1. How did life on Earth originate?
2. How common is life in the Universe?
3. Does life exist elsewhere in the Solar System?
4. Are the Martian " fossils" real?
5. How strange could alien life be?
6. Could silicon-based life exist?
7. Could some extraterrestrials have green blood like Mr. Spock?
8. What are the chances that current SETI programs will succeed?
9. If we encounter intelligent extraterrestrials, will they be friendly?
10. What religious consequences might the discovery of extraterrestrial life have?
11. Is travel to the stars possible?
12. Are UFOs alien spacecraft?
13. Is there life on Jupiter's moon Europa?

Intense controversy surrounds this question. One of the bones of contention is whether the prebiotic materials from which terrestrial life arose formed on Earth, after our planet cooled and solidified, or whether they came subsequently from space (delivered by impacting comets and meteorites, which in turn picked up their organics from interstellar space). Quite possibly, it was a combination of the two. Another question mark hangs over the location where life first appeared – the shores of primordial lagoons, the environs of hydrothermal vents, and hot underground rocks being among the various suggestions. The discovery of primitive life in extreme locations has fueled this speculation. Tied to this question of location is the issue of the biochemical steps involved in the origin of life. It' s relatively easy to conjure up a starting brew of simple organics, including amino acids and the like. But the stages involved in the manufacture of nucleic acids, proteins and the first single-celled life are far from clear. One possibility is that there was an "RNA World" before the DNA-protein environment of life as we know it today, although the fragility of RNA as a molecule makes this scenario questionable. Research into the origin of life is currently progressing along several fronts, including attempts to reproduce the steps leading to life in the laboratory, the study of extremophiles in an attempt to identify the earliest common ancestor of all extant terrestrial life, computer simulations of prebiotic evolution, and investigations of possible extraterrestrial connections (such as the transfer of organics between worlds by meteorites and the formation of complex molecules in space).

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The answer to this is closely tied to how life on Earth got started. One idea that' s attracting a lot of attention right now is that there may be a "Life Principle" at work in the Universe. In other words, wherever there exist the necessary prebiotic ingredients – simple carbon compounds, an energy source, and maybe water – evolution leading to primitive living things will proceed inevitably and automatically as a matter of course. This idea is an offshoot of complexity theory and has been pushed hard by researchers such as Manfred Eigen, Stuart Kauffman and Robert Shapiro (see Shapiro's Planetary Dreams, for example). I happen to think this makes a lot of sense. If there is a Life Principle, then we' d expect life – at least life at a microbial level – to be rampant across the cosmos. We' d certainly expect signs of it somewhere else in the Solar System (such as on Mars and Europa). And we would expect to find the early stages of the Life Principle in action on the surface of Titan, where there is probably a good prebiotic brew of chemicals and an internal energy supply. Contrasting with the Life Principle hypothesis, is the view that life came about as a result of a fantastically unlikely sequence of events – so unlikely that it may only have happened a few times (or not at all) elsewhere. At the moment, I think the evidence leans somewhat in favor of the ubiquitous life school of thought. That evidence includes the discovery of organic matter in comets, meteorites and interstellar clouds, and the ease with which biochemicals and primitive cell-like structures can be synthesized under plausible prebiotic conditions. Time will tell who is right, but I personally expect the first incontrovertible signs of extraterrestrial life to be found within the next two decades.

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We' re probably going to find out over the next 10 to 20 years, thanks to a small armada of probes that will explore the environs of biologically interesting worlds such as Mars, Europa, and Titan. Many astrobiologists suspect we may uncover evidence of extinct or even extant microbial life on Mars, extant oceanic life on Europa (assuming an ocean is present), and signs of prebiotic evolution on Titan. If Europa and Titan prove fruitful then Callisto, Ganymede, Triton and even Pluto offer other promising locations in the search for life or complex organic synthesis. Comets and the atmospheres of the gas giants are also still on the astrobiological agenda. Looking beyond the solar system, large space-based optical and infrared interferometers will be able to search not only for Earth-like planets but also for signs of biogenic activity in the spectra of those worlds.

