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Astronomy and science organizations have declared 30 June as Asteroid Day, with plans to talk up the danger of asteroids that might be on a collision course with Earth. One partner in the effort, the Sentinel mission, has an especially urgent need to drum up public support: it is struggling to raise the US$450 million it needs to launch a space telescope dedicated to finding hundreds of thousands of near-Earth objects.

Astronomers have long sought a spacecraft that would hunt for near-Earth objects full-time, but public funding for such a programme has never materialized. Ground-based surveys have identified nearly all of the largest asteroids, but many of the space rocks that measure between 50 and a few hundred metres in diameter — big enough to wipe out a city with a direct hit — remain uncharted.

“Without space-based assets, progress is going to stall,” says physicist Mark Boslough of Sandia National Laboratories in Albuquerque, New Mexico.

Sentinel, announced in 2012 by the B612 Foundation of Mill Valley, California, was supposed to provide that eye in the sky, and NASA had hoped that the privately run effort would supplement the agency’s own asteroid-hunting programme. NASA is considering funding an alternative mission, the Near-Earth Object Camera (NEOCam), initially proposed to the agency in 2006. But some scientists fear that Sentinel could spoil the chances of NASA supporting NEOCam. If Sentinel cannot fly, “then it’s just a distraction from getting a job done that needs to get done,” says planetary scientist Timothy Swindle of the University of Arizona in Tucson.

Sources: G. Longo et al., 2005 (treefall); NRC, 2010 (impact energies)

Ground-based surveys have located roughly 90% of the large asteroids that might be a concern — these are all bigger than one kilometre in diameter and pass by Earth’s orbit at a distance of 45 million kilometres or less. An impact by one of these behemoths would have global consequences; fortunately, such immense objects plummet to Earth on average only once every 700,000 years or so, and none of those known is heading this way any time soon.

The statistics are less reassuring for smaller objects. As of August 2014, according to data in a recent paper, surveys had spotted only 565 near-Earth asteroids ranging from roughly 45 to 55 metres across — out of an estimated total of up to 520,000 (A. W. Harris and G. D’Abramo Icarus 257, 302–312; 2015). An asteroid no larger than that damaged trees across 2,000 square kilometres of Siberian forest in 1908, in what is known as the Tunguska event (see ‘Direct hit’). Tunguska-sized bodies strike Earth once every 500 years or so, Boslough says. An object that exploded over Chelyabinsk, Russia, in February 2013 was estimated to have been about 20 metres in diameter.

Planetary protection

Sentinel aims to ensure that a similar fate does not unexpectedly befall a place the size of Lagos or London. Mission director Harold Reitsema, who helped to design instruments on the Hubble Space Telescope, says that Sentinel will use an infrared detector to find 90% of near-Earth objects larger than 140 metres across, and will also spot a good number of objects measuring 30 metres or more. If an asteroid were found to be headed for Earth long enough before impact, authorities could try to knock it off course. Or they could plan to evacuate the target area.

To help Sentinel along, in 2012 NASA signed an agreement with the B612 Foundation to provide analytical support and, once the spacecraft is in orbit, a data downlink. The space agency has an interest in Sentinel’s success; it is under orders from Congress to find 90% of all near-Earth objects 140 metres or bigger by 2020. NASA will miss that target whether Sentinel launches or not, but a good space telescope could help to build a catalogue more quickly than ground-based surveys alone, which would need perhaps several decades.

“I don’t want anybody to think that B612 is going to save the planet, but they are doing what we need to do,” NASA administrator Charles Bolden said in 2013 when Congress pressed him on asteroid-detection efforts.

Yet progress has been slow. The B612 Foundation raised donations of roughly $1.2 million in 2012 and $1.6 million in 2013 — far short of its annual goal of $30 million to $40 million. NASA says that Sentinel has also missed every development milestone laid out in the 2012 agreement. In a January statement to an advisory panel, NASA said that its “reliance on the private sector for a space-based NEO survey … is being re-examined”. NASA’s Lindley Johnson, director of the near-earth object programme, declined to speak to Nature, citing the ongoing discussions between the B612 Foundation and the agency.

NEOCam, meanwhile, would use an infrared telescope to search for asteroids from a vantage point between Earth and the Sun. In September, NASA will decide whether it is a finalist out of more than two dozen proposals being considered for launch by 2022 through the Discovery programme, which caps each mission’s cost at $450 million.

If Sentinel receives substantial funding soon, it could launch by late 2019, says B612 Foundation chief executive and former astronaut Edward Lu. Even if NASA terminates its agreement with the foundation, he vows to keep the project going. “Believe me, I could do a lot of other things,” he says. “But I feel this is extremely important.”