Death by Black Hole: And Other Cosmic Quandaries - Neil deGrasse Tyson (2014)
SECTION 2. THE KNOWLEDGE OF NATURE
Chapter 8. VAGABONDS OF THE SOLAR SYSTEM
For hundreds of years, the inventory of our celestial neighborhood was quite stable. It included the Sun, the stars, the planets, a handful of planetary moons, and the comets. Even the addition of a planet or two to the roster didn’t change the basic organization of the system.
But on New Year’s Day of 1801 a new category arose: the asteroids, so named in 1802 by the English astronomer Sir John Herschel, son of Sir William, the discoverer of Uranus. During the next two centuries, the family album of the solar system became crammed with the data, photographs, and life histories of asteroids, as astronomers located vast numbers of these vagabonds, identified their home turf, assessed their ingredients, estimated their sizes, mapped their shapes, calculated their orbits, and crash-landed probes on them. Some investigators have also suggested that the asteroids are kinfolk to comets and even to planetary moons. And at this very moment, some astrophysicists and engineers are plotting methods to deflect any big ones that may be planning an uninvited visit.
TO UNDERSTAND THE small objects in our solar system, one should look first at the large ones, specifically the planets. One curious fact about the planets is captured in a fairly simple mathematical rule proposed in 1766 by a Prussian astronomer named Johann Daniel Titius. A few years later, Titius’s colleague Johann Elert Bode, giving no credit to Titius, began to spread the word about the rule, and to this day it’s often called the Titius-Bode law or even, erasing Titius’s contribution altogether, Bode’s law. Their handy-dandy formula yielded pretty good estimates for the distances between the planets and the Sun, at least for the ones known at the time: Mercury, Venus, Earth, Mars, Jupiter, and Saturn. In 1781, widespread knowledge of the Titius-Bode law actually helped lead to the discovery of Neptune, the eighth planet from the Sun. Impressive. So either the law is just a coincidence, or it embodies some fundamental fact about how solar systems form.
It’s not quite perfect, though.
Problem number 1: You have to cheat a little to get the right distance for Mercury, by inserting a zero where the formula calls for 1.5. Problem number 2: Neptune, the eighth planet, turns out to be much farther out than the formula predicts, orbiting more or less where a ninth planet should be. Problem no. 3: Pluto, which some people persist in calling the ninth planet* falls way off the arithmetic scale, like so much else about the place.
The law would also put a planet orbiting in the space between Mars and Jupiter—at about 2.8 astronomical units† from the Sun. Encouraged by the discovery of Uranus at more or less the distance Titius-Bode said it would be, astronomers in the late eighteenth century thought it would be a good idea to check out the zone around 2.8 AUs. And sure enough, on New Year’s Day 1801, the Italian astronomer Giuseppe Piazzi, founder of the Observatory of Palermo, discovered something there. Subsequently it disappeared behind the glare of the Sun, but exactly one year later, with the help of brilliant computations by the German mathematician Carl Friedrich Gauss, the new object was rediscovered in a different part of the sky. Everybody was excited: a triumph of mathematics and a triumph of telescopes had led to the discovery of a new planet. Piazzi himself named it Ceres (as in “cereal”), for the Roman goddess of agriculture, in keeping with the tradition of naming planets after ancient Roman deities.
But when the astronomers looked a bit harder, and calculated an orbit and a distance and a brightness for Ceres, they discovered that their new “planet” was teeny. Within a few more years three more teeny planets—Pallas, Juno, and Vesta—were discovered in the same zone. It took a few decades, but Herschel’s term “asteroids” (literally “starlike” bodies) eventually caught on, because, unlike planets, which showed up in the telescopes of the day as disks, the newfound objects could not be distinguished from stars except by their motion. Further observations revealed a proliferation of asteroids, and by the end of the nineteenth century, 464 of them had been discovered in and around the swath of celestial real estate at 2.8 AU. And because the swath turned out to be a relatively flat band and did not scatter around the Sun in every direction, like bees around a hive, the zone became known as the asteroid belt.