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Members of the original discovery team at NASA continue to argue that the tiny worm-like structures in meteorite ALH 84001 are the remains of Martian microorganisms. Additional signs of biogenic activity in the rock, they maintain, include carbonate globules with magnetite and iron sulfide inclusions (similar in size and texture to carbonate precipitates that are often formed by terrestrial bacteria) and organic molecules known as PAHs (polyaromatic hydrocarbons). Other scientists have been more skeptical, preferring an inorganic explanation, and the controversy rages on. The purported fossils are extremely small – from 20 to 100 nanometers (nm) in length. This makes them smaller than the tiniest confirmed terrestrial bacteria, known as Mycoplasma. Recently, though, geologists at the University of Queensland announced that they had found living "nanobes" with diameters of 20 to 150 nm in sandstone taken from 3 km below the seabed off Western Australia. Whatever the outcome of the Martian "fossils" controversy it underscores the difficulty in detecting extraterrestrial life. Even when laboratory samples are available researchers can disagree over whether unambiguous signatures of life are present or not. The same kind of dispute surrounded – and continues to surround – the results of the Viking biology experiments, and it may well surface again when future planetary probes hunt for life in the solar system.

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Very strange indeed judging by how bizarre many terrestrial creatures (like deep-sea angler fish, giant anteaters, Venus flytraps, etc.) seem to us. Even if life is universally based on the same biochemical building blocks, local circumstances and the huge variation possible within genes will ensure an almost unimaginably wide diversity of appearances. At the same time, some anatomical components, including legs, wings and eyes, might occur over and over again simply because they are so useful (a phenomenon known to evolutionary biologists as convergence). We might not find anything that looks much like human beings on other worlds, but bipeds with heads and forward-looking eyes are a real possibility. If life can arise in very un-Earthlike environments (such as the atmosphere of a gas giant) or with a completely different biochemical basis, then we can expect to come across an incredibly broad spectrum of life. Among the weirder possibilities already entertained by scientists are creatures living on the surface of a neutron star and an intelligent interstellar cloud (in Fred Hoyle's The Black Cloud).

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People have speculated for over a century about whether life always has to be based on carbon. One seemingly promising alternative is silicon, the element which lies immediately below carbon in the periodic table and is chemically most similar to it. Silicon is certainly common enough in the Universe and its basic chemistry is a lot like that of carbon. But silicon' s biological credentials have to be challenged on a number of counts. One of these is its powerful affinity for oxygen. Wherever silicon turns up – on the Earth, on the Moon, in meteorites, or in interstellar space – it is combined with oxygen in highly stable compounds. And once it is combined in this way, either as silicon dioxide or as silicates, it is hard to see how it could be unlocked again to serve as the basis of complex silicon-based biomolecules. A second problem centers on the normal physical state of silicon dioxide. When carbon is oxidized during respiration in a terrestrial organism, it becomes the waste gas carbon dioxide, which is easy to get rid of. The oxidation of silicon, in stark contrast, yields a solid because, as soon as it forms, silicon dioxide organizes itself into a lattice in which each silicon atom is surrounded by four oxygens. Disposing of such a substance would, to say the least, pose a major respiratory challenge! But maybe not an impossible one. In his novel A Martian Odyssey, Stanley Weisbaum imagines a silicon creature that deposits a brick of silica every 10 minutes – respiring and building temporary living quarters at the same time. And Star Trek fans will no doubt recall the Horta in episode 26 of the original series – a creature which became virtually extinct every 50,000 years, leaving just one individual to look after the eggs of the next generation.

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A favorite theme of science fiction is the diversity of pigmentation that might exist among extraterrestrials due to differences in the color of their blood. Star Trek' s Mr. Spock, for example, has a greenish tinge to his skin on account of being a green-blooded Vulcan. The variation in the color of the blood of terrestrial species is due to differences in the metallic component of the blood from one species to another. Human blood, like that of all mammals and birds, is red because of a cocktail of iron-containing pigments, including hemoglobins (red), myoglobins (red), chlorocruorins (green), and hemerythrins (violet). Horseshoe crabs and certain other organisms, on the other hand, have blood in which the oxygen-carrier is hemocyanin, a copper-containing pigment that is blue. Perhaps Mr. Spock has blood which is similar to that of the humble sea cucumbers which is yellow-green and based on vanadium.

