Faint Echoes, Distant Stars

by Ben Bova

  Astrobiology is just beginning to make an impact on NASA’s exploration plans. To date, no space mission has been planned primarily from inputs by astrobiologists. This is changing, however, as the astrobiologists gain more knowledge and confidence and as the astrobiologists and mission engineers build up a mutual understanding and respect.

  FINDING EARTH-LIKE PLANETS

  The next generation of technology will allow astronomers to detect small, rocky planets circling other stars. Perhaps the Kepler system will make the first detection of an Earth-sized planet; perhaps astrobiologists will have to wait until the Terrestrial Planet Finder system is functioning in space.

  Once Earth-sized planets are found, the race will be on to determine if they are Earth-like. Do they have free oxygen in their atmospheres? Is there any trace of water vapor?

  Finally, if the SETI investigators train their radio telescopes on such worlds, will they detect radio signals? Will we at last eavesdrop on the day-to-day radio chatter of another civilization?

  Or will we find deliberate signals beamed out into the interstellar vastness, signals aimed at finding kindred thinking creatures, signals intended to end the long, lonely isolation of an intelligent species?

  And if we do detect such signals, will we dare to answer?

  SPANNING THE CHASM OF IGNORANCE

  Like that frontier scout peering across the chasm, astrobiologists are trying to determine what lies just beyond the reach of our instruments.

  This much seems clear: Unequivocal proof of the existence of life could be found within a decade or two. The timing of the discovery depends more heavily on political decisions about the funding of astrobiological research than the nature of the research itself. Deep beneath the surface of Mars, somewhere in the ice-covered seas of Jupiter’s Galilean satellites, in the hydrocarbon lakes of Saturn’s moon Titan, or perhaps on the icy body of a comet, living organisms could be found.

  To the general public they will not seem terribly exciting, at first glance. They will most likely be single-celled creatures, akin to the archaea and bacteria that have thrived on this planet since it cooled enough to begin forming a solid crust.

  But those microscopic creatures will be telling us a soul-shaking fact: We are not alone. Life is neither a chance occurrence nor a special, unique creation, a fluke that occurred once on a single small planet. Life is as much a part of the universe as the stars themselves.

  Is there intelligent life out there somewhere? That is the great unanswered question. Has life evolved into intelligence somewhere in the depths of space? How and when we find another intelligent species is entirely unguessable. We may have stumbled across it already and simply do not recognize it as such. Or we may be totally alone in our self-awareness and yearning to find others like us.

  Perhaps one day the human race will find its peers. Or its superiors. That thought may fill some with foreboding. But to those who search the universe for understanding, that fateful moment will profoundly change our attitudes about ourselves and one another. Faced with intelligent creatures from another world, creatures who look nothing like us, we will at last recognize that all humans are brothers and sisters, that each of us is part of the same family.

  That moment will mark the end of humankind’s adolescence and the beginning of true understanding.

  Whether we do or not, whether the universe is barren of life or teeming with star-spanning civilizations, it is our quest for understanding that is important. Whatever the ultimate answers are, we should not fear to seek them. The astrobiologists search for knowledge, for understanding, and that is what makes us human.

  The astrobiologists are engaged in the ultimate quest, and we are all a part of their mission.

  Postscript

  The Apocalypse to Come

  A STAR IS DYING, and the planets orbiting around it are burning to cinders.

  The star is called IRC+10216 in the arcane lexicon of the astronomers. It is more than 550 light-years from Earth, a mind-boggling distance of some 5,500 trillion kilometers.

  Like our Sun, IRC+10216 was a stable yellowish star that had beamed out its energy reliably for billions of years. Then it began to swell, bloating like some monstrous abscess, turning a sullen red as it grew larger and larger.

  Any planets that might have been orbiting IRC+10216 were doomed. Their atmospheres were boiled away, their oceans flashed into steam; eventually the swelling star swallowed the charred cinders of their remains. If life once existed on any planet of that star, it has been incinerated.

  Still the star grew, hotter, larger, glaring balefully as planet after planet was engulfed by its inexorable expansion.

  Far out in the icy darkness at the fringes of the star’s planetary system, an immense cloud of comets had been circling for billions of years, huge chunks of frozen water, massive icebergs endlessly floating through the emptiness of space.

