyages at nearly the speed of light, and relativistic time dilation would make the voyage seem much shorter for the traveller. However, it would be slow for the people on Earth interested in the results of the mission. Also, upon return, the travellers would experience that the full time had passed on Earth.
Issue #64 -
Astronomy Picture of the Day
2005 April 27
The Hercules Cluster of Galaxies
Explanation: These are galaxies of the Hercules Cluster, an archipelago of "island universes" a mere 650 million light-years distant. This cluster is loaded with gas and dust rich, star forming, spiral galaxies but has relatively few elliptical galaxies, which lack gas and dust and the associated newborn stars. Colors in the composite image show the star forming galaxies with a blue tint and ellipticals with a slightly yellowish cast. In this cosmic vista many galaxies seem to be colliding or merging while others seem distorted - clear evidence that cluster galaxies commonly interact. Over time, the galaxy interactions are likely to affect the the content of the cluster itself. Researchers believe that the Hercules Cluster is significantly similar to young galaxy clusters in the distant, early Universe and that exploring galaxy types and their interactions in nearby Hercules will help unravel the threads of galaxy and cluster evolution.
Credit & Copyright: Jim Misti (Misti Mountain Observatory) -
Interstellar travel - Cont'd
Speculative interstellar travel
Interstellar travel designs fall into two categories. The first, which we will call slow interstellar travel, takes a great deal of time, longer than a human lifespan. The second, which we will call fast interstellar travel assumes that the difficulties above can be conquered.
Slow interstellar travel
Slow interstellar travel designs generally use near future spacecraft propulsion techniques. As a result, voyages are extremely long, hundreds or thousands of years. Voyages would be one-way trips to set up colonies. Exactly one near-term interstellar propulsion technology is known: nuclear pulse propulsion, and it is controversial. All other proposals require substantial technology development, and many require massive investments in space-based infrastructure.
Many scientists and writers have postulated various techniques for suspended animation. These include human hibernation and cryonic freezing. While neither is currently practical, they offer the possibility of sleeper ships in which the passengers lie inert for the long years of the voyage.
A second possibility is generation ships, ships that would be very large, large enough to hold a colony of people. These people would live out their lives on board the ship, and their distant descendants would arrive at a new solar system. After living happily for hundreds or thousands of years inside the ship, they might have lost interest in establishing a colony, stopping only to resupply and perhaps to build other ships. Some science fiction even suggests that the descendants may eventually forget that they are on a spaceship.
Generation ships are not currently feasible, both because building such an enormous ship would have to be done in space, and because such a sealed habitat would be extremely difficult to construct. Biosphere 2, an artificial closed ecosystem, was built in an attempt to work out the engineering difficulties in such a system.
A third possibility is a robotic space mission carrying a large number of frozen early stage human embryos. This method of space colonization requires, among other things, the development of a method to replicate conditions in a woman's womb, the prior detection of another terrestrial planet with a nitrogen-oxygen, non-toxic atmosphere as well as advances in the field of fully autonomous mobile robots.
Fast interstellar travel
More interesting to scientists and to writers is the possibility of starships that reach the stars quickly (or at least, within a human lifespan). This would require some sort of exotic propulsion methods or exotic physics, but the idea captures the imagination.
A vehicle that would allow fast interstellar travel is called a starship. They are very common in fiction. A number of propulsion methods have been proposed that would allow this.
In 1957 it was deemed possible to build 8 million ton starships with nuclear pulse propulsion engines, even though they would be limited to about 1/10 the speed of light. One problem with it is that such a propulsion method uses nuclear explosives as fuel, and may be highly controversial due to the risk of radiation or other hazards in using such a method.
Another early proposal for an interstellar propulsion system was the Bussard ramjet, in which a huge scoop would collect the diffuse hydrogen in interstellar space, "burn" it using a proton-proton fusion reaction, and expel it out the back. As the fuel would be collected en route, the craft could have theoretically accelerated to near the speed of light. Proposed in 1960, later calculations with more accurate estimates suggest that the thrust generated would be less than the drag caused by any conceivable scoop design.
