What does the word "their "in paragraph 5 refer to?

Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct word or phrase that best fits each of the numbered blanks from 36 to 40.

 In 2010, the planetary defence team at NASA had (36) _____ and logged 90 percent of the asteroids near Earth measuring one kilometre wide. These 'near-Earth objects', or NEOs, are the size of mountains and include anything within 50 million kilometres of Earth's orbit. With an estimated 50 left to log, NASA says none of the 887 (37) _____ it knows about are a significant danger to the planet.

 Now NASA is working towards logging (38) _____ of the smaller asteroids, those measuring 140 metres wide or more. Of the 25,000 estimated asteroids of this size, so far about 8,000 have been logged, leaving 17,000 unaccounted for. Considering that a 19-metre asteroid that exploded above the city of Chelyabinsk in Russia in 2013 injured 1,200 people, these middle-sized asteroids would be a serious danger if they enter Earth's orbit.

 Whether NASA can find the remaining middle-sized NEOs depends on getting the money to build NEOCam, a 0.5-metre space telescope which would use infrared light to (39) _____ asteroids. If it did get the money, it could probably achieve its goal in ten years. (40) _____ logged, the planetary defence team would still need to work out how to defend the planet against being hit by the truly worrying asteroids - the PHAS.

(Source: https://learnenglish.britishcouncil.org)