Asteroids have become notorious as celestial menaces but are best appreciated in a positive light, as surreal worlds bearing testimony to the origin of the planets The Small Planets by Erik Asphaug rowing up in the Space Age, my friends cally modernizing our perception of these baffling objects. and I would sometimes play the gravity But in spite of careful observations and the occasional prox- game. One of us would shout, “Pretend imity of these bodies to Earth, we know less about asteroids you’re on the moon!” and we’d all take (and their relatives, the comets) than we knew about the the exaggerated slow strides we’d seen on moon at the dawn of space exploration. Minor planets ex- Gtelevision. “Pretend you’re on Jupiter!” another would say, hibit a delicate interplay of minor forces, none of which can and we’d crawl on our hands and knees. But no one ever be readily ignored and none of which can be easily simulated shouted, “Pretend you’re on an asteroid!” In that pre-Ar- in a laboratory on Earth. Are they solid inside, or aggregate mageddon era, who knew what “asteroid” meant? Now a assemblages? What minerals are they composed of? How do grown-up who studies asteroids for a living, I still don’t they survive collisions with other small bodies? Could a lan- know how to respond. der or astronaut negotiate an asteroid’s weird surface? Although we haven’t seen any of the largest asteroids up close, they probably resemble shrunken, battered versions of Half-Baked Planets the moon. In their weaker gravity, visiting astronauts would simply take longer strides. But below a few dozen kilometers y graduate studies began during the Bush administra- in diameter, gravity is too feeble to press these so-called mi- Mtion, when asteroids were mere dots—a thousand nor planets into even an approximately round shape. The points of light known to orbit primarily in a belt between smallest worlds instead take on a carnival of forms, resem- Mars and Jupiter. A few lesser populations were known to bling lizard heads, kidney beans, molars, peanuts and skulls. swoop closer to Earth, and then there were comets in the Because of their irregularity, gravity often tugs away from Great Beyond. From periodic variations in color and bright- the center of mass; when added to the centrifugal forces in- ness, asteroids were inferred to be irregular bodies ranging in duced by rotation, the result can seem absurd. Down might size from a house to a country, rotating every several hours not be down. You could fall up a mountain. You could jump or days. More detailed properties were largely the stuff of too high, never to return, or launch yourself into a chaotic scientific imagination. (though majestically slow) orbit for days before landing at Asteroids residing closer to Mars and Earth commonly have an unpredictable location. A pebble thrown forward might the spectra of rocky minerals mixed with iron, whereas aster- strike you on the head. A gentle vertical hop might land you oids on the Jupiter side are generally dark and red, suggesting 100 meters to your left or even shift the structure of the as- a primitive composition only coarsely differentiated from that teroid underfoot. Even the most catlike visitor would leave of the primordial nebula out of which the planets began to co- dust floating everywhere, a debris “atmosphere” remaining alesce 4.56 billion years ago [see illustration on page 48]. This aloft for days or weeks. timing is precisely determined from analysis of lead isotopes— These aspects of asteroid physics are no longer only theo- the products of the radioactive decay of uranium—in the old- retical curiosities or a game for children. Space missions such est grains of the most primitive meteorites. In fact, meteorites as the Near Earth Asteroid Rendezvous (NEAR), the first have long been suspected to derive from asteroids. The spectra probe to go into orbit around a minor planet, are dramati- of certain meteorites nearly match the spectra of certain class- AND NASA JOHNS HOPKINS UNIVERSITY LABORATORY APPLIED PHYSICS 46 Scientific American May 2000 The Small Planets Copyright 2000 Scientific American, Inc. GIANT PAW PRINT is a strange crater on the asteroid Eros, so dubbed by scientists now studying this 33-kilometer-long space rock with the NEAR space probe (center of lower image). On the other side of the body is a youthful, saddle-shaped gouge (left of upper image) full of unexplained markings. Through images such as these, asteroids are now turning from astronom- ical objects—mere points of light—into geologic objects—whole worlds whose exploration has only begun. Copyright 2000 Scientific American, Inc. Types of Asteroids Where They Roam TROJAN ASTEROIDS JUPITER 1,000 0.024 100 0.24 MAIN