Fusion-powered starships should be able to reach speeds of approximately 10 percent of that of light. Light sails powered by massive lasers could potentially reach similar or greater speeds. Finally, if energy resources and efficient production methods are found to make antimatter in the quantities required, theoretically it would be possible to reach speeds near that of light, where time dilation would shorten perceived trip times for the travellers considerably (though shielding the spacecraft from stray atoms in interstellar space would become a very serious issue as faster speeds were achieved). Even given the assumption of 10 percent of light speed, this would be enough to reach Alpha Centauri in forty years, only half a present human lifetime.
A second possibility is that wormholes would provide some way of connecting distant locations. Wormholes are warps in space-time which cause it to bend back on itself. It is not known whether or not wormholes are possible in practice. Although there are solutions to the Einstein equation of general relativity which allow for wormholes, all of the currently known solutions involve some assumption, for example the existence of negative mass, which may be unphysical.
There are two types of wormholes that may enable interstellar travel. The first kind originates with the same process as a black hole: the death of a star. Wormholes of this kind safe enough for a human being to navigate would probably have to be supermassive and rotating, on a similar scale to Sagittarius A* at the centre of the Milky Way Galaxy; smaller black holes produce intense tidal forces that would completely destroy anybody falling into them. Theorists postulate that wormholes connect arbitrary points in -- or between -- universes, across an Einstein-Rosen Bridge.
Another kind of wormhole is based on quantum gravity. Some have speculated that Euclidean wormholes that spontaneously come into being and disappear again, and exist at scales of Planck length. It may be that this wormhole could be "propped open" using negative energy (also known as vacuum energy), though the quantity of the energy would be immense. However, it is not clear that any of this is possible, largely because there is no widely accepted theory of quantum gravity.
A third possibility are starships that utilize Alcubierre's warp drive. In this theory, a starship warps space, expanding space behind it and contracting space ahead of it. The concept of manipulating space-time in this manner is based on Einstein's General Theory of Relativity, which states that gravitational fields cause space-time to be warped. Being able to warp space would probably require a control over gravity that we currently do not have.
Issue #65 -
Astronomy Picture of the Day
2005 April 28
M51: Cosmic Whirlpool
Explanation: Follow the handle of the Big Dipper away from the dipper's bowl, until you get to the handle's last bright star. Then, just slide your telescope a little south and west and you might find this stunning pair of interacting galaxies, the 51st entry in Charles Messier's famous catalog. Perhaps the original spiral nebula, the large galaxy with well defined spiral structure is also cataloged as NGC 5194. Its spiral arms and dust lanes clearly sweep in front of its companion galaxy (right), NGC 5195. The pair are about 31 million light-years distant and officially lie within the boundaries of the small constellation Canes Venatici. Though M51 looks faint and fuzzy in small, earthbound telescopes, this sharpest ever picture of M51 was made in January 2005 with the Advanced Camera for Surveys on board the Hubble Space Telescope.
Credit: S. Beckwith (STScI) Hubble Heritage Team, (STScI/AURA), ESA, NASA -
Interstellar communication
Interstellar communication is the transmission of signals across planetary systems. Interstellar communication is potentially much easier than interstellar travel, being possible with technologies and equipment which are currently available. However, successful communication requires someone at the other end to communicate with, and in that regard it is currently not known whether interstellar communication is possible at this time.
Radio
The SETI project has for the past several decades been conducting a search for signals being transmitted by extraterrestrial life located outside the solar system, primarily in the radio frequencies of the electromagnetic spectrum. Special attention has been given to the "water hole", the frequency of one of neutral hydrogen's absorption lines, due to the low background noise at this frequency and its symbolic association with the basis for what is likely to be the most common system of biochemistry.
The regular radio pulses emitted by pulsars were briefly thought to be potential intelligent signals; the first pulsar to be discovered was originally designated "LGM-1", for "Little Green Men." They were quickly determined to be of natural origin, however.
Several attempts have been made to transmit signals to other stars as well. One of the earliest and most famous was the 1974 radio message sent from the largest radio telescope in the world, Arecibo, in Puerto Rico. An extremely simple message was aimed at a globular cluster of stars known as M13 in the Milky Way Galaxy and at a distance of 30,000 light years from our solar system. These efforts have been more symbolic than anything else, however.
