Astronomy fanatica

last SG · 2005-02-27T00:18:37+00:00

u'll find it on page 4 of this thread

poon cho tang SG

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

poon cho tang SG

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

poon cho tang SG

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

poon cho tang SG

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)

poon cho tang SG

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

poon cho tang SG

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

poon cho tang SG

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

poon cho tang SG

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

poon cho tang SG

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

poon cho tang SG

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

poon cho tang SG

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

poon cho tang SG

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.

[email protected]

Another interesting UC Berkeley effort called [email protected] began in May 1999. The existence of the [email protected] 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 [email protected] 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 [email protected] 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. [email protected] classic will soon retire as the next generation of distributive computing advances.

Issue #68

poon cho tang SG

Astronomy Picture of the Day

2005 May 1

Planetary Nebula Mz3: The Ant Nebula

Explanation: Why isn't this ant a big sphere? Planetary nebula Mz3 is being cast off by a star similar to our Sun that is, surely, round. Why then would the gas that is streaming away create an ant-shaped nebula that is distinctly not round? Clues might include the high 1000-kilometer per second speed of the expelled gas, the light-year long length of the structure, and the magnetism of the star visible above at the nebula's center. One possible answer is that Mz3 is hiding a second, dimmer star that orbits close in to the bright star. A competing hypothesis holds that the central star's own spin and magnetic field are channeling the gas. Since the central star appears to be so similar to our own Sun, astronomers hope that increased understanding of the history of this giant space ant can provide useful insight into the likely future of our own Sun and Earth.

Credit: R. Sahai (JPL) et al., Hubble Heritage Team, ESA, NASA

poon cho tang SG

SETI - Cont'd

Optical SETI experiments

While most SETI sky searches have studied the radio spectrum, some SETI researchers have considered the possibility that alien civilizations might be using powerful lasers for interstellar communications at optical wavelengths. The idea was first suggested in a paper published in the British journal Nature in 1961, and in 1983 Charles Townes, one of the inventors of the laser, published a detailed study of the idea in the US journal Proceedings of the National Academy of Sciences.

Most SETI researchers were cool to the idea. The 1971 Cyclops study discounted the possibility of optical SETI, reasoning that construction of a laser system that could outshine the bright central sun of a remote star system would be too difficult. Now some SETI advocates, such as Frank Drake, have suggested that such a judgement was too conservative.

There are two problems with optical SETI, one of which is easy to deal with, the second of which is troublesome. The first problem is that lasers are highly "monochromatic", that is, they only emit light on one frequency, making it troublesome to figure out what frequency to look for. However, according to Fourier analysis, emitting light in narrow pulses results in a broad spectrum of emission, with the frequencies becoming higher as the pulse width becomes narrower, and an interstellar communications system could use pulsed lasers.

The other problem is that while radio transmissions can be broadcast in all directions, lasers are highly directional. This means that a laser beam could be easily blocked by clouds of interstellar dust, and more to the point, we could only pick it up if we happened to cross its line of fire. As it is unlikely an alien civilization would focus an interstellar laser communications beam on Earth deliberately, we would have to cross such a beam by accident.

However, as discussed earlier, the power requirements for omnidirectional interstellar radio broadcasts are tremendous, and narrow-beam radio communications are technically more plausible. As SETI researchers have adjusted to the idea that interstellar radio communications may be over narrow beams, the idea of hunting for interstellar laser beams has become no more troublesome.

In the 1980s, two Soviet researchers conducted a short optical SETI search, but turned up nothing. During much of the 1990s, the optical SETI cause was kept alive through searches by Stuart Kingsley, a British dedicated amateur living in the US state of Ohio.

Now the SETI old-timers have warmed to the concept of optical SETI. Paul Horowitz of Harvard and researchers with the SETI institute have conducted simple optical SETI searches using a telescope and a photon pulse detection system, and are considering further searches. Horowitz says: "Everyone's been mesmerized by radio, but we've done that experiment a lot and we're a little tired of it."

Optical SETI enthusiasts have conducted paper studies of the effectiveness of using contemporary high-energy lasers and a ten-meter focus mirror as an interstellar beacon. The analysis shows that an infrared pulse from a laser, whose light output is not bound by the inverse-square law of light emitted from a hot body like the Sun, would appear thousands of times brighter than the Sun to a distant civilization in the beam's line of fire. The Cyclops study proved incorrect in suggesting a laser beam would be inherently hard to see.

Such a system could be made to automatically steer itself through a target list, sending a pulse to each target at a rate, say, of once a second. This would allow targeting of all Sun-like stars within a distance of 100 light-years. The studies have also described an automatic laser pulse detector system with a low-cost, two-meter mirror made of carbon composite materials, focusing on an array of light detectors. This automatic detector system could perform sky surveys to detect laser flashes from civilizations attempting to contact us.

