Category Dreams of Other Worlds

Robert Burnham points out that it was not until the Space Age that astronomers realized just how pervasive cometary and asteroid impacts are throughout the Solar System. Beyond the Moon’s cra­tered and craggy surface, NASA’s planetary science missions have charted numerous bodies in our star system riddled with impact craters. Those missions reveal, explains Burnham, “that by an over­whelming margin the most common feature in the Solar System is the impact crater. Entire planets, moons, and asteroids turned out to be covered with them, at all scales ranging from global to micro­scopic. Craters or their traces can be found on every solid surface from Mercury to Pluto and surely beyond.”39 It’s likely the object that devastated the Siberian taiga near the Stony Tunguska River at approximately 7:15 a. m. on June 30, 1908, was a meteorite. However, the unknown projectile was not made of iron like the meteorite that excavated Meteor Crater in Arizona approximately 50,000 years ago. It is estimated the impactor exploded above ground with the force of 1,000 atomic bombs, more than adequate to destroy a modern city. The explosion reportedly flattened 830 square miles of forest, leveling 30 million trees, and was heard 500 miles away. Multiple remarkable witness accounts of this event included a man, 40 miles from the impact site, who reported being knocked off his feet by a searing blast wave.40 Atmospheric pressure waves registered on barographs after racing around the world at hundreds of miles per hour. The explosion apparently caused a local geomagnetic storm that lasted for days. People in London wondered about the strange, glowing atmospheric effects and pink-fringed clouds appearing in evening skies. One British meteor specialist published his observations in Nature in 1908, saying that the night sky over Bristol, England, was so bright few stars could be seen.41

In July 1994, Comet Shoemaker-Levy 9 broke into fragments that subsequently slammed into Jupiter with an estimated explo­sive force of six hundred times humanity’s entire nuclear arsenal, and spewed plumes of material thousands of miles above Jupiter’s clouds. Millions globally, including astronomers, world leaders and politicians, and a fascinated public, found themselves capti­vated by television news reports and images streaming from JPL’s designated website. That single comet impact raised global aware­ness of the very real danger such events pose for Earth’s popu­lations (figure 6.4). The comet began to impact Jupiter on July 16. Within three days, the U. S. House Committee on Science and Technology voted to establish a NASA program to track comets and asteroids that could threaten the Earth. As co-discoverer of the comet David Levy recalls, “This vote reflected the nation’s fascina­tion, and its growing awareness that such a trail of destruction could have been headed at Earth. The initiative was designed to protect future generations of people.” Should the threat of a comet or an asteroid impact be successfully deflected, Levy contends the global attention focused on Comet Shoemaker-Levy will have in effect “rescued our planet.”42

Though the devastation of comet impacts is obvious, these pri­mordial snowballs may very likely be the reason for life on Earth. Given their ubiquity, it’s believed that cometary impacts are a com­mon vector by which life may have emerged on Earth. As noted previously, researchers working with the Stardust data reported finding the amino acid glycine in Comet Wild 2 samples. Amino acids are comprised of what is commonly referred to as CHON, or carbon, hydrogen, oxygen, and nitrogen atoms, and the molecules that CHON, along with phosphorus and sulfur, can combine to

make are all chemical ingredients of DNA. Comet and meteorite impacts on Earth could have dispersed such organics or even syn­thesized them.43