Other methods
It has also been proposed that higher frequency signals, such as lasers operating at visible light frequencies, may prove to be a fruitful method of interstellar communication; at a given frequency it takes surprisingly small energy output for a laser emitter to outshine its local star from the perspective of its target.
Other more exotic methods of communication have been proposed, such as modulated neutrino or gravitational wave emissions. These would have the advantage of being essentially immune to interference by intervening matter, but are very difficult to generate or detect with current technology.
Sending physical mail packets (see link) between stars may prove to be optimal for many applications. While mail packets would likely be limited to speeds far below that of electromagnetic or other light-speed signals (resulting in very high latency), the amount of information that could be encoded in only a few tons of physical matter could more than make up for it in terms of average bandwidth. The possibility of using interstellar messenger probes for interstellar communication was first suggested by Ronald N. Bracewell in 1960, and the technical feasibility of this approach was demonstrated by the British Interplanetary Society's starship study Project Daedalus in 1978. Starting in 1979, Robert Freitas advanced arguments for the proposition that physical space-probes provide a superior mode of interstellar communication to radio signals, then undertook telescopic searches for such probes in 1979 and 1982.
FTL (Faster than Light) or STL (Slower than Light)
A question which has much impact on the way in that one would imagine future (interstellar) societies. How is society formed by the way it communicates? One could easily imagine alienation of different human tribes in remote star systems if it takes more than a thousand years to communicate.
Faster than light signals could also present the possibility of allowing communication with one's own past, according to current understanding of physics. This would present many of the same problems and opportunities provided for by physical time travel.
Issue #66 -
Astronomy Picture of the Day
2005 April 29
Small Moon Epimetheus
Explanation: Small saturnian moon Epimetheus (ep-ee-MEE-thee-us) is at most 116 kilometers across. Its cratered surface and irregular shape are highlighted by dramatic shadows in this composite close-up image from the Cassini spacecraft. However, orbiting 91,000 kilometers above Saturn's cloud tops, Epimetheus is not alone. Similar in size, saturnian moon Janus occupies an orbit separated from Epimetheus' by only about 50 kilometers. The two actually approach each other once every four years, but instead of colliding, the moons deftly exchange orbits and move apart again! In fact, co-orbiting Epimetheus and Janus both consist mostly of porous water ice and could have formed from the breakup of a single parent body. The small moons are also believed to play a role in maintaining the outer edge of Saturn's A ring.
Credit: Cassini Imaging Team, SSI, JPL, ESA, NASA -
SETI
SETI (rhymes with "Betty") stands for Search for Extra-Terrestrial Intelligence. Interstellar travel is a common theme in science fiction stories, but so far, in practice, the obstacles to such journeys are insurmountable. An alternative approach to interstellar exploration is to survey the sky in hopes of finding transmissions from a civilization on a distant planet, but such an effort has obstacles as well.
The SETI Institute has received limited telescope time at the Arecibo Observatory in Puerto Rico.
Overview
Visiting another civilization on a distant world would be fascinating, but at present is beyond our capabilities. However, it is perfectly within our capabilities to develop a communications system using a powerful transmitter and a sensitive receiver, and use it to search the sky for extraterrestrial worlds whose citizens have a similar inclination.
SETI is still no trivial task. The Milky Way galaxy is 100,000 light years across, and contains a hundred thousand million stars. Searching the entire sky for some far-away and faint signal is an exhausting exercise.
Some simplifying assumptions are useful to reduce the size of the task. One is to assume that the vast majority of life-forms in our galaxy are based on carbon chemistries, as are all life-forms on Earth. While it is possible that life could be based around atoms other than carbon, carbon is well known for the unusually wide variety of molecules that can be formed around it.
The presence of liquid water is also a useful assumption, as it is a common molecule and provides an excellent environment for the formation of complicated carbon-based molecules that could eventually lead to the emergence of life.
A third assumption is to focus on Sun-like stars. Very big stars have relatively short lifetimes, meaning that intelligent life would not likely have time to evolve on planets orbiting them. Very small stars provide so little heat and warmth that only planets in very close orbits around them would not be frozen solid, and in such close orbits these planets would be tidally "locked" to the star, with one side of the planet perpetually baked and the other perpetually frozen.
