From both direct sample analysis to direct remote sensing (with observations in the UV, Infrared, and Radio [telescope] wavelengths providing much of the detections of these molecules), a fair amount is now known about organic species beyond Earth in the solar, stellar, galactic, and intergalactic regions of space.
Regarding the Solar System, some information has already been introduced in Section 19, especially on pages dealing with Europa, Titan, and asteroids and comets. Overall, there seems to be a wide abundance of organic molecules, but so far none can be unequivocally equated with molecules that are or were once alive. The best evidence of the diversity of organic molecules is found in certain meteorites, mainly those classed as Carbonaceous Chondrites (CC). One in particular has proved a "standard" for direct proof of a broad variety of molecules. This is the Murchison CC which struck in 1969 north of Melbourne, Australia and was found within a day, allowing rapid sampling of the 100 kg collected that minimized contamination. Here is a piece of Murchison:
Murchison has been found to contain Carbon (as graphite) amd more than 70 extraterrestrial amino acids and several other classes of compounds including carboxylic acids, hydroxy carboxylic acids, sulphonic and phosphonic acids, aliphatic, aromatic and polar hydroCarbons, fullerenes, heterocycles as well as kerogen, Carbonyl compounds, alcohols, amines and amides (in all, about 250 hydrocarbon species). Most of the amino acids do not have terrestrial counterparts but 8 of the 20, principally glycine, that make up proteins were in the group; no RNA or DNA has yet been found in meteorites or anywhere else in the Universe.
Peculiar structures in Murchison are provocative in their resemblance to primitive microfossils (compare with ALH84001 on page 19-13). Consider this slice of Murchison:
The same year that Murchison was discovered, a brilliant fireball over the state of Chihuahua in Mexico led to the discovered of hundreds of fragments of a carbonaceous chondrite. Known as the Allende meteorite (named from a nearby town), this is by far the most (in weight) of any known carbonaceous chondrite, thus providing abundant material for scientific study. It contained 16 species of amino acids (some not found on Earth).
Murchison and Allende (and other meteorites) have a distinctive characteristic which bears on the origin of terrestrial life. Earth's amino acids have a "left-handed" chirality (the type of symmetry that is expressed as one holds his/her two hands together). It could just as well have had a "right-handed" symmetry. The vast majority of proteins in a terrestrial environment (including living things) also have left-handed amino acids (sugars, by contrast, possess right-handed symmetry). This strongly implies that the beginnings of life some 3.5+ billion years ago came about by synthesis of organic molecules that obtained their amino acids from extraterrestrial bodies such as meteorites, in which right-handed molecules have been found.
Other CCs have similar but fewer organic species. The Murray CC has simple sugar and sugar alcohol molecules. Some astrobiologists believe these CCs are cometary nuclei; others think they may have been part of the asteroid belt. Spectra obtained for Halley's comet show presence of Carbon dioxide, ammonia, and (with some uncertainty) purines and pyrimidines - the bases in RNA-DNA nucleotides. Water, HCN, formic acid, methane, and ethanol have been observed in the Solar System. The bottom line: the ingredients that were used in the Miller-Urey experiment are all present in the Solar System beyond Earth. If actual life has not developed in other parts of the Solar System, then at least those ingredients needed to produce life in Earth's early oceans could have been introduced by infall of comets, asteroids, and meteorites. (If life itself is extraterrestrial, this notion is embodied in the term "panspermia".)
Where once the Miller-Urey experiment had provided apparently definitive insight into the origin of life on Earth as a purely terrestrial event, the discoveries in the carbonaceous chondrites offer a tantallizing alternative - at least some of the primitive building blocks of life may have first originated in extraterrestrial environments. Those who favor this also point out that the CCs contain water. This suggests that the water in the Earth's surficial locations - mainly the oceans - could also have an extraterrestrial origin. (But degassing of the melted Earth, an earlier hypothesis, remains viable as a source of much/most(?) of the primordial water, provided that didn't escape into space.)
While organic molecules in the Solar System remain of high interest to exobiologists, their presence beyond our star's spatial confines is also of considerable significance, since this would demonstrate the universality of conditions that produce both organic molecules and possible organisms. Telescope data-gathering is proving effective in verifying the presence of organic molecules in protplanetary disks, around certain stars, and, most frequently, in stellar molecular clouds. This (rather fanciful) diagram suggests the variety that has been confirmed or may be expected to be discovered in future observations:
The most effective instrument in searching for signatures of organic molecules in this, and other, molecular clouds is the radio telescope, such as this 12 meter dish at the Kitt Peak Observatory:
Probably the most studied interstellar molecular cloud so far is Sagittarius B2, about 26000 l.y. from Earth, near the galactic center. Here is an optical telescope image (as a negative) and a Radio telescope view of this discrete object:
The average temperature of such a cloud is about 30° K. This seems quite low for active formation of organic molecules, implying that these formed (probably elsewhere) under much warmer conditions before being incorporated into the molecular cloud.