Science writer Connie Barlow claims: “Our ancestors include ancient stars. Stars are part of our genealogy.”44 She means that if we want to discover our origins, we can’t simply peer into a micro­scope at possible progenitor organisms. Instead, we need to look to the stars, where heavy elements necessary for life, like carbon, are forged. But we also can attribute our genetic makeup to com­ets and asteroids that may have deposited on Earth the chemistry for life. All known organisms are the products of their DNA, and some components of DNA have been detected in meteorites. In 2011, NASA scientists reported finding in meteorites “adenine and guanine, two of the four so-called nucleobases that, along with cytosine and thymine, form the rungs of DNA’s ladder-like struc – ture.”45 DNA is the coded information in our cells that determines biologically who and what we are. “At the center of the ladder-like DNA molecule lie ring-like structures called nucleobases. It’s these tiny rings that scientists at NASA and the Carnegie Institution for Science in Washington found in 11 of 12 meteorites they scruti­nized.”46 Meteorites, if not jettisoned from a neighboring planet, are remnant chunks of rocky debris left over from the Sun’s for­mation. Given that such meteorites deliver organic molecules to Earth, molecules from our star’s earliest days are inevitably in our DNA. Scientists reported in the journal Nature having found fossils of multicellular life dating back 2.1 billion years, roughly 1.5 billion years prior to the Cambrian explosion. These fossils in black shales in West Africa rewrite the history of when multicel­lular organisms first emerged.47 Microbial life on Earth predates even this. Biologists, astronomers, and astrobiologists now faced with recalibrating the timescales on which life began on Earth are reworking our appreciation for comets and their critical role in dispersing stardust throughout the Solar System.

Stardust’s mission to capture the interplanetary dust grains that pervade our star system is an attempt to understand not only how our Sun originated and evolved, but also the characteristics of the early Solar System that led to the origin of life. What we learned from the Stardust mission is that comets and asteroids harbor the building blocks of life. Twenty different amino acids, occurring naturally in a wide number of arrangements, produce the proteins that create all living organisms on Earth. John and Mary Gribbin report, “Formic acid (the stuff some ants squirt out as a defensive weapon, and the stinging ingredient in stinging nettles) and metha – nimine are two of the polyatomic organic molecules that have been identified in dense interstellar clouds. Together, they combine to form an amino acid, glycine.”48 As James Lovelock, originator of Gaia Theory, has observed, “It seems almost as if our Galaxy were a giant warehouse containing the spare parts needed for life.”49 Perhaps the universe is built for life. The Gribbins suggest that the chemistry for life is endemic to the entire universe. They assert that “one of the most profound discoveries made by science in the twentieth century” is that the universe comprises “the raw materi­als for life, and that these raw materials are the inevitable product of the processes of star birth and star death.”50

Though the normal matter that makes up stars, planets, and stel­lar dust clouds constitutes only a small fraction of the total matter in the universe, most of which is dark and still mysterious, the stuff of stars nevertheless provides all the components necessary for life. “The raw material from which the first living molecules were assembled on Earth was brought down to the surface of the Earth in tiny grains of interplanetary material, preserved in the fro­zen hearts of comets,” write Gribbin and Gribbin, who eloquently observe, “Those grains themselves literally, not metaphorically, formed from material ejected by stars. The ‘manna from heaven’ that carried the precursors of life down to the surface of the Earth was literally, not metaphorically, stardust. And so are we.”51 That we are made of stardust is not to be disparaged. It’s likely that life has emerged on planets orbiting other stars in galaxies far, far away. If so, it’s certain that the blood in their veins and the calcium in their bones, should they have them, were forged in the fires of their suns.

For a direct sense of what Hipparcos learned, ESA offers a sky map on its website that can easily fit in the palm of your hand.34 The “Hipparcos Star Globe” represents the brightest stars and the major constellations measured by the satellite as charts that can be printed out on two sheets of paper and then assembled into a sky sphere. In fact, for ease of construction, the sky is projected onto an icosahedron, a polyhedron with 20 triangle-shaped faces. Instructions for constructing this astronomical origami are also provided on the ESA website. This simple sky chart conveys no more than the “bony skeleton” of the night sky’s stars; Hippar – cos mapped the anatomy in exquisite detail. Its main instrument charted the positions of 118,218 stars with the highest precision.35

In addition, a beam-splitter was used with a secondary detector to map out the sky with slightly lower precision—the resulting Tycho Catalog lists 1,058,332 stars.36 Years after the satellite ceased op­eration, astronomers produced the definitive Tycho-2 Catalog, containing a prodigious 2,539,913 stars.37 That number is 99 per­cent of the stars down to 11th magnitude, which is a level 100,000 times fainter than the brightest star, Sirius. With this exquisite level of detail, Hipparcos has mapped our location in the “city of stars” called the Milky Way galaxy (plate 14).

