Faint Echoes, Distant Stars

by Ben Bova

  Have organic molecules—or even living organisms—rained down on Earth as comets sweep by? In the 1910 appearance of Halley’s Comet, the Earth actually passed through Halley’s tail, with no discernable effects. (Although hucksters on street corners hawked hard hats and fake “comet pills” to protect the gullible against the nonexistent dangers of the comet.)

  Even if organic material was delivered to Earth by comet impacts during the primeval “heavy bombardment” phase, could the long-chain molecules have survived the blazing heat of atmospheric entry and impact? Is it possible that organics came to Earth in the gentler “flyby” mode as our planet sailed through cometary tails?

  Cosmologist Hoyle, one of the most innovative thinkers in astrophysics, suggested that comets have indeed brought living organisms to Earth and other planets. Together with his colleague, the Sri Lankan Chandra Wickramasinghe, Hoyle maintained that comets have deposited actual bacteria into Earth’s atmosphere, bacteria that existed as dormant spores while riding the comet and then revived to become active once warmed up. Hoyle and Wickramasinghe claim that many global outbreaks of disease have been caused by “alien invasions” of bacteria delivered by comets. The influenza pandemic of 1918–1919, which killed 22 million people (including three of my four grandparents) followed Earth’s 1910 brush with Halley’s Comet, they point out.

  The Hoyle-Wickramasinghe hypothesis is actually a variant of Svante Arrhenius’s panspermia concept of 1908 (see Chapter 8), the idea that life on Earth originated from spores that drift through the interstellar depths. Hoyle and Wickramasinghe suggest comets as the transportation mode.

  The Oort Cloud stretches perhaps 100,000 AUs from the Sun. At that distance its outermost members may well be mingling with the cometary cloud of our nearest stellar neighbor, the triple-star system of Alpha Centauri, which lies 4.3 light-years (275,000 AUs) from the Sun. Comets might indeed be a mode for transportation from one star to another.

  Perhaps some of the comets that we have seen in our skies actually originated around our stellar neighbor.

  THE PLUTO MISSION: ON AGAIN, OFF AGAIN

  Politics makes strange bedfellows, and the politics of science and space exploration are sometimes the strangest of them all.

  Distant Pluto is the only planet that has not yet been visited by space probes. Even though Pluto might technically be considered a Trans-Neptunian Object and not a planet, that doesn’t make it any less interesting.

  NASA had plans for a robotic Pluto/Kuiper Express mission, but when the estimated cost of the program soared past $800 million, it was scrapped in the general budget-cutting caused by the huge overruns in the International Space Station. Thus, when NASA submitted its budget proposal for fiscal year 2002 to the White House, there was no Pluto/ Kuiper mission included. Planetary scientists were unhappy and so were astrobiologists, because Pluto and the TNOs are probably unchanged from the very earliest times of our solar system. These icy bodies could be deep-freeze laboratories in which the first steps in prebiotic chemistry took place. In the quest to learn how life began, Pluto and the TNOs could be a prime objective.

  But budget strictures forced NASA’s top management to make a difficult choice. There wasn’t enough money to fund both a mission to Pluto and a mission to the Jovian moon Europa. Since Europa is closer and has given evidence of having a water ocean beneath its mantle of ice, NASA chose to fund a Europa mission and shelve the plans for Pluto. After all, Europa is one of the most promising sites in the solar system as a possible abode of extraterrestrial life. Beneath the ice that covers it, Europa may harbor a thermally habitable ocean of water.

  Yet, although the White House’s budget request to Congress for fiscal year 2002 did not include the Pluto/Kuiper Express mission, Congress had other ideas.

  Part of the budget problem stemmed from the fact that NASA’s plans for both Europa and Pluto came out of the Jet Propulsion Laboratory. The space agency’s premier center for planetary exploration missions, JPL had a “lock” on the Pluto/Kuiper Express effort. Yet there are other organizations that have conducted successful planetary missions. The Applied Physics Laboratory (APL) of Johns Hopkins University, for example, had operated the successful NEAR-Shoemaker exploration of near-Earth asteroids and even landed the spacecraft on the asteroid Eros in 2001. TRW Corporation had operated the Pioneer 10 and 11 flybys of Jupiter and Saturn in the 1970s.