Propanal and propenal are two distinctive molecules found in molecular clouds, such as Sagittarius B2.
Recently, a two Carbon sugar molecule, glycoaldehyde, was discovered by radio astronomy data analysis of Sagittarius B:
The significance of this finding is that glycoaldehyde can react with propenal to form ribose, a molecule that is a central constituent of RNA.
The list of identified organic molecules discovered in molecular clouds, protoplanetary discs, and other source presently exceeds 140. We will consider several examples below. But, if you want to have a fuller itemization, check out this Web site: National Radio Astronomy Observatory (includes data on a comet analysis). From this site, one can visit a Primer on the spectroscopy of organic molecules in molecular clouds and stars by H.A. Wootten of the University of Virginia.
Among the more frequent constituents important to organic molecular species are water, ammonia, acetylene, methane, and ethanol. Several species or groups are particularly interesting. One is ethylene glycol, which we know as the main constituent in one type of anti-freeze used in autos. Its formula, C2H604 is shown in this ball model format:
Vinyl alcohol is another common molecule:
On Earth, Polycyclic Aromatic Hydrocarbons, or PAHs, comprise the largest superclass of organic molecules. Most are built as ring structures (e.g., benzene). PAHs have been found in measurements made by the Spitzer Space Telescope in distant galaxies, some as old as 10 billion years. Here is a set of structural diagrams of some common PAHs that have been found in interstellar molecular clouds (lower illustration).
From the foregoing, one vital conclusion can be drawn: If indeed there is life elsewhere in the Universe, it is highly likely that most forms will be primitive (compared with Earth), prokaryotic, and small (microbial, requiring high magnification tools [e.g., electron microscope] to examine if samples are ever recovered). This seems probable for Mars, for Titan, and for Europa in our Solar System. Despite this likelihood that life forms will on many planetary bodies (those that can support them) be small and not advanced in the evolutionary chain, we as humans retain a hope - almost a compelling desire - that intellectual forms exist elsewhere.
Driven by this last incentive, Science's attempts to determine whether life can exist throughout the Universe generally have followed the same approach as planetologists use to study the other planets of the Solar System - we use our knowledge of Earth to provide an information base and a methodology for comparative study of these planets. Thus Earth is the reference standard. Earth had the right conditions that permitted life to develop some 4 billion years ago (either spontaneously from chemicals derived from gases expelled from its interior and/or from organic materials brought in by meteorites, etc from elsewhere in the Solar System (panspermia concept). In general, in a planetary system the most critical factors are size of the central star (too big restricts the time available for evolution to work), the star's composition (a multi-generation star containing Carbon in its makeup and in the gas/dust cloud from which it is born), the size of the planet (large enough to retain an atmosphere; small enough not to develop a very thick atmosphere with poisonous gases) and distance from the stellar parent (so that thermal energy is available but temperature range falls above freezing of water but below its conversion to steam). The optimal conditions for life to appear seem to be a hydrosphere, an atmosphere, a critical range of temperatures (the "Goldilocks dictum" - not too hot, not too cold, just right), and appropriate incredients.
With the background gained from the above paragraphs, let us return to our more general speculations on life throughout the Universe, using chemical and other non-biological evidence: As hinted at in previous paragraphs, an underlying and perhaps critical condition most pertinent to possibilities for life elsewhere in the Universe is associated with the concept of the metallicity of the star groups. Metallicity defines the proportion or percentage of chemical elements with atomic numbers above 2 in the total mix of elemental and molecular gases and dust available for star formation. Smaller stars with metallicities between 60 and 200% of the Sun's value are favored. These stars comprise about 20% of the total in a galaxy. Other, more negative, factors include the frequency of Supernovae (which over time build up the supply of the "metal" atoms), excessive radiation (continuous or in bursts), and the distribution and numbers of objects capable of destroying protoplanets by impact. Giant planets seem less likely to foster conditions that would aid life's establishment. They conclude that 1) most stars don't have planets and 2) complex life is rare even on those stars with planets. While they don't propose that Earth is unique, they do caution that the statistical distribution from their GHZ and CHZ models support the notion that it may prove very hard to find evidence of life of any kind either in the Milky Way or (even more so) in more distant galaxies.