Several optical SETI experiments are now in progress. A Harvard-Smithsonian group that includes Paul Horowitz designed a laser detector and mounted it on Harvard's 155 centimeter (61 inch) optical telescope. This telescope is currently being used for a more conventional star survey, and the optical SETI survey is "piggybacking" on that effort.

Between October 1998 and November 1999, the survey inspected about 2,500 stars. Nothing that resembled an intentional laser signal was detected, but efforts continue. The Harvard-Smithsonian group is now working with Princeton to mount a similar detector system on Princeton's 91-centimeter (36-inch) telescope. The Harvard and Princeton telescopes will be "ganged" to track the same targets at the same time, with the intent being to detect the same signal in both locations as a means of reducing errors from detector noise.

The Harvard-Smithsonian group is now building a dedicated all-sky optical survey system along the lines of that described above, featuring a 1.8-meter (72-inch) telescope. The new optical SETI survey telescope is being set up at the Oak Ridge Observatory in Harvard, Massachusetts.

The University of California, Berkeley, home of SERENDIP and [email protected], is also conducting optical SETI searches. One is being directed by Geoffrey Marcy, the well-known extrasolar planet hunter, and involves examination of records of spectra taken during extrasolar planet hunts for a continuous, rather than pulsed, laser signal.

The other Berkeley optical SETI effort is more like that being pursued by the Harvard-Smithsonian group and is being directed by Dan Wertheimer of Berkeley, who built the laser detector for the Harvard-Smithsonian group. The Berkeley survey uses a 76-centimeter (30-inch) automated telescope and an older laser detector built by Wertheimer.

Issue #69

poon cho tang SG

Astronomy Picture of the Day

2005 May 2

Methusaleh Outcrop on Mars

Explanation: What the history of the outcropping of rock called Methuselah? The unusual rock group is visible on the left of the above image taken by the robot Spirit rover current exploring Mars. Methuselah was discovered while maneuvering over hilly terrain and shows unusual multiple layering that caught the attention of the rover science team. Since the above representative color image was taken about three weeks ago, Spirit has moved in to get a closer look. Also visible in the above image are another rock outcrop dubbed Larry's Lookout on the upper right and a larger Clark Hill in the left background. On the far right is a more distant peak of the Columbia Hills.

Credit: Mars Exploration Rover Mission, JPL, NASA

poon cho tang SG

SETI - Cont'd

Probe SETI and SETA Experiments

The possibility of using interstellar messenger probes in the search for extraterrestrial intelligence 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 are a superior mode of interstellar communication to radio signals, which, if true, would favor a more solarcentric Search for Extraterrestrial Artifacts (SETA). Much like the "preferred frequency" concept in SETI radio beacon theory, the Earth-Moon or Sun-Earth libration orbits might therefore constitute the most universally convenient parking places for automated extraterrestrial spacecraft exploring arbitrary stellar systems. A viable long-term SETI program may be founded upon a search for these objects.

In 1979, Freitas and Valdes conducted a photographic search of the vicinity of the Earth-Moon triangular libration points L4 and L5, and of the solar-synchronized positions in the associated halo orbits, seeking possible orbiting extraterrestrial interstellar probes, but found nothing to a detection limit of about 14th magnitude. The authors conducted a second more comprehensive photographic search for probes in 1982 that examined the five Earth-Moon Lagrangian positions and included the solar-synchronized positions in the stable L4/L5 libration orbits, the potentially stable nonplanar orbits near LI/L2, Earth-Moon L3, and also L2 in the Sun-Earth system. Again no extraterrestrial probes were found to limiting magnitudes of 17-19th magnitude near L3/L4/L5, 10-18th magnitude for L1/L2, and 14-16th magnitude for Sun-Earth L2.

In June 1983, Valdes and Freitas used the 26-m radiotelescope at Hat Creek Radio Observatory to search for the tritium hyperfine line at 1516 MHz from 108 assorted astronomical objects, with emphasis on 53 nearby stars including all visible stars within a 20 light-year radius. The tritium frequency was deemed highly attractive for SETI work because (1) the isotope is cosmically rare, (2) the tritium hyperfine line is centered in the SETI waterhole region of the terrestrial microwave window, and (3) in addition to beacon signals, tritium hyperfine emission may occur as a byproduct of extensive nuclear fusion energy production by extraterrestrial civilizations. The wideband- and narrowband-channel observations achieved sensitivities of 5-14 x 10^(-21) W/m²/channel and 0.7-2 x 10^(-24) W/m²/channel, respectively, but no detections were made.