  The inescapable heat from the star’s relentless expansion is melting those comets, even vaporizing them. Fifty-five hundred trillion kilometers away, astronomers on Earth can detect the water vapor as the giant comets are boiling into steam.

  Life may be part of the natural processes of stars and planets. But it can also be snuffed out by those impassive, relentless processes.

  One of the astronomers who discovered the tragedy of IRC+10216, David Neufield of Johns Hopkins University, said, “We believe we are witnessing the apocalypse that will engulf our solar system in six billion years.”

  Is this truly the eventual fate of the Earth? Will life be erased from our planet, boiled away by our Sun’s remorseless expansion? And if it is, will there be other intelligences watching our tragedy, mourning our star’s death, wondering if life existed on the worlds of that distant star, wondering if they are alone in the universe?

  Or will we, in the fullness of time, expand our habitat beyond the Earth, even beyond the solar system, and spread our seed through the stars themselves? This is the path to humankind’s immortality. Once we begin to build self-sustaining habitations beyond the Earth, we will have detached the fate of humanity from the fate of the Earth. Once we learn how to reach other stars, we will have detached our fate from the inevitable doom of the Sun.

  Is this the destiny of our kind? If it is, our success will depend strongly on what astrobiology can tell us about how to survive in space—and eventually to prosper there.

  Appendix 1

  Units and Conversions

  LENGTH

  1 millimeter (mm) = 0.03937 inch

  1 centimeter (cm) = 10 mm = 0.3937 inch

  1 meter (m) = 100 cm = 39.37 inches, or 3.28 feet, or 1.09 yards

  1 kilometer (km) = 1,000 m = 0.62137 mile

  1 mile = 1.6093 km

  1 inch = 2.54 cm = 25.4 mm

  WEIGHT

  1 gram (gm) = 0.0353 ounce = 0.0022046 pound

  1 kilogram (kg) = 1,000 gm = 2.2046 pounds

  1 metric ton (tonne) = 1,000 kg = 2,204.6 pounds

  TEMPERATURE

  Scientists usually measure temperature in the Celsius (also called Centigrade) scale or in the Kelvin (or Absolute) scale. Absolute zero is the point at which, theoretically, all molecular motion stops and the energy we call heat ceases to exist.

  KELVIN CELSIUS FAHRENHEIT

  Water boils 373 100 212

  Water freezes 273 0 32

  Absolute zero 0 273.15 459.67

  Note: The Kelvin and Celsius scales use degrees of the same size. To go from Celsius degrees to Fahrenheit, multiply the Celsius number by 9/5 and add 32. To go from Fahrenheit to Celsius, subtract 32 from the Fahrenheit number, then multiply by 5/9.

  ASTRONOMICAL

  Astronomers use the light-year as a unit of distance. One light-year equals the distance that light travels in one year in vacuum. Light’s speed in vacuum is nearly 300,000 kilometers per second. There are 31.5 million seconds in a year. Thus, a light-year is equal to a distance of 9.45 trillion kilometers. A slightly longer uni
t, the parsec equals 3.26 light-years.

  POWERS OF TEN

  In writing very large or very small numbers, the powers of ten notation saves work and space.

  10 = 1011 = 100

  100 = 1020.1 = 10-1

  1,000 = 1030.01 = 10-2

  1,000,000 = 1060.000001 = 10-6

  1,000,000,000 = 1090.000000001 = 10-9

  PREFIXES FOR NUMBERS

  101 = deca10-1 = deci

  102 = hecto10-2 = centi

  103 = kilo10-3 = milli

  106 = mega10-6 = micro

  109 = giga10-9 = nano

  1012 = tera10-12 = pico

  Thus, a megaton is 106 (one million) tons. A millimeter is 10-3 (one-thousandth) of a meter. There are a billion nanoseconds in one second, and a gigawatt is a billion watts.

  Appendix 2

  Refractors and Reflectors

  The earliest telescopes that astronomers used were refractors. They had a convex lens at the far end of the tube and a concave lens at the eyepiece. Since Galileo popularized the telescope’s use with his pioneering discoveries, this type of refractor is to this day called a Galilean telescope.

