When the search for asteroids is under the weather

The proposed NEOCam mission, years in the making, would search for near Earth objects using a space-based infrared telescope. (credit: NASA/JPL-Caltech)





To scan the night sky for potentially hazardous near Earth asteroids, you have to be able to see the sky. And, lately, that’s been a problem.

For the last several years, the number of near Earth objects (NEOs) discovered each year has been steadily increasing thank to a number of search efforts, many supported by NASA. Those efforts are part of a mandate, established by Congress, to detect 90 percent of NEOs at least 140 meters in diameter by 2020.

However, 2018 appears likely to break that streak of increasing discoveries. In 2017, 2,055 NEOs were discovered, but with less than a month to go in 2018 only 1,703 have been found, according to data presented at a planetary defense workshop held by NASA December 9 in Washington, just before the Fall Meeting of the American Geophysical Union.

“That’s quite a drop. What happened?” said Kelly Fast, NEO observations program manager for NASA’s Planetary Defense Coordination Office. “Well, the weather was really bad in Hawaii,” where telescopes like ATLAS and Pan-STARRS used to search for NEOs are located. “That’s one of the hazards of ground-based telescopes: you are subject to the weather.”

That drop is also seen in the numbers of NEOs discovered with diameters more than 140 meters across: 567 such bodies were found in 2017, versus only 408 detected so far in 2018. That decline, she said, is also presumably due to poor weather.

Fast and others believe that 2018 is just an “anomalous” year. “The drop is primarily due to weather. I don’t think we’ve reached a limit” of current telescopes, said Vishnu Reddy of the University of Arizona. “We are capable of reaching the 2017 levels, if not more, in 2018, but the weather has been just terrible.”

While astronomers are optimistic for a rebound next year, the limitations imposed by weather have provided an additional impetus for space-based search efforts immune to the weather. “Having a telescope above the Earth’s atmosphere like NEOCam,” he said, “definitely helps us offset weather issues on the Earth.”

Well, it would if NEOCam existed. For now, the space telescope is still a proposal. “One of the things that has come out of studies by science definition teams and the National Academies and other groups is that you really need a space-based capability and you want to be looking in the infrared,” Fast said. Observing in the infrared, she said, allows for a more accurate determination of asteroid sizes.

The NEOCam concept calls for a spacecraft with a telescope 50 centimeters in diameter, placed at the Earth-Sun L1 Lagrange point. From that position it could look in different directions than groundbased telescopes, which observe in the opposite direction from the sun. Fast said NEOCam would be able to look to the sides, so to speak, along Earth’s orbit, providing more advanced warning about potentially hazardous NEOs that would otherwise be difficult to observe.

“It will improve the ability to provide [planetary] defense,” said Tim Spahr, a former director of the Minor Planet Center who works on the NEOCam team. He cited as one example an object on an orbit like the Chelyabinsk event in 2013, scaled up to 50 meters in diameter. “At 50 meters in size, it’s visible months ahead of the impact from this type of survey.”

NEOCam is not a new idea. It was most recently proposed in the latest competition for NASA’s Discovery-class low-cost planetary science missions. It was one of five finalists, but the agency instead selected in January 2017 two missions, Lucy and Psyche, that would travel to asteroids instead.

NEOCam, though, did get a consolation prize: some funding to continue an “extended Phase A” study of the mission, with a particular focus on development of its infrared instrument.

“With the budget that we have for the planetary defense program at NASA, we’ve been able to put the instrument for NEOCam into development,” said Lindley Johnson, the head of NASA’s Planetary Defense Coordination Office, with the formal title of Planetary Defense Officer.

“We are engaged on an instrument development program right now with the hopes in the future that the budget will be identified to go into a full mission development,” he said. “We’re moving things forward as rapidly as we can.”

It’s unclear if NEOCam’s future is as a “directed” mission, one NASA decides to carry out without competition, or if NEOCam will need to compete once again as part of the Discovery program. That may become clear soon: NASA released last week a draft version of the announcement of opportunity—a call for proposals—for the next round of the Discovery program. The official announcement is scheduled for release at the end of February, with finalists selected in December 2019 and one or more missions picked by March 2021.

At the annual meeting of the Division for Planetary Sciences of the American Astronomical Society in October, scientists used a NASA “town hall” meeting during the conference to lobby for NEOCam. They noted an opportunity to fly NEOCam along with a space science mission, IMAP, sharing the same launch in 2024 to the Earth-Sun L1 point. Doing so, they argued, could save $100 million.

Lori Glaze, acting director of NASA’s planetary science division, was noncommittal about that particular opportunity. “What we’re looking to do is to focus on the instrument development and put that into the project phase,” she said at that October town hall meeting.

NEOCam will come up this week at the first meeting of a new National Academies committee on Near Earth Object Observations in the Infrared and Visible Wavelengths. The committee will “investigate and make recommendations about a space-based telescope’s capabilities” about visible and infrared observations of NEO, including ways to obtain sizes of NEOs using infrared observations.

But while those committees meet and NASA ponders the future of a space-based telescope like NEOCam, the search for NEOs continues with ground-based telescopes—weather permitting. For Johnson, every little bit helps.

“By finding each object, we’re reducing the risk that Earth will get impacted by something we didn’t know about,” he said at the workshop concluded. “Every object we find brings the risk level of an un-warned impact down one increment. It’s very encouraging to me.”


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