Many if not all the meteorites found on Earth came from collisions between asteroids. Asteroids with diameters of around 12 miles (20 km) suffer a strike from another body once every 10 million years on average. If the crackup happens at high speed, the asteroid shatters to pieces with rocks and dust blasted into space to form a new asteroid family.
Sometimes material from the collision finds its way to our planet and collides again — this time with the atmosphere and the surface, landing as heat-blackened meteorites. If the asteroid collision is a gentle one, the two bodies can join together as a single asteroid with two prominent lobes.
The rarity of asteroid collisions on a human timescale makes the discovery of a possible impact on 493 Griseldis a stroke of good fortune. Griseldis, a city-sized object 29 miles wide (46 km), orbits the sun in the main belt of asteroids between Mars and Jupiter. Observations taken with the 8-meter Subaru Telescope from the summit Mauna Kea on Hawaii’s Big Island on March 17th this year revealed a wisp of a tail extending from Griseldis where no wisp had ever been seen before.
Unlike comet tails, which are pushed in the direction opposite the sun by the solar wind and the slight pressure of sunlight itself, this tail pointed in a different direction and only lasted a short time. Additional observations taken with the 256-inch (6.5-meter) Magellan telescope in Chile four nights later still detected the extension, though it had begun to fade. Astronomers took additional photos later that month and again in April and May but failed to see the tail. A check back through the archives in 2010 and 2012 also turned up negative.
It’s unknown if any rocks – potential future meteorites – might have been knocked loose during the collision, but the dust wafting into space from the event will become part of the zodiacal light, that shoe-shaped cloud of glowing dust seen at nightfall in the west in spring and before dawn in the east in fall. Comets and asteroids both contribute a share of their dust to the phenomenon, made visible by reflected sunlight.