NASA’s controversial plan to capture an asteroid and study it is facing a challenge beyond the obvious technical feat: the potential shuttering of the Spitzer Space Telescope, whose observations can help calculate an asteroid’s size.

The Spitzer telescope’s ability to observe in infrared light is potentially crucial. Doing so allows it to measure absolute brightness, which tracks directly with asteroid size. Images taken in visible light can’t reveal the true dimensions of an asteroid, because a highly reflective rock might appear to be larger than it actually is. And NASA needs to accurately know the size of an asteroid before sending a spacecraft there.

But the agency’s astrophysics division, facing tight budgets, has proposed turning off Spitzer next year. It scored lowest in a recent ‘senior review’ of all the astrophysics missions the agency is trying to keep operating.

“We have to look at other ways to fund operations of Spitzer,” said Lindley Johnson, near-Earth object programme manager at the Johnson Space Center in Houston, Texas, during a 19 June update of the asteroid mission. The telescope costs about US$17 million a year to operate. NASA is exploring several possible scenarios, Johnson said, including running Spitzer only part of the time or getting extra money from other institutions or funding sources.

The other big challenge is to find the right space rock to visit. Two possible projects are on the table: grab a single small asteroid, or fly to the surface of a large asteroid and grab a boulder. In both cases, the sample would be dragged near the Moon, where astronauts could visit it for close-up study.

NASA currently has six asteroids on its short list — three of the single small variety, and three that are on the order of 100–500 metres across, large enough to have boulders on their surfaces that could be retrieved.

The small-rock option includes 2011 MD, an asteroid about 6 metres across that zipped past Earth three years ago. Its orbit is very similar to that of Earth, but it travels more slowly. Over time, 2011 MD falls behind and is no longer visible from Earth. In February of this year, it passed close to Spitzer.

Spitzer stared at 2011 MD for 20 hours, said David Trilling, an astronomer at Northern Arizona University in Flagstaff. He and his colleagues, led by Michael Mommert, used those observations to calculate the size and then the density of the rock. It turns out to be very porous (about 65% empty space) and about as dense as water. “This object might swim if you put it in a swimming pool,” said Trilling. The work appeared today in Astrophysical Journal Letters. The best time to grab the asteroid would be in 2024, when Earth will again catch up to it.

Candidates for the boulder retrieval attempt include the asteroid Itokawa, which the Japanese spacecraft Hayabusa visited in 2005; one known as 2008 EV5; and Bennu, which the OSIRIS-REx spacecraft aims to visit.

“We are looking to have a fairly large list of potential candidates,” said Johnson. “It won’t be dozens, but it might be ten or so by the time we need to make the decision.”

NASA plans to choose between the small- and large-rock approaches by December, said Michele Gates, programme director for the asteroid redirect mission. It won’t have to pick an actual target until about a year before launch, currently targeted for 2019.