  Scientists and a few politicians, including Senator Barbara Mikulski of Maryland, who headed the subcommittee that oversees NASA’s budget, argued that the Pluto/TNO mission should be opened for competition instead of being considered JPL’s exclusive territory.

  According to planetary scientist Alan Stern of the Southwest Research Institute in Boulder, Colorado, “The trouble with having only one outfit to go to [i.e., JPL] is that if they come in with a price that NASA can’t afford, the only alternative is to cut the mission altogether.”

  With Congress’ prodding, NASA opened the Pluto mission to competition. Aerospace industry giants such as Lockheed Martin and TRW were invited to bid on the program. With Stern as their lead investigator, Southwest Research Institute teamed with Johns Hopkins Applied Physics Laboratory—and won the nod.

  “It was a David and Goliath situation,” quipped the happy Stern. “Around here, we say that APL means ‘Assured Pluto Lab.’ “ The fact that the Applied Physics Laboratory is situated in Senator Mikulski’s Maryland might also have been a factor in the decision.

  Stern estimated the mission would cost $488 million, well under the $500 million cap set for it. The Pluto/TNO spacecraft was to be launched in 2006 on a flight path that will take it to the Pluto/Charon system by 2014 or 2018, depending on which booster rocket was chosen for the launch.

  But there was no money in NASA’s FY 2002 budget for Pluto. In a somewhat rare move, Congress added $30 million to NASA’s budget to fund the first year of the Pluto/TNO program.

  Then came Daniel Goldin’s resignation and the appointment of Sean O’Keefe as the new NASA administrator. Under pressure from the Office of Management and Budget, early in 2002 O’Keefe chopped both the Europa and Pluto/TNO missions out of the FY 2003 budget. This meant that there would be no missions to any planet beyond Mars for the foreseeable future.

  But wait! As Yogi Berra said, “It ain’t over ‘til it’s over.” In February 2002, NASA administrator O’Keefe unveiled the New Horizons program for planetary exploration, in which individual projects would be capped at $650 million, well above the figure Stern and his colleagues envisioned for the Pluto/TNO mission. The National Research Council’s Decadal Survey put the Pluto/TNO mission at the top of its list for planetary exploration.

  Both the House and Senate appropriations committees kept the New Horizons Pluto/TNO mission in the FY 2003 budget, and NASA then selected once more the Southwest Research Institute/Johns Hopkins APL team to start preliminary design studies for the Pluto/TNO mission. In April 2003, NASA gave the Johns Hopkins/APL team approval to begin building the New Horizons Pluto/TNO spacecraft and related ground equipment, aiming for a launch in 2006. Whether it will survive future budget battles is anyone’s guess.

  Funding such programs is always a year-to-year proposition. The budget battle must be fought on the floor of Congress every year. Stern and his colleagues worry that if the mission is postponed beyond its planned 2006 launch date, Pluto will have moved so far from the Sun that its “winter” will have set in and its atmosphere of nitrogen gas will freeze out entirely on the cold, cold ground, making it impossible to study the planet’s atmosphere until “spring” rolls around—two hundred years from now.

  If the Southwest/APL program gets off the ground, Stern and his colleagues regard this mission as a first reconnaissance of the Trans-Neptunian region. Their spacecraft will be equipped with mapping cameras and spectrometers that will search for signs of organic materials on the surfaces of Pluto, Charon, and other TNOs.

  “Maybe there’s an ocean beneath Pluto’s surface,” Stern speculates
. When asked how an ocean could exist at Pluto’s frigid temperature of -250°C, with nitrogen frozen solid on its surface, Stern says hopefully that perhaps an ocean heavily laced with anti-freeze ingredients, such as alcohol, might exist there. Some species of Arctic fish have anti-freeze in their blood, here on Earth.