- from the Extraterrestrial Encyclopedia Back

The answer depends on how many intelligent species are presently sending out signals that we' re capable of picking up. People have been playing this entertaining guessing game ever since Frank Drake wrote down his famous equation and have come up with answers ranging from zero to enormous. It all hinges on the initial assumptions you make. At least now we can be pretty confident that planetary systems are fairly common, and the scientific consensus is leaning toward the view that life will come about whenever it gets the chance. The big questions concerning which we have few clues are: (1) How often does life become intelligent (human level or greater)? (2) How often does intelligence develop advanced technology (capable of interstellar communication?)(3) How often does a communications-capable species make a sustained effort to send messages? (4) How often, on average, does a communicating race survive? An optimist might argue that intelligence confers such a big survival advantage that it will be commonly selected for – and has been more than once on Earth (humans plus cetaceans). But if we use the dolphin argument for (1), then we have to admit that intelligence doesn't t always lead to technology. The example of the human race (so far) suggests that we might need to be conservative about the answer to (3). We've done quite a lot of listening but hardly any shouting. If the Galaxy is full of listeners like us, they are all wasting their time. In fact, they might be wasting their time anyway if they' re only at about our technological level. Electromagnetic waves (including radio waves and laser light) represent the main of line of attack in SETI searches. But the chances are, given our current rate of technological progress, that we will develop a superior, superluminal method within a few centuries (physics allowing!) As soon as this happens, we will switch over to that new method. Even if there are a lot of communications-capable races out there, the number falling within our current technological window is tiny – maybe no more than 50 in the whole Galaxy, with the nearest one perhaps several thousand light-years away. That doesn't allow time for even a single question and answer before we pass out of the window. So why bother? I think we should bother because the prize at stake is so great. But I also suspect we are only likely to make contact when our technology is significantly more advanced.

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There are a couple of schools of thought on this one, and they tend to be reflected in our science fiction portrayals (compare E.T. with Alien for the polar extremes!) The first is that any race which manages to get a message through to us or actually arrives here in a starship will have matured beyond its violent, combative stage. The opposite view is that "top" races, like humans, are the ultimate products of billions of live-and-let-die evolution, so they are bound to have an aggressive streak. But who knows? Every creature on this planet looks primarily after itself. Life seems almost bound to have a selfish streak. So, I suspect other intelligent races in the Galaxy will be a bit like us in character – a complex mixture of passivity and aggression, altruism and selfishness. And, as with our species, there is likely to be plenty of variation from one individual of a species to another.

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It depends what religious belief, if any, you start out with. Some fundamentalists might be surprised by the discovery of alien life since they maintain that terrestrial life was a one-off creation of God. (Actually, such a discovery would surprise some evolutionary biologists, too, who argue that while life came about naturally, it did so as a result of an incredibly unlikely chain of events.) Fundamentalists could accommodate the discovery by suggesting that God had the capacity to create life wherever he chose. In fact, such arguments have been rehearsed many times by philosophers over the past millennium. A more profound discovery would be that of extraterrestrial intelligence since it' s a fundamental tenet of Judeo-Christian belief that God has a special relationship with mankind. If there are other intelligent, moral creatures out there, theologians would have a lot of agonizing to do over the issues of incarnation and redemption. Could Jesus appear in alien forms? On a different front, SETI itself has some of the elements of a quasi-religion, including the search for a higher power that may be able to reveal to us the secrets of the Universe and a hoped for epiphany in the form of "first contact."

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Yes, it' s happening right now. Four spacecraft from Earth – Pioneers 10 and 11 and Voyagers 1 and 2 – are already heading out of the solar system on trajectories that will eventually carry them into interstellar space. Simply escaping the Sun' s gravitational pull is not a problem. The real challenge is to cross the vast distances between stars in a reasonable amount of time. Only by achieving speeds that are a substantial fraction of the speed of light will this be possible. Various interstellar propulsion systems, relying on Newton' s 3rd Law (i.e., the rocket principle) have been suggested and some may well be developed during the 21st century to the stage where they will be capable of powering a robot mission to a target such as the Alpha Centauri system or Barnard's Star. Crewed starships, however, seem a much more remote prospect. Two basic strategies for achieving human interstellar travel suggest themselves. The first involves traveling at sufficiently high speed (probably greater than 90 percent of light-speed) that relativistic effects come prominently into play. For the crew of a highly relativistic starship, time is significantly slowed down, so that even far flung missions are possible in much less than a human lifetime. The big problem with this, of course, is that many decades or even centuries may have elapsed back on Earth, effectively hurling the star travelers into the future. The second strategy avoids this disturbing effect of "time dislocation" by resorting to travel outside of normal space and time. One such ploy, for example, might be to utilize a convenient wormhole "subway" as Jodie Foster' s character does in the movie Contact. Another might be to exploit the effect of quantum entanglement to teletransport instantly from one place to another. Whether such techniques will ever become practicable remains to be seen.

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