This map depicts the locations of known asteroids, including ones whose orbits come close to that of Earth. Credit: NASA/JPL-Caltech

Far behind schedule in its congressional charge to find nearly all the near-Earth objects (NEOs) that could pose an “existential threat” to the US, NASA is seeking advice on the technology options for a proposed space-based telescope dedicated to the search.

NASA officials emphasized the need for the satellite, designated the Near Earth Object Camera (NEOCam), at a 19 December meeting of a National Academies of Sciences, Engineering, and Medicine committee on NEO observations. Even with NEOCam, meeting a congressional mandate to locate 90% of near-Earth objects that are 140 m or larger in size is expected to take until 2034 at the earliest—well past the 2020 deadline established in the 2005 NASA authorization act (see Physics Today, February 2017, page 31).

At the current rate of discovery, using ground-based telescopes alone, 30 years or more of observations would be needed to reach the 90% requirement, NASA chief scientist Jim Green told the committee. In two decades of searching, NASA and its partners have cataloged about a third of the estimated 25 000 NEOs larger than 140 m, according to a June 2018 interagency report. The large objects are being discovered at a rate of around 500 annually. The wide-field Large Synoptic Survey Telescope in Chile will accelerate the rate of ground-based asteroid discovery when it comes on line by around 2023, but not by nearly enough to meet the 90% target, even by the early 2030s.

Green asked the committee to evaluate the pluses and minuses of IR and visible-light observations of asteroids and to recommend techniques that could be used to determine the objects’ sizes. Edward Wright, a UCLA astronomer who was the principal investigator on NASA’s Wide-Field Infrared Survey Explorer (WISE), told the committee that IR is better suited for detecting asteroids, many of which are too dark to be observed in the visible spectrum. After completing its original mission, the WISE spacecraft was reactivated as NEOWISE in 2013 as a proof of concept for using IR to detect NEOs. That probe probably won’t last another six months, said Lindley Johnson, NASA’s planetary defense officer.

Astronomers have made steady progress identifying near-Earth objects, including ones at least 140 m in size (orange). More discoveries have been made since this chart was produced earlier this year. Credit: NASA/JPL-Caltech

Some initial studies of the NEOCam mission have already been performed, Johnson told the committee. The project, led by NEOWISE principal investigator Amy Mainzer, is awaiting a decision to enter the preliminary design phase. The spacecraft could be designed, built, and launched within four years. But the $150 million NASA has requested for its planetary defense budget in fiscal year 2019 can’t accommodate both significant funding for NEOCam and the $150 million allocated for the Double Asteroid Redirection Test (DART) mission, a $313 million planetary protection demonstration mission that’s due for launch in 2020.

According to Johnson, Congress never followed up its 2005 legislation with adequate appropriations for asteroid detection, and NASA, through multiple years and administrations, never requested additional funding. An annual budget of about $250 million would be sufficient to move both NEOCam and DART forward and to continue asteroid cataloging efforts, Johnson said.

The DART spacecraft will intercept and collide with the smaller member of the binary asteroid Didymos in 2022. The impact is expected to redirect the 150 m object’s orbit around the primary asteroid by a fraction of a percent, enough to be observed from Earth. DART will demonstrate one of the three mitigation methods being investigated for deflecting an asteroid that threatens to collide with Earth. The other two are nuclear explosives and a gravity tractor, in which the mass of a spacecraft placed in close proximity to an asteroid tugs the rock into a new course over a long period. Last year the Trump administration canceled the Asteroid Redirect mission, which would have demonstrated the gravity tractor method.

The Department of Energy is keeping long-retired five-megaton warheads warehoused for potential use in a planet-saving mission (see Physics Today, May 2018, page 29). But Johnson said it’s likely that any type of US warhead would have to be repackaged to ensure it survives a lengthy spaceflight to rendezvous with an asteroid.

The 140 m threshold specified in the congressional directive is somewhat arbitrary, officials acknowledged. Johnson said if an object that size struck Earth, the 60-megaton energy release “could be an existential threat” to the nation’s economy. The explosion of a 50 m asteroid—the estimated size of a 1908 object that flattened 2000 square kilometers of forest in Siberia—would devastate an area the size of New York City.

The Siberian event is believed to have been caused by an airburst. With the exception of the 5% of asteroids that are metal-rich, most NEOs 50 m or less in diameter would break up in the atmosphere. An asteroid estimated at 20 m exploded over Chelyabinsk, Russia, in 2013 (see the article by David Kring and Mark Boslough, Physics Today, September 2014, page 32) and created a shock wave that shattered windows over a wide area and injured more than 1000 people, most from flying glass.