By now, many tens of thousands of asteroids have been catalogued, with hundreds more discovered every year. Altogether, by some estimates, more than a million measure a half-mile across and up. As far as anyone can tell, even though Roman gods and goddesses did lead complicated social lives, they didn’t have 10,000 friends, and so astronomers had to give up on that source of names long ago. So asteroids can now be named after actors, painters, philosophers, and playwrights; cities, countries, dinosaurs, flowers, seasons, and all manner of miscellany. Even regular people have asteroids named after them. Harriet, Jo-Ann, and Ralph each have one: they are called 1744 Harriet, 2316 Jo-Ann, and 5051 Ralph, with the number indicating the sequence in which each asteroid’s orbit became firmly established. David H. Levy, a Canadian-born amateur astronomer who is the patron saint of comet hunters but has discovered plenty of asteroids as well, was kind enough to pull an asteroid from his stash and name it after me, 13123 Tyson. He did this shortly after we opened our $240-million Rose Center for Earth and Space, designed solely to bring the universe down to Earth. I was deeply moved by David’s gesture, and quickly learned from 13123 Tyson’s orbital data that it travels among most of the others, in the main belt of asteroids, and does not cross Earth’s orbit, putting life on Earth at risk of extinction. It’s just good to check this sort of thing.
ONLY CERES—the largest of the asteroids, at about 580 miles in diameter—is spherical. The others are much smaller, craggy fragments shaped like doggy bones or Idaho potatoes. Curiously, Ceres alone accounts for about a quarter of the total asteroidal mass. And if you add up the masses of all the asteroids big enough to see, plus all the smaller asteroids whose existence can be extrapolated from the data, you don’t get anywhere near a planet’s worth of mass. You get about 5 percent the mass of Earth’s moon. So the prediction from Titius-Bode, that a red-blooded planet lurks at 2.8 AU, was a bit exaggerated.
Most asteroids are made entirely of rock, though some are entirely metal and some are a mixture of both; most inhabit what’s often called the main belt, a zone between Mars and Jupiter. Asteroids are usually described as being formed of material left over from the earliest days of the solar system—material that never got incorporated into a planet. But that explanation is incomplete at best and does not account for the fact that some asteroids are pure metal. To understand what’s going on, one should first consider how the larger objects in the solar system formed.
The planets coalesced from a cloud of gas and dust enriched by the scattered remains of element-rich exploding stars. The collapsing cloud forms a protoplanet—a solid blob that gets hot as it accretes more and more material. Two things happen with the larger protoplanets. One, the blob tends to take on the shape of a sphere. Two, its inner heat keeps the protoplanet molten long enough for the heavy stuff—primarily iron, with some nickel and a splash of such metals as cobalt, gold, and uranium mixed in—to sink to the center of the growing mass. Meanwhile, the much more common, light stuff—hydrogen, carbon, oxygen, and silicon—floats upward toward the surface. Geologists (who are fearless of sesquipedalian words) call the process “differentiation.” Thus the core of a differentiated planet such as Earth, Mars, or Venus is metal; its mantle and crust are mostly rock, and occupy a far greater volume than the core.
Once it has cooled, if such a planet is then destroyed—say, by smashing into one of its fellow planets—the fragments of both will continue orbiting the Sun in more or less the same trajectories that the original, intact objects had. Most of those fragments will be rocky, because they come from the thick, outer, rocky layers of the two differentiated objects, and a small fraction will be purely metallic. Indeed, that’s exactly what’s observed with real asteroids. Moreover, a hunk of iron could not have formed in the middle of interstellar space, because the individual iron atoms of which it’s made would have been scattered throughout the gas clouds that formed the planets, and gas clouds are mostly hydrogen and helium. To concentrate the iron atoms, a fluid body must first have differentiated.
BUT HOW DO solar system astronomers know that most main-belt asteroids are rocky? Or how do they know anything at all? The chief indicator is an asteroid’s ability to reflect light, its albedo. Asteroids don’t emit light of their own; they only absorb and reflect the Sun’s rays. Does 1744 Harriet reflect or absorb infrared? What about visible light? Ultraviolet? Different materials absorb and reflect the various bands of light differently. If you’re thoroughly familiar with the spectrum of sunlight (as astrophysicists are), and if you carefully observe the spectra of the sunlight reflected from an individual asteroid (as astrophysicists do), then you can figure out just how the original sunlight has been altered and thus identify the materials that comprise the asteroid’s surface. And from the material, you can know how much light gets reflected. From that figure and from the distance, you can then estimate the asteroid’s size. Ultimately you’re trying to account for how bright an asteroid looks on the sky: it might be either really dull and big, or highly reflective and small, or something in between, and without knowing the composition, you can’t know the answer simply by looking at how bright it is.