Space missions produce data of such complexity and abundance that it’s often years before all the results are known. Hipparcos is no exception. The Tycho-2 Catalog was published in 2000, seven years after the satellite returned its last data. As recently as 2007, Dutch astronomer Floor Van Leeuwen re-analyzed the Hipparcos data. He diagnosed many small effects that had been overlooked in the original analysis, such as tiny jogs in the spacecraft’s orienta­tion due to micrometeorite impacts and subtle changes in the image geometry each time the satellite went into Earth’s shadow and then emerged into sunlight. He also took advantage of great gains in the power of computers to improve the calculation of positions. With a million stars, the number of angles between any star and all of the others is a million squared, or a trillion. To pin down the errors in positions, those trillion angles must be calculated many times, a procedure that took six months at the end of the mission but only a week when Van Leeuwen did his work using much faster proces­sors. His analysis has shrunk the errors by a factor of three from the initial goal of 0.002 arc seconds, and a factor of ten for the brightest stars.38 This minuscule angle would be formed by draw­ing lines from the top and bottom of Lincoln’s eye on a penny in New York and having them come to an apex in Paris.

The trick of Hipparcos is to measure positions across the entire sky rather than picking off stars one by one. Thus it gains from the power of large numbers. Imagine you had to cover the floor of a large room with irregular but similar-sized tiles. You could lay them out tile by tile with a good chance of keeping the separa­tion of adjacent tiles uniform, but as you covered a larger region it would become very difficult to control the uniformity. Whether you worked from the center out or the edges in or from side to side, it’s likely you’d either have tiles left over or leave a gap. The optimum solution would be to be aware of the distance between any two tiles and regulate it over the whole area, thereby filling it uniformly. If the tiles are now stars on the sky, that’s what scien­tists did with the Hipparcos data. They made an optimal solution for all stars simultaneously. In practice it was a calculation that taxed the best computers at that time.

Behind the numbers are the people who work to make a mission successful, often devoting their entire scientific careers to the task. Michael Perryman was born in the dreary industrial town of Luton, just north of London, and he was interested in math and numbers from an early age. His math teacher at school advised him to study a subject with better employment potential. He ignored the advice and studied theoretical physics at Cambridge University. For his PhD he stayed at Cambridge but switched to radio astronomy, joining a group that was still buzzing with excitement from the discovery of pulsars and the award of the Nobel Prize in Physics to Martin Ryle and Tony Hewish in 1974. At this point he has spent thirty years of his career on the unglamorous but very important work of mapping star positions, exceeding the amount of time the illustrious Tycho Brahe spent on his observations.39 Appropriately, in 2011 he was awarded the prestigious Tycho Brahe Prize by the European Astronomical Society.

Perryman was just twenty-six when he was selected to be the project scientist for the Hipparcos mission, a great honor and re­sponsibility for someone so young.40 Soon he found himself re­sponsible for the coordination of two hundred scientists and for all the headaches that go along with a complex multinational project. The biggest challenge came soon after launch when a motor on the Ariane launch rocket malfunctioned and the satellite didn’t reach its desired geostationary orbit. The unplanned orbit exposed Hip- parcos to high levels of radiation twice a day and it was thought the satellite might not last more than a few months. The team adjusted and made the best of the situation, but for over two years Perry­man lived under a “sword of Damocles” as gyros were knocked out by the radiation. In the end, the mission exceeded its design goal of both lifetime and science. Away from the project, Perryman enjoyed hiking and caving, choosing to escape underground from his upward-looking day job.