Issue #70

poon cho tang SG

Astronomy Picture of the Day

2005 May 3

Solar System Rising Over Fire Island

Explanation: If you wait long enough, the entire Solar System will rise before you. To see such a sight, however, you will need to look in the direction of the ecliptic. All of the planets and their moons orbit the Sun in nearly the same plane, the ecliptic plane. From the Earth, this means that each day they will all rise in nearly the same direction - and later set in the opposite direction. Ten years ago, a series of time exposures caught, left to right, the Sun, Venus, the Moon, and Jupiter, all rising in the ecliptic plane behind Fire Island, New York, USA. Exposures were taken every six minutes and digitally superposed on an image taken from the same location at sunrise. Smaller members of our Solar System, including most comets and many asteroids, do not always move along the ecliptic plane. The picturesque Fire Island Lighthouse, visible in the foreground, was built in 1826 and is still in use today.

Credit & Copyright: Larry Landolfi (Landolfi Photography)

poon cho tang SG

SETI - Cont'd

Where are they? / The interstellar Internet

SETI experiments performed so far have not found anything that resembles an interstellar communications signal. Says Frank Drake of the SETI Institute: "All we know for sure is that the sky is not littered with powerful microwave transmitters."

The great Italian physicist Enrico Fermi suggested in the 1950s that if there was an interstellar civilization, its presence would be obvious once we bothered to look. This is known as the Fermi paradox.

The paradox can be illustrated as follows:

While faster than light, or "superluminal", flight is ruled out by contemporary physics, no law of physics absolutely rules out interstellar flight at "subluminal" speeds, though the physical requirements are formidable. Assuming that stars are on the average about ten light-years apart; that an interstellar mission can be conducted at a speed of 10% of the speed of light; and that it takes four centuries for an interstellar colony to grow to the point where it can launch a pair of interstellar missions, then the "doubling time" of the interstellar colonies created by this advanced civilization would be 500 years. This would allow colonization of the entire galaxy in five million years.

Even limiting an interstellar mission to 1% of the speed of light and assuming it takes a millennium for a society to get to the point where it can mount two interstellar missions, this still means the galaxy would be completely populated in 20 million years. That is a very short interval on a cosmic time scale.

Given the lack of observable signals, as well as the lack of any persuasive evidence that extra-terrestrials have ever visited this planet, Fermi's argument suggests that there is no such interstellar civilization. This argument, depressing for many SETI enthusiasts, is called the "Fermi paradox".

The fact that radio-based SETI searches have not come up with anything very interesting so far is not cause to rule out the existence of contactable alien intelligence. As the previous sections of this document show, trying to find another civilization in space is a difficult proposition, and we have only searched a small fraction of the entire "parameter space" of targets, frequencies, power levels, and so on.

The negative results do place limits on the proximity of certain "classes" of alien civilizations, as specified in a scheme proposed by Soviet SETI researcher Nikolai S. Kardashev in the early 1960s called the Kardashev scale and later extended by Carl Sagan. In this scheme, a "Type I" civilization is one capable of using all the sunlight falling on the surface of an Earthlike planet for an interstellar signal; a "Type II" civilization is capable of harnessing the power of an entire star; and a "Type III" civilization is capable of making use of an entire galaxy. Intermediate civilizations can be numerically defined on a logarithmic scale.

Assuming that an alien civilization is actually transmitting a signal that we could pick up, the searches so far rule out a Type I civilization within a spherical radius of 1,000 light-years, though there may be many civilizations comparable to our own within a few hundred light years that have remained undetected.

A similar analysis using the same assumption shows that there is no detectable Type II civilization in our Galaxy. In the early days of SETI, researchers assumed that such advanced civilizations were very common in our Galaxy. It is discouraging that this does not seem to be so.

However, it is important to emphasize that our SETI hunts have been based on assumptions on communications frequencies and technologies that may be laughable to alien societies, if they have the concept of humor. It is possible that intelligent species abandon radio when new technologies are discovered, making the length of time a world is transmitting on conventional radio extremely short. The lack of results do not say that alien civilizations don't exist. They only say that if they do, our most optimistic assumptions for getting in touch with them have proven unrealistic.

There is another issue that provides another possible explanation as to why we don't see evidence of a large number of alien societies. That issue is time.

Our Sun is not a first-generation star. All first-generation stars are either very small and dim, or have exploded, or have burned out. This first generation synthesized the heavy elements needed to create planets and lifeforms. Later generations of stars, including our Sun, have been born and have died or will die in their turn.

Our galaxy is more than 10 billion years old. Intelligent life and technological societies may have arisen and died out many times during this ten billion years. Assuming that an intelligent species survives for ten million years, that means that only 0.1% of all societies that have arisen during the history of our galaxy are in existence now.