  Meanwhile, because the transit times to the outer planets take years using conventional chemical rockets, O’Keefe has proposed a new joint NASA/Energy Department program for research in nuclear propulsion. A nuclear rocket could cut the transit time for missions to the outer planets down to months instead of years. However, bitter political resistance stopped NASA’s work on nuclear rockets in the 1970s, and the “no nukes” forces might stymie this new effort even before it gets started.

  On the other hand, engineers have refigured the flight plan for a booster using chemical rockets and shaved a year off the transit time. If launched in 2006, the new flight plan will have the Pluto/TNO craft at Pluto by 2015. Pluto will be slightly closer to the Sun at that time, so there will be more light for photographing its surface and Charon’s.

  At this point, the politics look trickier than the technical challenges of a Pluto/TNO mission.

  Section IV

  Life Beyond the Solar System

  17

  Extrasolar Planets:Good News and Bad News

  The Imp of the Perverse

  —Edgar Allan Poe

  THE SEARCH for life elsewhere in the universe is not confined to our solar system, of course. In particular, the search for intelligent aliens, SETI, aims at other stars because there is no evidence that intelligent life exists on any planet in our own solar system other than Earth.

  Astrobiologists assume that other stars harbor planetary systems and that some of those planets are much like Earth. Yet until 1995, not even the first half of that assumption was backed by any credible evidence.

  Finding planets orbiting other stars has been a difficult, frustrating business. Planets are much smaller than stars, and the only light they give off is the reflected light of the stars around which they orbit. For planet-searchers, the stars are too bright: The dim light reflected by a nearby planet is lost in the brilliant glare of the star’s radiance.

  Planet-searchers refer to the “firefly on a searchlight” problem. In essence, they are looking at big, bright searchlights (stars) and trying to find small, flickering fireflies (planets) that might be fluttering near them.

  The problem is basically distance. The distances between stars are enormous. Although light travels at some 300,000 kilometers per second, it still takes more than four years for light to span the distance between the nearest star, Alpha Centauri, and the Sun. Small wonder that the search for planets orbiting other stars did not find anything until a mere few years ago.

  WHY BOTHER TO LOOK?

  Until the middle of the twentieth century, astronomers had no hope of finding extrasolar planets. Their instruments were simply not powerful enough for the job. Besides, the prevailing explanation for how the solar system was created, the steller encounter hypothesis we examined in Chapter 6, maintained that planetary formation was such a rare phenomenon that we should not expect other stars to have planets. Not much sense hunting for extrasolar planets if there aren’t any to be found.

  Yet, by the middle of the twentieth century, the stellar encounter hypothesis was being criticized by a new generation of astrophysicists who conceived the idea that the Sun and planets developed together out of a cold interstellar cloud. By the time the Sun began to shine, it was surrounded with a flattened accretion disk of dust-laden gas, which was already “lumpy” with planetesimals on their way to becoming the planets we see today. Perhaps planet-building was as natural as star-building. That meant that there should be plenty of extrasolar planets to be found.

  VAN DE KAMP AND THE “RUNAWAY”

  No matter what the theorists proposed, the hard physical facts of the firefly-and-searchlight problem were still there: Extrasolar planets are too small, too dim, and too far away to be seen.

  But could they be detected even though they could not be seen?

  Peter van de Kamp (1901–1995) pioneered the astrometric technique for finding extrasolar planets. Astrometric means, literally, “star measuring.” The technique depends on the fact that a planet’s gravitational field exerts a force on the star it orbits. True, the planet’s tug on its star is minuscule, rather like the impact of a flea on an elephant’s back, but it is real and—in a precious few cases—it might be measurable.

  The astrometric technique had been used earlier to find small, faint stars that accompanied more normal stars. In 1844, the German astronomer Friedrich Wilhelm Bessel (1784–1846) detected a slight wobble in the motion of the bright star Sirius, which indicated it was accompanied by a faint companion. Called Sirius B in astronomical nomenclature,22 the dwarf was not actually seen until 1862; it gives off less than a thousandth of the light that Sirius A does. Still, Sirius B is a star, much more massive than a planet, and it emits light. Planets are smaller and glow only from the reflected light of a nearby star.