  Another great astronomer, Johannes Kepler, who was plagued with poor eyesight, suggested placing a convex lens at the eyepiece to increase the telescope’s magnification power. While this produced an inverted image, astronomers did not mind seeing the heavens upside down. Inverted astronomical images can confuse the unwary backyard astronomer, however.

  There was a serious problem with refracting telescopes. Their lenses produced images that were often tinged with annoying rings of color, usually red and blue. This is called chromatic aberration, meaning essentially “colored distortion.” The lenses in those earliest optical instruments refracted (bent) the different components of white light at slightly different angles; they acted as prisms as well as magnifying lenses, spreading the white light into a mini-rainbow of colors and distorting the image under study. The more powerful the lenses, the greater the distortion. This limited the size of refracting telescopes for many generations.

  Nearly seventy years after Galileo’s first astronomical observations, Isaac Newton (1642–1727) hit upon the idea of using a mirror to “collect” light and reflect it to an eyepiece. The mirror could be made as large as contemporary technology could produce, without chromatic aberration, since the mirror merely reflects the light and does not bend it through a lens.

  Newton thus invented the reflecting telescope. In the Newtonian reflector a primary mirror gathers in the light and reflects it to a secondary mirror, which in turn reflects the light into a magnifying lens that serves as the eyepiece.

  The great advantage of the reflector is that it can be made big. Astronomers refer to them as “light buckets,” and the bigger the bucket (the primary mirror) the more light the telescope can gather. All the major astronomical telescopes are reflectors: The largest to date are the twin Keck telescopes in Hawaii, with segmented mirrors of 10 meters’ diameter. The Hubble Space Telescope in orbit nearly 500 kilometers above the Earth has a primary mirror of 2.4 meters.

  Chromatic aberration was finally solved in the 1880s by the use of achromatic lenses, where two lenses made of different types of glass, each with a different index of refraction, are cemented together. Where one part of the lens spreads the incoming white light, the other part concentrates it. One half of the lens counteracts the aberration caused by the other half, and the resulting image is free of chromatic aberration.

  Appendix 3

  Cosmic Abundances of the Elements

  ELEMENT SYMBOL NUMBER OF ATOMS PER MILLION HYDROGEN ATOMS

  Hydrogen H 1,000,000

  Helium He 80,000

  Oxygen O 690

  Carbon C 420

  Neon Ne 130

  Nitrogen N 87

  Silicon Si 45

  Magnesium Mg 32

  Iron Fe 32

  Sulfur S 16

  Aluminum Al 3

  Calcium Ca 2

  Sodium Na 2

  Nickel Ni 2

  Argon Ar 1

  All others, 1 or less

  The universe is more than 98 percent hydrogen and helium, the two lightest elements. The elements that are fundamental to living creatures—hydrogen, oxygen, carbon, and nitrogen—are the first, third, fourth, and sixth most abundant elements in the universe.

  Appendix 4

  Acid and Alkaline: The pH Story

  Citrus fruits have a tart, acidic flavor. Soap, on the other hand, is made from materials such as lye that are alkaline, the opposite of acidic. Chemists have developed a system for classifying substances by their acidity or alkalinity: the pH value.

  The term pH stands for “potential of hydrogen”: that is, how strongly the substance will attract and hold hydrogen atoms. The scale runs from pH = 0, which is as acidic as possible, to pH = 14, which is maximum alkalinity. Pure distilled water is pH = 7, exactly in the middle of the scale. A few examples:

  MATERIAL pH

  Human gastric juices 1.0 to 5.0

  Citrus fruits around 4

  Battery acid around 2

  Fertile soil 6.5 to 7

  Distilled water 7.0

  Human blood plasma 7.35 to 7.45

  Common soap 9 to 10

  Sodium hydroxide 13

  The pH scale is logarithmic: that is, each step is ten times larger than the step that preceded it. Thus a pH of 5 is ten times as acidic as a pH of 6, and pH 12 is 100 times more alkaline than pH 10.