If Earth defines the standard state, from which there can be no major deviations if life is to form and survive, then life-supporting planets are constrained to vary in their physical and chemical properties only within a narrow range. A planet must have accessible and reactive Carbon capable of polymerizing (some have postulated an alternative element, silicon, as the keystone in complex life-sustaining molecules but no such compounds have been successfully synthesized and made to function like Carbon life). It appears (but is not totally certain) that water is also essential. If so, in an envIron like Earth, this limits the range of temperatures at which life originates to water's freezing and vaporization (boiling) point values of 0 to 100 °C (these values are changed somewhat by prevailing pressures). However, life, once formed, has been found to exist at temperatures that are higher and lower, but generally in the presence of liquid water; witness, the "smokers", which host symbiotically specialized life, that eject superheated water and steam on active divergent ridges on the ocean floor. Atmospheres of Oxygen and Nitrogen favor many forms of life; planets that are either airless or contain hostile chemicals such as methane and sulphur compounds tend to suppress life.
If such conditions, and their ranges, are indeed limiting factors, then only a few, if any, planets in a given planetary system are properly suited to the origin, development, and persistence of living creatures. Thus, while billions of planets are potentially existent universally today, only a small fraction are suited to supporting life. These will be confined to those at a distance from their star where temperatures are in appropriate ranges to allow water in a suitable state (not chemically bound or heated so that all has evaporated and escaped to space). They will have proper sizes to maintain fostering atmospheres. Their chemistry will allow production of molecules (most likely Carbon-based) that can, over time, evolve into organic ones of sufficient complexity to merit the status of "living".
Water has been detected in the Solar System mainly on Earth, on Mars, on icy satellites, and in comets. A search for this compound beyond the Solar System has finally met with success. Astronomers have now detected water around CW Leonus, a Carbon-rich star in its waning life, some 500 l.y. from Earth. This water is believed to now be vapor derived from billions of comets about the star, as it rapidly releases its heat during an explosive phase.
The astronomer Carl Sagan addressed this question of how remote sensing similar to what we have described in this Tutorial could be used to detect life on another planet. His reasoning was based on how such sensors actually detect life on Earth (at the time of his writing an article for Scientific American in 1994, resolution of civilian space images generally could not clearly identify the works of Man, so he confined his surmise to looking at non-human indicators). He pointed out that a Landsat TM type sensor orbiting another planet could detect water, Oxygen, Carbon dioxide, chlorophyll absorption, and the bright response of vegetation in the Near-IR. Today, a high resolution hyperspectral sensor could do these things much better.
Now, to a topic of great popular interest (especially as it ties in with our fascination with UFOs). One of the prime motivations that has stemmed from space exploration continues to the the Search for ExtraTerrestrial Intelligence (SETI) - which is a much higher goal than simply seeking evidence that lower levels of life exist elsewhere. This has so far been something of an ad hoc effort by a small number of dedicated astronomers who have had limited support from private sources. Now, NASA and other large organizations have become involved and a more concerted and systematic hunt for advanced life forms is being funded.
In fact, the hunt has turned serious in that significant funding is now being poured into the search. The best hope still remains to seek out radio signals that have some non-random and intelligible pattern. In Carl Sagan's "Contact", a book adapted into a movie, the heroine, an astronomer (played convincingly by Jodie Foster), was using an array of radio wave receivers in New Mexico that actually has been functional for several decades. SETIites are now awaiting the full deployments of a much larger array, up to 350 dishes, each 6.1 meters in diameter, at the Hot Creek Radio Observatory site 470 km (290 miles) northeast of San Francisco. This is known as the Allen Telescope Array (ATA), which will produce a Very Long Baseline configuration and will initially monitor the centimeter wavelength region. The first 42 dishes are now being emplaced. Here are an artist's drawing of an individual dish and what the full array may look like:
Astronomers have now selected the first five stars to be monitored by the ATA, as listed here:
* beta CVn, similar to the Sun, 26 light years from Earth, in the Constellation Canis Venatui (Hound Dog).
* HD 10307, another solar analogue about 42 light-years away. It has almost the same mass, temperature and metallicity of the Sun. It also has a benign companion star.
* HD 211415, about half the metal content of Sun and a bit cooler, this star is in just a little farther away than HD 10307.
* 18 Sco, a popular target for proposed planet searches. The star, in the constellation Scorpio, is almost an identical twin to the Sun.
* 51 Pegasus. Already famous. In 1995, Swiss astronomers reported they had detected the first planet beyond our Solar System in orbit around 51 Pegasus. An American team soon verified the finding of the Jupiter-like object and the rush to find more extra-solar planets was on.