  Van de Kamp was for many years the director of the Sproul Observatory at Swarthmore College outside Philadelphia. Using the observatory’s 61-centimeter refractor telescope, he started in 1938 to observe a few of the stars closest to the Sun. In particular, he amassed thousands of photographic plates of Barnard’s Star, a faint red dwarf six light-years away.

  Known as “Barnard’s Runaway,” it is moving across the sky faster than any other star. While the stars seem to us to be firmly set in their positions in the sky, they are actually moving. It is because they are so far away that they show no discernable change in position during a human lifetime. Yet, as Galileo would say, they do move. Barnard’s Star has a proper motion (i.e., motion across our field of view) of 10.25 arc seconds per year, far above any other star in our sky.23

  Van de Kamp was looking for a slight wobble in the proper motion of Barnard’s Star that would betray the presence of an unseen planetary companion. He was also examining other nearby stars, patiently measuring their positions night after night, year after year, searching for the telltale perturbations in their proper motions that would reveal the presence of a planet.

  He had to make very precise measurements, out at the extreme edge of what his instruments were capable of achieving. Once again, like that hypothetical scout gazing across the chasm, we face a situation where a scientist is pushing the limits of available technology and techniques. Van de Kamp needed precise measurements, enormous patience, and time.

  To understand the problems he faced, consider an alien astronomer who is trying to determine if there are planets circling the yellowish star we call the Sun. By far the biggest gravitational tug on the Sun comes from the largest planet of our solar system, Jupiter. Jupiter completes one orbit around the Sun in just under twelve years. Our alien astronomer would need twelve years, then, to detect the slight wobble in the Sun’s proper motion caused by the unseen planet. To make his measurement certain, the astronomer would want to have data on several Jovian orbits, so he would continue to measure the Sun’s perturbations for several dozen years.

  Thus van de Kamp measured. And waited.

  But he wasn’t the only planet-hunter on the scene.

  “UNSEEN COMPANIONS”

  In January 1943, Dirk Reuyl and Erik Holmberg of the McCormick Observatory at the University of Virginia announced that they had detected an “invisible companion” orbiting the binary (double) star 70 Ophiuchi, slightly more than sixteen light-years from our Sun. A month later, the Danish American Kaj Aage Strand of van de Kamp’s own Sproul Observatory reported that he had found an unseen planet sixteen times more massive than Jupiter orbiting another binary star, 61 Cygni, about eleven light-years away.24 Strand had also studied 70 Ophiuchi and found no evidence for an unseen companion.

  The following year van de Kamp revealed that he had evidence for an unseen companion of Barnard’s Star that was sixty times more massive
than Jupiter: too small to be a star, but too big to be a planet. A conservative observer, van de Kamp undoubtedly made his announcement before he really wanted to because the claims of his colleagues caused an impressive stir in the astronomical community.

  By 1963, van de Kamp had amassed some 2,400 photographic plates of Barnard’s Star, taken over a quarter of a century. He announced that Barnard’s Star was orbited by an object 60 percent more massive than Jupiter, not sixty times bigger: clearly a planetary-sized body. The unseen planet’s orbital period was twenty-four years, which meant it must be roughly the same distance from Barnard’s Star as Saturn is from the Sun.

  While the earlier discoveries of companions for 70 Ophiuchi and 61 Cygni were still doubted by most astronomers, van de Kamp’s careful, patient accumulation of data satisfied the scientific world—and even The New York Times—that an extrasolar planet had at last been found.

  Or had it?

  Van de Kamp kept refining his data and by 1969 came to the conclusion that there were two planets circling Barnard’s Star. Yet there was a time bomb buried in that data, and it exploded in 1973.

  One of van de Kamp’s colleagues at Sproul, John L. Hershey, was studying the motions of the star Gliese 793 (so named because it is the 793rd star in a list compiled by the German astronomer Wilhelm Gliese). Gliese 793 showed exactly the same wobbles that Barnard’s Star did. Wherever there was a perturbation in the proper motion of Barnard’s Star, the same perturbation appeared in the motion of Gliese 793. Since they both had been photographed through the same Sproul telescope, the wobbles in the photographic images were apparently caused by a glitch in the equipment attached to the telescope.