Asteroids are weakly bound piles of rubble, and if one comes close to Earth, tides can cause the object to undergo landslides and structural rearrangement. The outcome of this encounter is a body with meteorite-like colours.
We once thought that small asteroids were solid 'chips off the old block'. Instead, in a developing story, they are now known to be ephemeral piles of boulders in near-zero gravity, constantly evolving from delicate forces never before thought important. In a study described on page 331 of this issue1, Binzel and colleagues now provide observational proof that near-Earth asteroids (NEAs) are transformed by tidal forces when they come anywhere near our planet.
NEAs were once regarded as rocky fragments (chips, splinters) derived from collisions between larger asteroids. Indeed, large main-belt asteroids (those beyond Mars's orbit) were thought to be shaped, after their primordial formation, mainly by rare collisions and by smaller crater-inducing impacts. Asteroid traits that were once ascribed solely to impacts include their size, non-spherical shape, range of spin period and axial tilt, and the evolution of surficial layers of dust and rocks similar to the lunar regolith. NEAs and smaller meteoroids (meteorites when they reach the ground) were thought to be delivered in two steps. First, a main-belt asteroid in near orbital resonance with Jupiter collides with another asteroid. The resulting fragments are then ejected into the resonance region and deflected by Jupiter's repeated gravitational perturbations into the inner Solar System and Earth's vicinity.
Collisions do affect asteroid traits, but for small- to moderate-sized asteroids, other long-ignored forces now seem much more important. The Yarkovsky effect, conceived a century ago, forgotten then rediscovered several times, is a subtle force due to the asymmetric absorption and re-radiation of solar energy by a body in space. Just a decade ago, it was realized that meteoroids are moved to resonances mainly by the Yarkovsky effect, not by collisions2,3. A related force, suggested in the 1950s to affect asteroids and resurrected in 2000 by David Rubincam (who called it the YORP effect), changes the spins of small bodies, sometimes spinning them so fast that they fly apart. Central to Binzel and colleagues' study1 is yet another subtle force: tides on NEAs passing close to planets.
For context, we must (as Donald Davis and I suggested in the 1970s) picture asteroids not as monolithic, solid bodies, but as 'rubble piles', whether left over from the time they accreted or smashed by catastrophic collisions. We now know (for example, from the Hyabusa spacecraft's study of the NEA called Itokawa; from Arecibo radar 'delay-Doppler mapping' of the binary system NEA 66391 (1999 KW4); and from data on asteroid spins) that NEAs of diameter greater than about 150 metres are just such rubble swarms, loosely bound by mutual gravity. Because NEA gravity is extremely weak, these bodies are non-intuitive assemblages that behave wholly unlike the rocky 'flying mountains' we had imagined.
When the YORP effect spins up a small rubble-pile NEA to a period of less than 2 hours, the centripetal force at its equator exceeds its gravitational binding so that it begins to fly apart. Recent research4,5 shows that surficial rubble moves in landslides from polar latitudes to the equator and then leaks off, often forming a satellite orbiting the main body. The two bodies may come together again, forming a double-lobed or 'contact-binary' shape. Alternatively, the satellite may escape into an independent orbit around the Sun. By running current orbits backwards in time, dozens of pairs have been found6; nearly one-third of NEAs have satellites or contact-binary shapes. The evolution of these configurations may be rapid compared with the several million years that NEAs last in the inner Solar System before they collide with a planet or the Sun, or are ejected. So an NEA may undergo many generations of satellite formation/loss and evolving shapes, spins and appearances.
Although planetary impact commonly dooms an NEA, it usually has many near-misses before hitting a planet — which the NEA Apophis will do in 2029 when it comes even closer to Earth than geosynchronous satellites. Such events alter NEA orbits but would leave a solid body's properties unchanged. However, tidal forces can contort a loosely bound rubble pile7, changing its shape, sometimes pulling it apart, just as the famous comet Shoemaker–Levy 9 was disrupted in 1992 by Jupiter's enormous tidal forces; two years later, the pieces spectacularly slammed into the giant planet.
In 2005, to explain inconsistencies in NEA spectral properties, Nesvorný and colleagues proposed8 that rubble-pile NEAs passing within 3 to 5 Earth diameters might have their properties observably modified by tidal forces. For decades, an asteroidal puzzle was that the colours (spectral properties) of asteroids do not usually resemble the spectra of meteorites derived from such asteroids. We now know that 'space-weathering' processes (for example, impacting solar-wind particles and micrometeorites) darken and discolour surface grains on asteroids, the Moon and other airless bodies. Nesvorný and colleagues suggested that distortions of an NEA as it passes near Earth might strip off the discoloured surficial material, revealing fresh, un-space-weathered material, thus changing the NEA's spectrum; later observational hints supported the idea9.
In 1996, Binzel and colleagues showed10 that whereas some NEAs resemble meteorites, others have maturely space-weathered colours similar to those of large main-belt asteroids, and still others have spectra intermediate between the extremes. (Here I omit discussion of the black, carbonaceous asteroids, whose opaque materials show little spectral character or change.) Binzel surmised that collisional disruption of small NEAs, which occurs more often than for large ones, freshened the NEA surfaces: some had recently been collisionally turned inside-out whereas others remained discoloured. More recently, theoretical work by Nesvorný et al. and observations by Vernazza's team11 have shown that asteroids become space-weathered and thoroughly discoloured unexpectedly rapidly — in less than a million years — so it is perplexing that there are so many fresh NEAs.
Testing the rapidity of space weathering and the tidal-freshening hypothesis, Binzel and colleagues1 now compare spectral data for two NEA groups: those that probably passed by Earth recently and those that cannot come close. The correlation between NEA colours and near-Earth passes is excellent. Most NEAs that probably passed within 8 diameters of Earth in the past several hundred thousand years exhibit fresh, meteorite-like spectra, whereas all of those that have not remain space-weathered. It is surprising that the weak tidal forces so comparatively distant from Earth (more than a quarter of the way to the Moon) would cause freshening landslides on NEAs, but our intuition and simplified computer models have so far failed to fully reveal how these fragile, ever-changing bodies behave.
Thus, many small NEAs have fresh surfaces because they came fairly close to Earth. To be observed by Earth-based telescopes, Binzel and colleagues' sample is biased towards NEAs in Earth's general neighbourhood; but what about inner Solar System asteroids that rarely or never come near Earth or Venus? Eventually, YORP spin-up-induced landslides must briefly freshen their surfaces. Future research will reveal the relative importance of YORP spin-up, binary formation and re-impact, and of collisions, compared with distortion by planetary tides, in shaping these bodies. For now, tides seem to be winning. Maybe many meteorites were finally liberated not by impacts, or even by YORP-induced landslides or satellite formation, but by tidally induced landslides and leakage of rubble into interplanetary space. Our perceptions of NEAs are rapidly changing. Perhaps soon, robotic or piloted docking missions to some of these NEA rubble piles will reveal the beautiful complexity of their evolving behaviour.