Death by Black Hole: And Other Cosmic Quandaries by Neil deGrasse Tyson Page A
Authors: Neil deGrasse Tyson
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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
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
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