  Appendix 5

  Major Planets of the Solar System

  NAME AVG. DIST. FROM SUN (AUS*) / 106 KM / 106 MILES LENGTH OF DAY LENGTH OF YEAR

  Mercury 0.39/57.91/35.98 58.65 days 87.97 days

  Venus 0.72/108.21/67.24 243.02 days** 224.7 days

  Earth 1.00/149.6/92.96 23.9345 hrs 365.242 days

  Mars 1.52/227.94/141.64 24.2630 hrs 1.88 yrs

  Jupiter 5.20/778.30/483.63 9.9250 hrs 11.86 yrs

  Saturn 9.55/1,429.39/888.22 10.6562 hrs 29.42 yrs

  Uranus 19.21/2,875.04/1,786.55 17.240 hrs** 83.75 yrs

  Neptune 30.11/4,504.50/2,799.10 16.110 hrs 163.72 yrs

  Pluto 39.54/5,915.80/3,676.07*** 6.39 days 248.02 yrs

  NAME DIAMETER KM/MILES MASS (EARTH = 1) DENSITY (WATER = 1)

  Mercury 4,878/3,031 0.0553 5.43

  Venus 12,102/7,520 0.81 5.20

  Earth 12,756/7,927 1.00 5.52

  Mars 6,792/4,221 0.1074 3.91

  Jupiter 142,984/88,850 317.710 1.33

  Saturn 120,536/74,901 95.162 0.69

  Uranus 51,118/31,765 14.535 1.318

  Neptune 49,532/30,779 17.141 1.638

  Pluto 2,300/1,429 0.002 2.0

  *The Astronomical Unit (AU) is the average distance between the Earth and the Sun, 149.6 million kilometers (92.96 million miles). Astronomers use the AU as a convenient yardstick for distances within the solar system.**Venus’ and Uranus’ rotation on their axes is retrograde, opposite the direction of the other planets’ rotations. Seen from their north poles, the other planets rotate counterclockwise, while Venus and Uranus rotate clockwise.***Pluto’s orbit is very eccentric, so much so that between 1979 and 1999 Pluto was actually closer to the Sun than Neptune.

  Appendix 6

  Radioactive Dating: Nature’s Alchemy

  Many centuries ago, alchemists sought to transform the common element lead into rare, expensive gold. They were never able to do it. But nature, over long periods of time, transforms much more expensive elements such as uranium into ordinary lead.

  This has given geologists and planetary astronomers an extremely effective tool for determining the age of rocks, whether they are rocks from Earth, the Moon, or meteorites that have fallen from space. The technique i
s called radioactive dating.

  Some elements, such as radium and uranium, are unstable. The nuclei of their atoms break down over time and slowly transform themselves into the nuclei of different, lighter atoms. This process is known as radioactive decay.

  Physicists have measured the rates of these radioactive decays and can tell from a sample of such elements how long their nuclei have been undergoing decay; in other words, how old the rocks that contain the elements are. For example, geologists can measure the amount of uranium in a sample of rock and compare it to the amount of lead in the sample and thereby calculate the age of the rock—the number of years that the atoms of uranium have been decaying into lead. Uranium-238, the most common type of uranium, has a half-life of 4.51 billion years. That is, in any given quantity of uranium-238, half of it will have decayed in 4.51 billion years. Knowing the decay rate of various radioactive elements, it is possible to measure the age of any rock that contains such elements.

  The radiation emitted by decaying atoms can be dangerous, although there is a certain amount of radioactive radiation everywhere, because atoms of unstable elements are breaking down all around us. In fact, some are breaking down inside our bodies. The human body contains tiny amounts of radioactive potassium-40 and carbon-14. Every second, some 38,000 of these atoms emit a bit of ionizing radiation inside your body. This amount is so small, however, that its effect on your health is negligible.

  The age of fossils of once-living creatures can be determined by radiocarbon dating, often abbreviated as carbon dating. When plants take up carbon dioxide from the air, the carbon they “inhale” includes a smattering of the radioactive form of carbon, carbon-14, as well as the normal, nonradioactive carbon-12. Carbon-14’s half-life is precisely known: 5,730 years. By comparing the amount of carbon-12 in a fossil to the remaining amount of carbon-14, the age of the fossil can be determined. Animal fossils (including human) can be dated this way because the animals eat plant matter. This technique only works to an age of about 120,000 years because beyond that time so little carbon-14 remains that it is impossible to measure it.