Before it became a crater of saurian doom, the space rock that ended the age of the dinosaurs most likely was a near-Earth object (NEO), an asteroid that occasionally came within striking distance of our planet as it orbited the sun. NASA and other space agencies are now developing ways to deflect and redirect asteroids should they approach, but those techniques will be useful only if we find dangerous NEOs before they find us. Yet NASA's search is not going as planned.

In 2010 NASA completed a congressionally mandated inventory of more than 90 percent of NEOs with a diameter of one kilometer or greater—objects that are big enough to create a planetary-scale disaster. No known objects of such cataclysmic size are now on collision courses with Earth, but smaller NEOs are still out there undiscovered by the millions. Even puny ones can cause big regional problems, such as the 18-meter rock that exploded over the Russian city of Chelyabinsk in 2013, inflicting more than $30 million in damage and injuring at least 1,600 people.

Recognizing the threat of smaller NEOs, in 2005 Congress upped the ante for NASA, giving the agency until 2020 to catalogue 90 percent of midsized NEOs at or above 140 meters in diameter. Congress, however, failed to provide the agency with sufficient new funding to achieve this ambitious goal.

So NASA is far behind. “With the current capabilities we have, the deadline of 2020 is not achievable,” says Lindley Johnson, program officer for NASA's NEO survey. The agency largely relies on three ground-based optical telescopes to hunt for NEOs—objects that, even at their largest and closest, are still very dim and difficult to find. This approach limits the search to hours when the skies above the observatories are dark and clear. NASA's Wide-field Infrared Survey Explorer (WISE) spacecraft also hunts NEOs by looking for their telltale thermal glow as they are warmed by sunlight, but WISE is predicted to cease functioning as early as 2017.

As of early 2013, NASA had mapped about 1,400 “potentially hazardous asteroids.” None are labeled worrisome for the next 100 years. COURTESY OF NASA AND JPL-CALTECH

All these limitations mean that the present survey would require another 30 to 35 years to find the several tens of thousands of midsized NEOs estimated to lurk undetected in the solar system. “There are no penalties for missing the deadline,” Johnson says, “so long as there's nothing big out there that's going to hit us.”

Now the best hope for global asteroid awareness may be a proposed infrared space telescope called NEOCam, which the agency short-listed in September 2015, along with four other proposals competing for funding as part of its Discovery program of highly focused science missions. NASA will select one or two of those proposals later this year for continued development toward a launch as early as 2020.

If flown, NEOCam would use innovative new infrared detectors to discover 10 times more NEOs than all those found to date, fulfilling Congress's 2005 mandate. But NEOCam's selection is not a foregone conclusion. In the high-stakes environment of federally funded space science, it could be argued that the money for a mission to seek out dangerous space rocks should come from somewhere besides NASA's planetary science division, which has been a regular target for cuts in recent federal budgets. Although Congress recently increased the financial support for the NEO survey from $4 million a year to $40 million, the price tag for a space mission to hunt for midsized objects is estimated to be about half a billion dollars. So without another big boost to the NEO program's funding, there is nowhere else within NASA that the money can easily come from except for the agency's planetary science programs.

The struggle for money is not a problem that should even exist, says Michael A'Hearn, an astronomer at the University of Maryland and former principal investigator for NASA's Deep Impact spacecraft, a Discovery-class mission. Given that Congress considers the search an important public policy task for NASA, it should appropriately fund the mission itself, he explains: “Can NASA afford not to select NEOCam? That is precisely what I worry about.”

Amy Mainzer, an astronomer at the NASA Jet Propulsion Laboratory and NEOCam's principal investigator, emphasizes that the mission is an excellent candidate for the Discovery program because it has scientific objectives that go beyond protecting Earth from asteroids. For example, the telescope would characterize the orbits, shapes, compositions and spin rates of some near-Earth objects—information that would help researchers trace the history of the solar system as well as select new targets for future human and robotic deep-space missions.

But even if NEOCam was purely about planetary defense, it or another mission like it still would be worthwhile, Mainzer maintains. “In terms of risk versus consequences, the possibility of an asteroid strike isn't as worrisome as something like climate change, which is very real and must be addressed right away,” she says. “But this doesn't mean we shouldn't go out and look for potentially hazardous NEOs. Looking will go a long way toward quantifying what we're dealing with and how worried we should be. It's a reasonable thing to do.”