About 10% of the stars in our galaxy are Sun-like, and there are about a thousand such stars within 100 light-years of our Sun. These stars would be useful primary targets for interstellar listening. However, we only know of one planet where life exists, our own. There is no way to know if any of the simplifying assumptions are correct, and so as a second priority the entire sky must be searched.
Searching the entire sky is bad enough. To find a radio transmission from an alien civilization, we also have to search through most of the useful radio spectrum, as there is no way to know what frequencies aliens might be using. Trying to transmit a powerful signal over a wide range of wavelengths is impractical, and so it is likely that such a signal would be transmitted on a relatively narrow band. This means that a wide range of frequencies must be searched at every spatial coordinate of the sky.
There is also the problem of knowing what to listen for, as we have no idea how a signal sent by aliens might be modulated, and how the data transmitted by it might be encoded. Narrow-bandwidth signals that are stronger than background noise and constant in intensity are obviously interesting, and if they have a regular and complex pulse pattern are likely to be artificial.
However, while studies have been performed on how to send a signal that could be easily decoded, there is no way to know if the assumptions of those studies are valid, and deciphering the information from an alien signal could be very difficult.
There is yet another problem in listening for interstellar radio signals. Cosmic and receiver noise sources impose a threshold to power of signals that we can detect. For us to detect an alien civilization 100 light-years away that is broadcasting "omnidirectionally", that is, in all directions, the aliens would have to be using a transmitter power equivalent to several thousand times the entire current power-generating capacity of the entire Earth.
It is much more effective in terms of communication to generate a narrow-beam signal whose "effective radiated power" is very high along the path of the beam, but negligible everywhere else. This makes the transmitter power perfectly reasonable, but the problem then becomes one of having the good luck to be in the path of the beam.
Such a beam might be very hard to detect, not only because it is very narrow, but because it could be blocked by interstellar dust clouds or garbled by "multipath effects", the same phenomenon that causes "ghosted" TV images. Such ghosts occur when TV transmissions are bounced off a mountain or other large object, while also arriving at our TV antenna by a shorter, direct route, with the TV picking up two signals separated by a delay.
Similarly, interstellar narrow-beam communications could be bent or "refracted" by interstellar clouds to produce multipath effects that could obscure the signal. If interstellar signals are being transmitted on narrow beams, there is nothing we can do at this end to deal with this problem other than to be alert.
Modern SETI efforts began with a paper written by physicists Giuseppe Cocconi and Philip Morrison and published in the science press in 1959. Cocconi and Morrison suggested that the microwave frequencies between 1 and 10 gigahertz would be best suited for interstellar communications.
Below 1 gigahertz, "synchrotron radiation" emitted from electrons moving in galactic magnetic fields tends to drown out other radio sources. Above 10 gigahertz, radio noise from water and oxygen atoms in our atmosphere tends to also become a source of interference. Even if alien worlds have substantially different atmospheres, quantum noise effects make it difficult to build a receiver that can pick up signals above 100 gigahertz.
The low end of this "microwave window" is particularly attractive for communications, because it is in general easier to generate and receive signals at lower frequency. The lower frequencies are also desirable because of the "Doppler shifting" of a narrow-band signal due to planetary motions.
Doppler shifting is a change in the frequency of a signal due to the motion of the source of that signal. If the source is approaching, the signal will be shifted up in frequency, while if the source is moving away, the signal will be shifted down in frequency. The rotation of a planet and its orbit around a star causes a Doppler shift in the frequency of any signal generated from that planet, and over the course of a day the signal can drift in frequency far out of its intended bandwidth. The problem gets worse with higher frequencies, and so lower frequencies are preferred.
Cocconi and Morrison suggested that the frequency of 1.420 gigahertz was particularly interesting. This is the frequency emitted by neutral hydrogen. Radio astronomers often search the sky on this frequency to map the great hydrogen clouds in our galaxy. Transmitting a communications signal near this "marker" frequency would improve the chances of its detection by accident. This frequency is sometimes called the "watering hole" by SETI enthusiasts.