On the drawing boards, but with the possibility of postponement or cancellation owing to budget problems, is TPF (the Terrestrial Planet Finder), an advanced and specialized space telescope. Its top five candidates for the planetary search are:
* Epsilon Indi A: Turnbull�s top TPF mission choice; this star is only about one-tenth as bright as the Sun and about 11.8 light-years away in the constellation Indus.
* Epsilon Eridani: This star is a bit smaller and cooler than our Sun; it is located about 10.5 light-years away in the constellation Eridanus.
* Omicron2 Eridani: A yellow-orange star about 16 light-years away that is roughly the same age as our Sun.
* Alpha Centauri B: This triple star system is located just 4.35 light-years away and one of the Sun�s closest stellar neighbors.
* Tau Ceti: This star is a G-class star and is in the same brightness category as the Sun. Despite being relatively metal-poor, it is long-lived enough for complex life forms to evolve.
The goals of this mission are to answer these questions:
* Are there Earth-like planets in the "habitable zones" around their parent stars where the surface temperature is capable of supporting liquid water over a range of surface pressures?
* What are the compositions of the atmospheres of terrestrial planets orbiting nearby stars? Is water, Carbon monoxide, or Carbon dioxide present?
* Are there atmospheric components or conditions attributable to primitive life, such as ozone or molecular Oxygen, seen in the Earth's atmosphere?
* How do planets form out of disks of solid and gaseous material around young stars?
The TPF at present will consist of four individual spacecraft that remain near each other as they 'formation fly'. They will sense in the Infrared and use Interferometry to obtain the signals. These two illustrations indicate the concept:
Writer's comment: Finding life elsewhere in the Universe ranks high among the most important tasks (opportunities) open to society. One hopes that Congress and/or the general public will clamor for more missions like this (which at the moment is facing the "axe") and opt for less capital spent in foreign martial involvements.
The ways in which SETI now seeks evidence for intelligent life are diverse and innovative. This is a very daunting and intriguing subject that could warrant considerable coverage space on this page but reluctantly must be confined to a synopsis of a few key ideas. However, we choose to guide you to several sites on the Internet for many of the omitted details. The starting point is the SETI site itself. The SETI Institute is currently being directed by Dr. Frank Drake, the originator of what is now known as Drake's Equation. Here is another Internet site that discusses in some depth that equation: Drake Equation.
For the record, we now state the Drake Equation (in its "dimensional analysis" format) and add some comments on values used in its terms (you can choose your own set of value in (2) above to see how changes affect the outcome):
N = the number of communicating civilizations in the Universe.
R = the rate of formation of stars of types around which planets can form
ne = the number of Earth-like bodies in the planetary system
fl = the fraction of planets with life
fi = the fraction with intelligent life
fc = the fraction that has developed interstellar communication systems
L = the lifetime (span) of civilizations (up to extinction)
Of course, none of these parameters has yet been fixed with reasonably certainty, so that many values (and ranges thereof) have been proposed. One common set (but still provisional) has R = 10; fp = 0.5; ne = 0.2; fl = 0.2; fi = 0.2; fc = 0.2, and L = 50000 yrs (source: Article "Why ET hasn't Called", by M. Shermer; Scientific American, August 2002). For our galaxy, this set of values give N = 400 civilizations.
Each of these term inputs is easily challenged. For fp, any number chosen would depend on such variables as the total number of stars in a galaxy and the type of star suited to planetary formation (usually limited to G and K types) and its percentage of the total population. Recent estimates center around 10% (factor = 0.1). At the 2001 Annual Meeting of the American Astronomical Society, Dr. J. Bally of the Univ. of Colorado presented an argument which concludes that only about 5% of the stars in the Universe are capable of producing (surviving) planetary systems. Massive stars will blow away the gas and dust needed for planets to form. Binary or ternary star systems, which are the most common arrangement, also are unfavorable for planetary growth, especially since one of the pair or trio is likely to be a Giant, and matter is collected as jets owing to attraction. He concludes that planets need to form soon after a star is born in order to have a reasonable chance for survival.
Plausible arguments can be made for values/ranges of each of the others. For example, fl will be sensitive to such related variables as the presence and importance of water, the nature of the evolved atmosphere, and the time needed for higher order life forms to evolve (relative to planet age; rate can vary as these others assume values other than that of our Earth). The possibility of non-Carbon-based life forms must be factored in. In Shermer's article, he calls attention to the great uncertainty of L, the time over which any intelligences will persist before extinction. Pessimists feel that this number can be small. For Earth, civilized life is only about 5000 years old (based on the time at which agriculture and earliest urbanization become practiced). With the advent of the atomic bomb, these doomsayers consider that total mass destruction may be likely. On the other hand, one can conceptualize human society as overcoming its self-destruction tendencies and lasting (into the future) for millions of years (the upper limit then may relate either to a catastrophic impact or the stage in which the Sun burns out and expands to envelop the inner planets).