This method of spectral analysis led initially to a simplified three-way classification scheme, with carbon-rich C-type asteroids, silicate-rich S-type asteroids, and metal-rich M-type asteroids. But higher precision measurements have since spawned an alphabet soup of a dozen classes, each identifying an important nuance of the asteroid’s composition and betraying multiple parent bodies rather than a single mother planet that had been smashed to smithereens.
If you know an asteroid’s composition then you have some confidence that you know its density. Curiously, some measurements of the sizes of asteroids and their masses yielded densities that were less than that of rock. One logical explanation was that those asteroids weren’t solid. What else could be mixed in? Ice, perhaps? Not likely. The asteroid belt sits close enough to the Sun that any species of ice (water, ammonia, carbon dioxide)—all of whose density falls below that of rock—would have evaporated long ago due to the Sun’s heat. Perhaps all that’s mixed in is empty space, with rocks and debris all moving in tandem.
The first bit of observational support for that hypothesis appeared in images of the 35-mile-long asteroid Ida, photographed by the space probe Galileo during its flyby on August 28, 1993. Half a year later a speck was spotted about 60 miles from Ida’s center that proved to be a mile-wide, pebble-shaped moon! Dubbed Dactyl, it was the first satellite ever seen orbiting an asteroid. Are satellites a rare thing? If an asteroid can have a satellite orbiting it, could it have two or ten or a hundred? In other words, could some asteroids turn out to be heaps of rocks?
The answer is a resounding yes. Some astrophysicists would even say that these “rubble piles” as they are now officially named (astrophysicists once again preferred pith over polysyllabic prolixity) are probably common. One of the most extreme examples of the type may be Psyche, which measures about 150 miles in overall diameter and is reflective, suggesting its surface is metallic. From estimates of its overall density, however, its interior may well be more than 70 percent empty space.
WHEN YOU STUDY objects that live somewhere other than the main asteroid belt, you’re soon tangling with the rest of the solar system’s vagabonds: Earth-crossing killer asteroids, comets, and myriad planetary moons. Comets are the snowballs of the cosmos. Usually no more than a couple of miles across, they’re composed of a mixture of frozen gases, frozen water, dust, and miscellaneous particles. In fact, they may simply be asteroids with a cloak of ice that never fully evaporated. The question of whether a given fragment is an asteroid or a comet might boil down to where it formed and where it’s been. Before Newton published his Principia in 1687, in which he laid out the universal laws of gravitation, no one had any idea that comets lived and traveled among the planets, making their rounds in and out of the solar system in highly elongated orbits. Icy fragments that formed in the far reaches of the solar system, whether in the Kuiper Belt or beyond, remain shrouded in ice and, if found on a characteristic elongated path toward the Sun, will show a rarefied but highly visible trail of water vapor and other volatile gases when it swings inside the orbit of Jupiter. Eventually, after enough visits to the inner solar system (could be hundreds or even thousands) such a comet can lose all its ice, ending up as bare rock. Indeed, some, if not all, the asteroids whose orbits cross that of Earth may be “spent” comets, whose solid core remains to haunt us.
Then there are the meteorites, flying cosmic fragments that land on Earth. The fact that, like asteroids, most meteorites are made of rock and occasionally metal suggests strongly that the asteroid belt is their country of origin. To the planetary geologists who studied the growing number of known asteroids, it became clear that not all orbits hailed from the main asteroid belt.
As Hollywood loves to remind us, someday an asteroid (or comet) might collide with Earth, but that likelihood was not recognized as real until 1963, when the astrogeologist Eugene M. Shoemaker demonstrated conclusively that the vast 50,000-year-old Barringer Meteorite Crater near Winslow, Arizona, could have resulted only from a meteorite impact, and not from volcanism, or some other Earth-based geologic forces.
