Выберите требуемую по смыслу форму из предложенных в скобках и объясните свой выбор. armageddon unlikely the universe may be a safer place for planet earth than astronomers / 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.
Previously, astronomers had estimated (это действие было совершено до другого действия в тут возможность в здесь нет времени для past simple и нам важен результат самого действия) that the total number of potentially devastating asteroids is about half the earlier estimates - about 700.