Still another factor that is not stated in the Drake Equation (or commonly discussed online) is the nature and strength of the communication signal. Present-day radio waves are probably too weak to have much effect in all but the nearest part of our galaxy. Higher energy waves (such as Gamma radiation) would be more powerful but we on Earth have not yet devised suitable transmitters that efficiently send out these rays. And any signal transmitted must move away in many directions from it planetary (spherical) source; if only directional beams are emitted, the number of planetary receivers (made by other intelligent recipients) in the right position to pick the signals will be greatly reduced. But, directional beams (those using laser light are especially promising) have one advantage - they remain concentrated over a small angular volume. As we on Earth continue to find many more planets (most would be in our Galaxy at this stage of our detection capabilities), we can systematically send signals to these with a good chance to intercept them in the narrow field of view encompassing our signals. Likewise, any intelligent and technically capable life on any one(s) of these may have by now spotted our Solar System and have, or will, send signals to us. What the "message" should be, owing to uncertainties of language recognition and translation, is probably dictated by the need to transmit something of a universal nature: sending intermittent signals (sound or light) that consist of a series of prime numbers is a favorite suggestion. Since mathematics has a unifying generality to it - all intelligent beings should find some of the same fundamental theorems and expressions - the message we receive (or send) is most likely to be in that format (spoken/written anguage is ruled out because each one on Earth has a unique set of meanings attached to words that therefore precludes universality). Or, the message could be made up of musical notes (example: "Close Encounters of the Third Kind").
Start with hypothetical observers at two points A and B not then in contact in early spacetime. Over expansion time, their light cones would eventually intersect, allowing each to see (at time t1) other parts of the Universe in common but not yet one another. At a later time, beyond t2 ("now") in the future, the horizons of A and B (boundaries of the two light cones) will finally intersect, allowing each to peer back into the past history of the other.
All in all, we are still a "far cry" removed from having any reasonable estimate of probabilities of other civilizations actually extant. Shermer develops arguments that produce numbers as low as 2 to 3 for our galaxy (if really strong signals can reach Earth from other galaxies, this number would then greatly rise). T.R. McDonough of the Planetary Society arrives at 4000 as a reasonable "guess" for the number of planets in our galaxy; Carl Sagan during an optimistic moment, came up with 1,000,000. Today, no one is arriving at zero, so those seeking ET can remain hopeful, even optimistic.
In October 2001, the Scientific American magazine carried a review article (pages 61-67) by G. Gonzalez, D. Brownlee, and P. D. Ward bearing the provocative title of "Refuges for Life in a Hostile Universe". We will not paraphrase its many intriguing statements and conclusions but urge you to track the article down and read through it. Its bottom line is that there appears to be only a narrow range of conditions around stars and within galaxies that would likely harbor life (organic matter; not necessarily intelligent). They define CHZ and GHZ as Circumstellar and Galactic Habitable Zones respectively. In systems like our Sun's this is confined to a narrow inner zone. In spiral galaxies the GHZ is beyond the inner bulge, halo, and thick disk that consist mostly of old stars; the more favorable region is an annulus about midway from the center to the edge defined by the spiral arms, in that part known as the thin disk.
A splendid review of the origin and occurrence of life beyond Earth is the 1999 book by Paul Davies, The Fifth Miracle: The Search for the Origin and Meaning of Life, Simon & Schuster.
Whether the evolutionary mechanisms found to operate on Earth - namely, change into diverse and usually more complex forms in response to changing conditions, aided by genetic processes and natural selection - lead to intelligent life elsewhere can only so far be the subject of speculation (devoid of any direct evidence). But, again, considering the large number of favorable planets - almost certainly in the millions - spread throughout the billions of galaxies, it would not be surprising, and is almost to be expected, that organisms with consciousness and other aspects of intelligence will someday be communicated with, thus supporting the thesis of Universal life. It is provocative to conjecture whether these "alien" thinkers have some insight into the concept of an Intelligent Designer (Creator or God) and whether they believe, as most here still do, in the special gift of that God of the "soul" destined to persist in some form of immortality.