As we will see futher in Section 6, Shoemaker’s discovery triggered a new wave of curiosity about the intersection of Earth’s orbit with that of the asteroids. In the 1990s, space agencies began to track near-earth objects—comets and asteroids whose orbits, as NASA politely puts it, “allow them to enter Earth’s neighborhood.”
THE PLANET JUPITER plays a mighty role in the lives of the more distant asteroids and their brethren. A gravitational balancing act between Jupiter and the Sun has collected families of asteroids 60 degrees ahead of Jupiter in its solar orbit, and 60 degrees behind it, each making an equilateral triangle with Jupiter and the Sun. If you do the geometry, it places the asteroids 5.2 AU from both Jupiter and the Sun. These trapped bodies are known as the Trojan asteroids, and formally occupy what’s called Lagrangian points in space. As we will see in the next chapter, these regions act like tractor beams, holding fast to asteroids that drift their way.
Jupiter also deflects plenty of comets that head toward Earth. Most comets live in the Kuiper Belt, beginning with and extending far beyond the orbit of Pluto. But any comet daring enough to pass close to Jupiter will get flung into a new direction. Were it not for Jupiter as guardian of the moat, Earth would have been pummeled by comets far more often than it has. In fact, the Oort Cloud, which is a vast population of comets in the extreme outer solar system, named for Jan Oort, the Danish astronomer who first proposed its existence, is widely thought to be composed of Kuiper Belt comets that Jupiter flung hither and yon. Indeed, the orbits of Oort Cloud comets extend halfway to the nearest stars.
What about the planetary moons? Some look like captured asteroids, such as Phobos and Deimos, the small, dim, potato-shaped moons of Mars. But Jupiter owns several icy moons. Should those be classified as comets? And one of Pluto’s moons, Charon, is not much smaller than Pluto itself. Meanwhile, both of them are icy. So perhaps they should be regarded instead as a double comet. I’m sure Pluto wouldn’t mind that one either.
SPACECRAFT HAVE EXPLORED a dozen or so comets and asteroids. The first to do so was the car-sized robotic U.S. craft NEAR Shoemaker (NEAR is the clever acronym of Near Earth Asteroid Rendezvous), which visited the nearby asteroid Eros, not accidentally just before Valentine’s Day in 2001. It touched down at just four miles an hour and, instruments intact, unexpectedly continued to send back data for two weeks after landing, enabling planetary geologists to say with some confidence that 21-mile-long Eros is an undifferentiated, consolidated object rather than a rubble pile.
Subsequent ambitious missions include Stardust, which flew through the coma, or dust cloud, surrounding the nucleus of a comet so that it could capture a swarm of minuscule particles in its aerogel collector grid. The goal of the mission was, quite simply, to find out what kinds of space dust are out there and to collect the particles without damaging them. To accomplish this, NASA used a wacky and wonderful substance called aerogel, the closest thing to a ghost that’s ever been invented. It’s a dried-out, spongelike tangle of silicon that’s 99.8 percent thin air. When a particle slams in at hypersonic speeds, the particle bores its way in and gradually comes to a stop, intact. If you tried to stop the same dust grain with a catcher’s mitt, or with practically anything else, the high-speed dust would slam into the surface and vaporize as it stopped abruptly.
The European Space Agency is also out there exploring comets and asteroids. The Rosetta spacecraft, on a 12-year mission, will explore a single comet for two years, amassing more information at close range than ever before, and will then move on to take in a couple of asteroids in the main belt.
Each of these vagabond encounters seeks to gather highly specific information that may tell us about the formation and evolution of the solar system, about the kinds of objects that populate it, about the possibility that organic molecules were transferred to Earth during impacts, or about the size, shape, and solidity of near-earth objects. And, as always, deep understanding comes not from how well you describe an object, but from how that object connects with the larger body of acquired knowledge and its moving frontier. For the solar system, that moving frontier is the search for other solar systems. What scientists want next is a thorough comparison of what we and exosolar planets and vagabonds look like. Only in this way will we know whether our home life is normal or whether we live in a dysfunctional solar family.