Issue #67 -
Astronomy Picture of the Day
2005 April 30
The Moons of Earth
Explanation: While orbiting the planet during their June 1998 mission, the crew of the Space Shuttle Discovery photographed this view of two moons of Earth. Thick storm clouds are visible in the lovely blue planet's nurturing atmosphere and, what was then Earth's largest artificial moon, the spindly Russian Mir Space Station can be seen above the planet's limb. The bright spot to the right of Mir is Earth's very large natural satellite, The Moon. The Mir orbited planet Earth once every 90 minutes about 200 miles above the planet's surface or about 4,000 miles from Earth's center. The Moon orbits once every 28 days at a distance of about 250,000 miles from the center of the Earth.
Credit: STS-91 Crew, NASA -
SETI - Cont'd
Radio SETI experiments
Early work
In 1960, Cornell University astronomer Frank Drake performed the first modern SETI experiment, named "Project Ozma", after the Queen of Oz in L. Frank Baum's fantasy books. Drake used a 25-meter-diameter radio telescope at Green Bank, West Virginia, to examine the stars Tau Ceti and Epsilon Eridani near the 1.420 gigahertz marker frequency. A 400 kilohertz band was scanned around the marker frequency, using a single-channel receiver with a bandwidth of 100 hertz. The information was stored on tape for off-line analysis. Nothing of great interest was found.
The first SETI conference took place at Green Bank in 1961. The Soviets took a strong interest in SETI during the 1960s and performed a number of searches with omnidirectional antennas in the hope of picking up powerful radio signals beginning in 1964. Legendary American astronomer Carl Sagan and Soviet astronomer Iosif Shklovskii together wrote the pioneering book in the field, Intelligent Life in the Universe which appeared in 1966.
In 1971, the U.S. National Aeronautics and Space Administration (NASA) funded a SETI study that involved Drake, Bernard Oliver of Hewlett-Packard Corporation, and others. The report that resulted proposed the construction of an Earth-based radio telescope array with 1,500 dishes, known as "Project Cyclops". The price tag for the Cyclops array was $10 billion USD, and, not surprisingly, Cyclops was not built.
Arecibo message
In 1974, a largely symbolic attempt was made to send a message to other worlds. To celebrate a substantial upgrading of the 305 metre Arecibo Radio Telescope in Puerto Rico, a coded message of 1,679 bits was transmitted towards the Globular Cluster M13, some 25,000 light years away.
The pattern of 0s and 1s contained in the message defined a 23 × 73 grid which when plotted revealed some data about our location in the Solar System, a stylised figure of a human being, chemical formulae and an outline of the radio telescope itself. The 23 by 73 grid was chosen because both 23 and 73 are prime numbers and it was thought that this could aid any hypothetical alien listener to recognize the grid representation.
Given the limitations of the speed of light, no reply would be possible for 50,000 years and hence has been dismissed by some as a publicity stunt. A controversy arose because the transmission raised the serious question of whether a small group should be allowed to speak for Earth.
SERENDIP, Sentinel, META, and BETA
In 1979, the University of California, Berkeley launched a SETI project named "Search for Extraterrestrial Radio from Nearby Developed Populations (SERENDIP)". In 1980, Sagan, Bruce Murray, and Louis Friedman founded the U.S. Planetary Society, partly as a vehicle for SETI studies.
In the early 1980s, Harvard University physicist Paul Horowitz took the next step and proposed the design of a spectrum analyzer specifically intended to search for SETI transmissions. Traditional desktop spectrum analyzers were of little usefulness for this job, as they sampled frequencies using banks of analog filters and so were restricted in the number of channels they could acquire. However, modern integrated-circuit digital signal processing (DSP) technology could be used to build autocorrelation receivers to check far more channels. This work led in 1981 to a portable spectrum analyzer named "Suitcase SETI" that had a capacity of 131,000 narrowband channels. After field tests that lasted into 1982, Suitcase SETI was put into use in 1983 with the 25-meter Harvard/Smithsonian radio telescope at Harvard, Massachusetts. This project was named "Sentinel", and continued into 1985.
