Q. Scientists say the newly found meteorite Tissint broke off from Mars 700,000 years ago. What has it been doing all this time? And how do they know how long?
A. ?The short answer is, just orbiting the Sun till the Earth got in the way,? said Gareth V. Williams, associate director of the Minor Planet Center of the International Astronomical Union?and an astronomer at the Harvard-Smithsonian Center for Astrophysics.
?It is extremely unlikely that a meteorite blasted off Mars will fly off in?an orbit that intersects the Earth on its first orbit,? Dr. Williams said. ?It takes time for?the initial orbit to be perturbed into an orbit that crosses the orbit of?the Earth.?
Then, the Earth and the meteorite had to reach the same?point at the same time, he said.
As for the timeline, Michail?I.?Petaev, senior scientist at the Center for Astrophysics, said that the slow, spiraling orbit of such a small object was very complex, as it was pulled by the gravity of ?how many bodies we will never know.? But as Tissint was knocked loose, its freshly exposed surface began to be hit by cosmic rays, very high-energy particles from long-ago stellar explosions.
The particles produced radioactive isotopes that decay at a known rate, Dr. Petaev said. These indicate the time since Tissint was knocked off Mars, or at least since its most recent major collision.
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