Left unsaid up to now is the questions "What kind of intelligence?" and "How intelligent?" Will the aliens be significantly smarter than humans presently are? The answer is probably YES - for two obvious reasons: 1) 'Homo' has been around for only about 2 million years, and has only really developed a sophisticated intelligence in the last few thousand (or hundred!) years, so it is sensibly likely that evolution culminating in intelligence has proceeded farther on many planets; and 2) since getting to Earth from planets light years away requires some very advanced techniques not yet discovered and developed by terrestrial scientists, any aliens who do arrive or visit would be much more complexly evolved mentally.
This is perhaps a propitious moment to interject the writer's (NMS) personal biases about the likelihood of extraterrestrial life and, in particular, intelligent life. I, and others, believe, with almost absolute certainty, that life exists elsewhere in the galaxy, and even more certainly, in most other galaxies. I suspect that some of that life has a form or degree of intelligence. My argument (much like ones put forth by experts in this field) for this view is largely statistical. With so many billions of stars and billions of galaxies, the probability that planets are the norm and on at least some the conditions favoring life - evolved to varying degrees - are in place is high enough to conclude safely that life exists. The implications for this are enormous, and were part of my speculation near the bottom of page 20-10: The scientific model for a Universe (or a Multiverse) contains a strong likelihood that evolution in the broadest sense has produced creatures that include intelligent beings; all of this is purely natural, the consequence of the inevitable occurrence of life-forming environments in a diverse Universe. The metaphysical model differs in a fundamental way: it holds that a Creator or Intelligent Designer either directly made intelligent life in a miraculous way or decided to produce that life by a master plan based on evolutionary principles. But, it seems ridiculous to have produced humans only on one of the likely billions of planets existant, with all other stars/galaxies being there just for our amusement or to give astronomers something to do. If God exists, I have come to believe that the grand scheme the Creator Being imposed on the Universe (Multiverse) was to set into the creation model the inherent predisposition for life to have developed on a vast myriad of planets. In simple terms: God intended for intelligent life to be widespread universally and not just on a single planet - Earth. This idea, and related topics, is explored more fully on the next page.
Notice that we have progressed well down this page without dwelling in detail on the favorite topic among those - scientists and laypersons - who speculate on the possibilities and ramifications of intelligent "alien" life in our galaxy, and by reasonable inference, in most other galaxies: the reality of whether we have really been visited by these creatures in their spaceships (UFO's) at times in the past, and a corollary, whether we on Earth will ever have the means to visit other planets by some means of space travel. In light of present knowledge, any extraterrestrials will almost certainly not originate in any other Solar System planet but would reach us from beyond - well into outer space. We, in turn, must gain experience in space travel by first journeying to one or more of our neighboring planets. Probably in the lifetime of some who read these words this will happen. But extending this travel to other stars - interstellar travel within the Milky Way - may not happen in this timeframe, although 100, 500, 1000, a million years hence, the advances in science and technology should bring this about. But, complications and limitations must be overcome.
By far the biggest problem in interstellar travel is distance. A simple example illustrates the difficulty. The nearest star is Proxima Centauri, 4.2 light years away (actually, Alpha Centauri, 0.1 l.y. farther away, is a better choice owing to its size), or about 42 trillion kilometers (26 trillion miles) from Earth, would be a reasonable first target. To help you visualize these distances, look at this diagram (the distances are in Astronomical Units [A.U.])
If a manned spacecraft were to leave the Solar System at the same velocity that Pioneer 10 had when it actually did escape the system, namely, 60000 km/hr (37000 mph), it would take approximately 80000 years to reach one of these two stars. (And, the same time to return to Earth, unless a one-way trip is the choice, then at least double that total.) This obviously would be impractical under the psychology of today's thinking. The solution should be obvious: Earthlings must build a spacecraft that contains all those materials and provisions needed to sustain life for eons. Chief among these would be foodstuffs, water, Oxygen and other essentials. Then, even if human life spans can be extended, the strategy would still have to be: to continually recreate people on board through breeding, so that those who arrive at Alpha Centauri (hopefully, we will discover planets there; none have been detected as yet) will be many generations down the time path of continuing life of the future. Trips to other more distant stars may be more enticing if these show evidence of planets that can sustain life. Surely, from the billions of humans on Earth when this time of launch comes there may be volunteers who are agreeable to setting forth on this voyage; if and when we develop the appropriate technology, the travelers may need to consent to being placed in some kind of suspended animation (a body freeze technique is commonly proposed to put living creatures into hibernation). Such a trip is likely to fulfill at least one of four motivations: 1) either the innate need for man to explore; (2) and/or a desire to establish contact and exchange knowledge with other civilizations; (3) and/or a compelling need to survive if Earth should threatened to become uninhabitable); (4) and/or a decision to colonize another (uninhabited) planet with suitable living conditions for organisms, or if intelligent beings are found, to settle with them.