Even 131,000 channels weren't enough to search the sky in detail at any fast rate, and so Suitcase SETI was followed in 1985 by Project "META", for "Megachannel Extra-Terrestrial Array". The META spectrum analyzer had a capacity of 8 million channels and a channel resolution of 0.5 hertz. The project was led by Horowitz with the help of the Planetary Society, and was partly funded by moviemaker Steven Spielberg. A second such effort, META II, was begun in Argentina in 1990 to search the southern sky. META II is still in operation, after an equipment upgrade in 1996. Also in 1985, Ohio State University began their own SETI program, named Project "Big Ear", which later received Planetary Society funding. The next year, in 1986, UC Berkeley initiated their second SETI effort, SERENDIP II, and has continued with two more SERENDIP efforts to the present day.
The Planetary Society is now pursuing a follow-on to the META project named "BETA", for "Billion-Channel Extraterrestrial Array". This is a dedicated DSP box with 200 processors and 3 gigabytes of RAM. BETA is about a trillion times more powerful than the signal processing equipment used in Project Ozma. BETA actually only scans 250 million channels, with a range of 0.5 hertz per channel. It scans through the microwave range from 1.400 to 1.720 gigahertz in eight hops, with two seconds of observation in each hop.
MOP and Project Phoenix
In 1992, the U.S. government finally funded an operational SETI program, in the form of the NASA "Microwave Observing Program (MOP)". MOP was planned as a long-term effort, performing a "Targeted Search" of 800 specific nearby stars, along with a general "Sky Survey" to scan the sky. MOP was to be performed by radio dishes associated with the NASA Deep Space Network, as well as a 43-meter dish at Green Bank and the big Arecibo dish. The signals were to be analyzed by spectrum analyzers, each with a capacity of 15 million channels. These spectrum analyzers could be ganged to obtain greater capacity. Those used in the Targeted Search had a bandwidth of 1 hertz per channel, while those used in the Sky Survey had a bandwidth of 30 hertz per channel.
MOP drew the attention of the U.S. Congress, where the program was strongly ridiculed, and was cancelled a year after its start. SETI advocates did not give up, and in 1995 the nonprofit "SETI Institute" of Mountain View, California, resurrected the work under the name of Project "Phoenix", backed by private sources of funding. Project Phoenix, under the direction of Dr. Jill Tarter, previously of NASA, is a continuation of the Targeted Search program, studying 1,000 nearby Sunlike stars, and uses the 64-meter Parkes radio telescope in Australia. Backers believe that if there is any alien civilization among those thousand stars broadcasting toward us with a powerful transmitter, the search should be able to detect it.
Issue #68 -
SETI - Cont'd
Radio SETI experiments - Cont'd
Allen Telescope Array
The SETI Institute is now collaborating with the Radio Astronomy Laboratory at UC Berkeley to develop a specialized radio telescope array for SETI studies, something like a mini-Cyclops array. The new array concept is named the "Allen Telescope Array" (ATA) (formerly, One Hectare Telescope [1HT]). It will cover 100 meters on a side. The array is being constructed at the Hat Creek Observatory in rural northern California.
The array will consist of 350 or more Gregorian radio dishes, each 6.1 meters (20 feet) in diameter. These dishes will essentially be commercially available satellite television dishes. The ATA is expected to be completed by 2005 at a very modest cost of $25 million USD. The SETI Institute will provide money for building the ATA while UC Berkeley will design the telescope and provide operational funding. Berkeley astronomers will use the ATA to pursue other deep space radio observations. The ATA is intended to support a large number of simultaneous observations through a technique known as "multibeaming", in which DSP technology is used to sort out signals from the multiple dishes. The DSP system planned for the ATA is extremely ambitious.
SETI@home
Another interesting UC Berkeley effort called SETI@home began in May 1999. The existence of the SETI@home project means that any individual can become involved with SETI research by simply downloading screensaver software over the Internet. The software performs signal analysis on a downloaded 350 kilobyte "work unit" of SERENDIP IV SETI radio survey data, and then reports the results back over the Internet.
Over 5 million computer users in hundreds of countries have signed up for SETI@home and have collectively contributed with over 19 billion hours of computer processing time. The project is widely praised in the computer press as an effective exercise in home-grown distributed computing. As of June 22, 2004 the next generation of SETI@home was released to the public. It is based on the Berkeley Open Infrastructure for Network Computing (BOINC), which is being developed out of the UC Berkeley. SETI@home classic will soon retire as the next generation of distributive computing advances.
Issue #68