LARGE ASTEROIDS NEVER ROTATE BELT FASTER THAN ABOUT 10 TIMES PER DAY 10 2.4 MARS EARTH 1 24 SUN Spin Period (hours) Spin Period Spin Rate (rotations per day) Spin Rate (rotations 0.1 240 S CLASS TRANSITION (200 METERS) C CLASS 0.01 2,400 0.1 1 10 100 1,000 D CLASS Diameter (kilometers) TWO GROUPS OF ASTEROIDS emerge on a plot of their ro- MAIN ASTEROID BELT lies between the orbits of Mars tation rates (vertical axis) versus size (horizontal axis). No and Jupiter, but stragglers cross Earth’s orbit (and some- known asteroid larger than 200 meters across rotates faster times collide with Earth) or revolve in sync with Jupiter (in than once every 2.2 hours. The cutoff is easy to explain if these two groups known as the Trojan asteroids). The inner main asteroids are piles of rubble that fly apart if spun too fast. Small- belt consists mainly of stony or stony-iron asteroids (S class); er asteroids, which can turn once every few minutes, must be sol- farther out the asteroids are darker, redder and richer in car- id rocks. The transition probably arose because of collisions. bon (C class and D class). es of asteroids. We therefore have pieces some meteorites reveal that liquid water oids is waxing as new observations trans- of asteroids in our possession. was often present. form them from dim twinkles in the sky Many astronomers used to think that Encounters among the planetesimals into mind-boggling landforms. For this, telescope observations, combined with became increasingly violent as Jupiter asteroid scientists can thank National meteorite analysis, could substitute for randomized the orientation and elliptic- Aeronautics and Space Administration spacecraft exploration of asteroids. Al- ity of their orbits. Instead of continuing administrator Daniel S. Goldin and the though the puzzles proved more stub- to grow, the would-be planets were chis- dinosaurs. born than expected, researchers have eled or blasted apart by mutual colli- Goldin’s “faster, better, cheaper” man- been able to piece together a tentative sions. Their pieces often continued to tra has been a boon to asteroid science, outline of solar system history. For the orbit the sun in families with common because a visit to a tiny neighbor is both planets to have accreted from a nebula orbital characteristics and related spec- faster and cheaper than a mission to a ) s of dust and gas, there had to be an ini- tra. Many asteroids and meteorites are major planet. The specter of fiery death A tial stage in which the first tiny grains the rock- or metal-rich debris of these from above has also focused minds. The oid/3FR coagulated into growing bodies known differentiated protoplanets. Other aster- discovery of the Chicxulub crater in the er as planetesimals. These became the oids (and most comets) are more primi- Yucatán vindicated the idea that the im- .cz/~ast as building blocks of planets. But in the tive bodies that for various reasons nev- pact of an asteroid or comet 65 million .c zone beyond Mars, gravitational reso- er differentiated. They are relics from years ago extinguished well over half the l.asu nances with massive Jupiter stirred the the ur-time before planets existed. species on Earth [see “An Extraterrestri- om sunk fr cauldron and prevented any body from al Impact,” by Walter Alvarez and Frank ( growing larger than 1,000 kilometers The Sky Is Falling Asaro; Scientific American, October epublic ech R across—leaving unaccreted remnants to 1990; “Collisions with Comets and As- z become the present asteroids. decade ago no asteroid had been teroids,” by Tom Gehrels; Scientific The largest of these would-be planets A imaged in any useful detail, and American, March 1996]. es of the C nonetheless accumulated enough inter- many astronomers had trouble taking A repeat is only a matter of time, but nal heat to differentiate: their dense met- them seriously. The first asteroids, dis- when? Until we completely catalogue all ademy of Scienc c als percolated inward, pooling and per- covered in the early 1800s, were named significant near-Earth asteroids—a job A haps forming cores, leaving behind in the grand mythological manner. But we have just begun—poker analogies VEC A lighter rocky residues in their outer lay- with the tenth, the hundredth and the must suffice. (We will never completely ETR PR ers. Igneous activity further metamor- thousandth, asteroids began taking on catalogue the comet hazard, because P phosed their rock types, and volcanoes the names of their discoverers, and then each comet visits the inner solar system CE: SOUR erupted on some. Although none grew of discoverers’ spouses, benefactors, col- so rarely.) The chance of a global calam- CE; A large enough to hold on to an atmo- leagues and dogs.