Science writer Timothy Ferris has suggested that since galactic societies could be only transitory, then if there is in fact an interstellar communications network, it consists mostly of automated systems that store the cumulative knowledge of vanished civilizations and communicate that knowledge through the galaxy. Ferris calls this the "Interstellar Internet", with the various automated systems acting as network "servers".

Ferris suspects that if such an Interstellar Internet exists, communications between servers are mostly through narrow-band, highly directional radio or laser links. Intercepting such signals is, as discussed earlier, very difficult. However, the network could maintain some broadcast nodes in hopes of making contact with new civilizations. The Interstellar Internet may be out there, waiting for us to figure out how to link up with it.

Another theory which has been proposed to explain the apparent lack of interstellar communication is the suggestion that the galaxy may contain predatory (or otherwise aggressive) species. Those species smart enough to maintain radio silence are those that survive such predation. Yet another theory proposed by physicist Arnon Dar, and described in the PBS Nova show 'Death Star' (see link), proposes that gamma-ray burst events are sufficiently frequent to sterilize vast swaths of galactic real-estate.

Issue #71

poon cho tang SG

Astronomy Picture of the Day

2005 May 4

Cassini Spacecraft Crosses Saturn's Ring Plane

Explanation: If this is Saturn, where are the rings? When Saturn's "appendages" disappeared in 1612, Galileo did not understand why. Later that century, it became understood that Saturn's unusual protrusions were rings and that when the Earth crosses the ring plane, the edge-on rings will appear to disappear. This is because Saturn's rings are confined to a plane many times thinner, in proportion, than a razor blade. In modern times, the robot Cassini spacecraft orbiting Saturn now also crosses Saturn's ring plane. A series of plane crossing images from late February was dug out of the vast online Cassini raw image archive by interested Spanish amateur Fernando Garcia Navarro. Pictured above, digitally cropped and set in representative colors, is the striking result. Saturn's thin ring plane appears in blue, bands and clouds in Saturn's upper atmosphere appear in gold, and dark shadows of the rings curve across the top of the gas giant planet. Moons appear as bumps in the rings.

Credit: Cassini Imaging Team, SSI, JPL, ESA, NASA

poon cho tang SG

SETI - Cont'd

Criticism of SETI

Not all scientists think that SETI is proper science, labelling it pseudoscience. While many of the arguments against SETI are themselves often viewed as unscientific, there is a line of attack on SETI which has credibility and has yet to be adequately answered in the opinions of many scientists.

The core issue in this line of attack is falsifiability. Karl Popper's criterion for distinguishing science from pseudoscience is short and succinct:

"Scientific methodology exists wherever theories are subjected to rigorous empirical testing, and it is absent wherever the practice is to protect a theory rather than to test it."

It is argued that SETI, by this criterion, is not science. (Extreme versions of this criticism label it a religion, an accusation which is aided somewhat by nigh-spiritual levels of importance attached to the finding of extraterrestrial intelligence by prominent SETI advocates present and past.) The key problem with SETI, according to its detractors, is that it lacks falsifiability. Predictions of positive results of the SETI experiments are explained at great length in a wide variety of ways and places, but a definitive failure condition is not provided.

Ironically, the need for such a failure condition can even be found in a former SETI advocate's own writing. Dr. Carl Sagan, a famous scientist, prominent skeptic and affable science populist, in his book The Demon-Haunted World: Science as a Candle in the Dark, provides his so-called "Baloney Detection Kit". SETI critics have sometimes charged that the following entries of this mental kit are violated by SETI:

- Arguments from authority carry little weight (in science there are no "authorities").

- "Occam's razor" - if there are two hypotheses that explain the data equally well, choose the simpler.

- Ask whether the hypothesis can, at least in principle, be falsified (shown to be false by some unambiguous test). In other words, is it testable? Can others duplicate the experiment and get the same result?

Aside from the falsifiability issue, the first item is also often violated--ironically enough, by invoking Dr. Sagan as an authority whose belief in SETI vindicates the SETI programmes. The second is violated by the very conception of SETI. All available data from astronomy research is adequately explained by an assumption that there is no detectable intelligent life in our (or any) galaxy. SETI is, by this criticism, asking a question which does not arise from available data.

SETI advocates respond to this criticism by pointing out that no mainstream scientist claims proof of intelligent life. The advocates merely claim that it is possible or likely that it exists. By such reasoning, any search for anything not yet discovered can be called unscientific. In terms of Occam's Razor, it is debatable whether it is indeed a simple hypothesis that Earth is the only planet in the entire universe that harbors intelligent life.

Given SETI's popularity in the public arena, it is unlikely that its critics will prevail. Nevertheless, the criticisms associated with SETI have made it difficult for the project to acquire public funding, so it relies primarily on private donors.

Issue #72

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