However, most readers of this Tutorial would judge the long duration travel scenario (with or without hibernation) to be rather undesirable even if altruistic. Is there any alternative. Yes, if we can find new means of propulsion through space at much greater speeds than presently achievable. For instance, let the inertial velocity reach 0.2 the speed of light 'c' (60000 km/sec). If that occurs soon after launch, the spacecraft should reach Alpha Centauri in 5 x 4.3 l.y., or 21 years (earthtime), or 42+ years roundtrip. Theoretically, this transit time can be greatly shortened if the spacecraft attains a velocity near light speed. However, relativistic effects will come into play (see Preface to this Section), including differential aging between space travelers and those remaining on Earth. Thus, time dilation would make the high speed trip appear to those on Earth to have taken longer than the 42 years plus adjustments for being somewhat under light speed.
There is another problem that may even supercede the time constraint on long distance travel. This is the radiation - cosmic rays and other particles - that pervade space. Calculations based on studies of humans in near Earth missions and of objects in space then recovered indicate that unless the spacecraft is properly sealed within a protective radiation-shielding material, the humans on board would receive lethal doses after traveling for several years. This might even apply to trips as short as Mars and back. The quantity of protective material is large, adding so much weight to the spacecraft that known and existing propulsion systems fall way short of being powerful enough to launch the craft into the long journey. Technology must find acceptable means to overcome this problem. The solution is not yet in sight but future discoveries hopefully will neutralize the radiation hazards.
There are many other factors required for success and safety to take into consideration. Perhaps at the top of that list is to find propulsion systems that can reach these high speeds (significant fractions of the speed of light); to get to such speeds requires huge expenditures of energy. Presently, no energy supply is known, but some sound proposals for possibilities already have surfaced: ion engines, anti-matter engines, controlled nuclear processes, gravitational "slings", laser beams pushing on light sails, and various other innovative but speculative mechanisms for powerful propulsion systems. Quantum-minded thinkers can conjure up schemes that depend on "wormholes" and "quantum tunneling", "timewarps", and the like. A long shot depends on the proof of existence of the hypothesized "tachyons", particles that travel at faster than the speed of light; if real and accessible, some technique would be needed to harness them as aids to propulsion.
Whatever propulsion system is eventually proven feasible, one requisite is that it be one that travels with the spacecraft (instead of a "one-shot" push at the outset) so that there would always be a means for course corrections, maneuvering, handling the unforseen, possible landing, and eventual return to Earth if that is a mission requirement. If we base our predictions for space travel eventuality on the huge advances in science and technology over the last two centuries - now seeming to occur as though growing exponentially - it is reasonable to expect that the possibility of interstellar travel will turn into reality in the not too distant future (say, in this millenium). If that is achieved, then the counterpoint argument is that aliens "out there" may be ahead of us and have in fact visited Earth - if we are judged to be interesting enough.
So, is there any "bottom line" to this speculation on aliens. There certainly is no consensus among scientists. The writer has a strong bias: Earth has never been visited by alien intelligents. Why do I say that? Because the distances between inhabited planets (as probabilities imply they exist) are just too great for space travel. Propulsion systems near the speed of light probably are unattainable. Sending creatures who reproduce, and thus arrival occurs generations later, is probably not appealing to the erstwhile travelers. Instead, the aliens would more likely send unmanned robotic spacecraft and/or would send signals into space to communicate. But, personal appearances by the aliens are (so far) just "figments" of human imaginations.
To sum up the hard realities of space travel: 1) present and foreseeable technologies are still far short of making such trips plausible, safe and worthwhile 2) in time (probably many centuries), humans may learn enough to engage in such endeavors; 3) almost certainly, a manned trip will be preceded by unmanned spacecraft to prove the workability of the technology; 4) if mankind survives itself (or evolves into some form of superintelligence that can control the various threats to its self-destruction (wars; envIronmental nihilism) on Earth, trips to other stars seems someday to be inevitable; and 5) in the meantime, we should continue to inventory suitable planets and search for intelligent life elsewhere (SETI), so as to develop the incentive for undertaking travel to candidate planets; we should also hunt for any evidence that Earth has previously been visited (stated in the Fermi Paradox: If aliens have already come to Earth, as might be expected since statistically there could well be many more advanced civilizations than those on Earth today if intelligent life is widespread in the Universe, then why haven't we found any valid signs of their visit?)
