In 2012, NASA's Spaceguard Survey concluded that astronomers had now identified 90% of the asteroids with diameters greater than 1 km. How could astronomers know that they had reached this goal?
A) Asteroids larger than 1 km are so rare and so big, they have always been easy to keep track of
B) NASA is now able to keep track of everything in the solar system out to Pluto with great precision; nothing remains to be discovered in that region
C) Most of the near-Earth objects that big are comets, and they all have big tails which are easy to see
D) The people doing the survey began to find the same objects over again, indicating they were reaching the limits of their survey
E) This was fake news; in fact, we can't ever know if we have found 90% of the larger near-Earth objects

Question

Quizplus · Accepted Answer

The people doing the survey began to find the same objects over again, indicating they were reaching the limits of their survey. This means that they had already identified almost all of the large near-Earth asteroids, and were running out of new ones to discover.

In 2012, NASA's Spaceguard Survey Concluded That Astronomers Had Now