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Armageddon Unlikely

The universe may be a safer place for planet Earth than astronomers ________(thought/ have thought/had thought). A new survey ____________ (has found/found/ has been found) that there are far fewer dangerous asteroids hurtling through space that ______________ (can/could) hit Earth and cause widespread devastation.
Previously, astronomers _________________ (estimated/have estimated/had estimated) that there were between 1,000 and 2,000 asteroids at least a kilometer in diameter that ___________________ (can/ could) potentially slam into our planet. That meant there was a 1 percent chance of a catastrophic collision with Earth in the next million years. Based on the findings, researchers from Yale University and the Jet Propulsion Laboratory in California ____________________ (calculated/had calculated/have calculated) that the total number of potentially devastating asteroids is about half the earlier estimates - about 700.