An excellent article for the layperson on space travel feasibility, simply titled Star Trek, by W.S. Weed, appeared in the August 2003 issue of Discovery Magazine. It is well worth reading to realize both potential and problems in visiting even the nearby stars. Its gist is summed here: This is the list of the 5 propulsion systems reviewed in the article: Atomic Rockets; Nuclear Fusion: Antimatter; Laser Sail; and Fusion Ramjet. Within several decades one or more of these systems, and probably others yet conceived, may become practical working modes that can propel space travelers at speeds from 0.1 to perhaps 0.8 that of light. Most of the systems discussed would require that humans onboard would have to spend from 40 to 100 years to reach stars close-by. The various problems they would face: food/water; air; deletorious gravitational effects; radiation threats, and, not the least, psychological adjustments are all solvable but require considerable refinement from present-day capabilities. Of course, the age problem (not helped if the speed of light is not approached) can be handled by onboard breeding and birth, i.e, resorting to multiple generations.
Perhaps this is the point to introduce a note of caution. Based on some current trends, we ourselves may do intelligent life in, or a super asteroid, might extinguish us. That is one motivation for continuing to seek means to escape the confines of our Solar System and seek out new planetary systems around other stars. There is another compelling reason, purely natural in scope, suggested by this illustration:
Thus, the Earth and its siblings are doomed ultimately by the exhaustion of fuel in the Sun which will lead to a catastrophic (if life still exists then) event associated with the Red Giant phase (mentioned on page 20-5a). Even before the next 5 billion years, severe warming may wipe out life if countermeasures haven't been found in the next billion years. Or, the ultimate Doomsday scenario: Man, or some successive genus or new type of intelligence, destroys "himself" through acts of his own (common example - total nuclear holocaust). A frightening yet plausible mode of destruction is for humans to develop thinking machines that may become "smarter" that people and develop the capability to subordinate their makers or even gain a control that leads to the demise of the flesh-and-blood race. (This is already happening at levels below us humans as we are driving lower forms of life into premature extinction). Yet, somehow I fail to feel much personal concern at this time, as I happily believe that humans will mature enough in the future to assure their control over their survival and to become good stewards of other life.
In closing Section 20, contemplation of Cosmology, and its astronomical substructure, is a truly humbling experience for one's brain. The wonder is: that there exists on one tiny point in a humongous Universe something called the "conscious" and that the human mind (yours, for instance) can conceive of, and begin to understand, the truths of this Universe's attributes and history that are continually being discovered and refined. Finally, it is both astonishing and reassuring to realize that all beings - be they people or animals/plants or inanimate matter - are remarkably cosmological in nature: All the atoms in our bodies were once contained in stars or interstellar space; our parts in a sense are variously billions of years old and their atomic constituents, even after multiple dispersions and reassemblies, will last at least as long as the present Universe - estimated to continue for perhaps 50 b.y or more. In one way, then, our essences will have achieved some kind of plausible Immortality in view of the many incarnations that preceded our current atomic arrangement and are yet to happen. But, from the humble side, perhaps "We are not alone" and certainly we are not at the center of the known Universe; our importance is sui generis and our rank among the Universe's populations is probably just average.
Despite this humbling thought, you, the reader, and me, the writer, and all of humanity possess a fundamental trait in common: We wonder about the meaning of it all, and especially what has caused mankind and all of the Universe(s) to have come into existence. This inclination - which comes under the heading of Teleology (referring to prime causes and purposes) is not the normal purview of Science. Instead, the speculations are the mainstays of Philosophy and Metaphysics. The writer has long engaged in trying to relate Science to the Ultimate Meaning of it all. I have added a page (Next button below or page 20-12b) to this Section which documents much of my thinking, to which you are invited to visit next.
This Section on Cosmology has doubtless been heavy going for most readers. Some may be inspired to wish to learn more. Refer to the Preface link accessed from page 20-1 for the references to books consulted by the writer in preparing this overview. We want to steer you to a Web site that allows you to seek more pictures and textual information about most of the topics that have been covered or touched upon so far in this Appendix. NASA Goddard astronomers have put together a Web site that features many previous Astronomy Pictures of the Day (APOD). In the Search box, you simply type in a topic, e.g., young stars; supernova; black holes; spiral galaxies, planet, etc. and if the category is there a running text with links to subtopics and pictures will be delivered. This can be an adventure. Another web site is Wipedia's Physical Cosmology. The next site gives a Brief history of Cosmology . Both history and metaphysics are found at this PBS site. Finally, a series of useful topical links are embedded in this AIP site.