Another near miss: Is Earth ready for an incoming space rock?

Two days. That’s how much warning astronomers had that asteroid 2017 AG13 was coming. Barely enough time to call up Bruce Willis.

Discovered on Saturday, 2017 AG13 passed between Earth and the moon on Monday morning. At an estimated 36 to 111 feet wide, it’s no dinosaur killer, but even bus-sized rocks can do damage, as Russia learned in 2013.

The latest close asteroid pass comes less than a month after the White House released its “National Near-Earth Object Preparedness Strategy,” outlining how the administration hopes to develop a global capacity to respond to the threat of asteroids.

The document states seven main objectives. They include scientific goals such as improved tracking, deflecting, and modeling technologies, and policy objectives such as developing emergency procedures and recovery protocols. The plan also describes the need to improve communication and cooperation, both domestic and international.

But these self-described “high-level goals” are but “a step in addressing the myriad challenges of managing and reducing the risks posed by both large and small” near-Earth objects. What concrete plans exist for protecting Earth from asteroids?

NASA has consolidated these efforts under the title of the Planetary Defense Coordination Office. "Asteroid detection, tracking and defense of our planet is something that NASA, its interagency partners, and the global community take very seriously," said John Grunsfeld, associate administrator for NASA's Science Mission Directorate, in a press release.

The first step is to know what’s out there. NASA's database of all known asteroids has registered nearly 40 new objects this month so far. Tracking more than 700,000 space rocks in total, statistical analysis suggests that NASA knows where more than 90 percent of the dinosaur-killer sized asteroids are, NBC reports.

And there’s always room for improvement. The NEOWISE infrared space telescope continues searching for new objects, and you can even become an asteroid hunter in your free time from home.

Others believe the best defense is a good offense. A number of plans exist for deflecting an asteroid that’s been confirmed to be on an Earth collision course, depending on the timeframe.

On the slow-but-steady end of the scale sits the gravity tractor. Just parking a spacecraft next to an asteroid would gradually draw the rock into a new trajectory by the attractive force of gravity. One advantage of this technique is that it would be equally effective regardless of whether the asteroid was solid metal or a loose pile of dirt. The bigger the object, however, the slower the change.

Another strategy would be to exploit dark-colored asteroids’ uneven intake and release of solar heat, known as the Yarkovsky effect. A 2012 analysis of the one-third mile wide 1999 RQ36 found that this phenomenon had changed its orbit by about 100 miles, or 160 kilometers over the course of 12 years. "If it was up to me, I would send a rocket with some titanium dioxide on it and paint it white, and have that Yarkovksy effect accelerate, and move it away from the Earth," Jason Dworkin, a project scientist at NASA's Goddard Space Flight Center told NBC.

But these deterrents work incrementally, take years to have a measurable effect. Shorter timescales call for more active measures, such as giving the asteroid a shove. 

The Asteroid Impact and Deflection Assessment (AIDA) Mission is a collaboration between the European Space Agency and NASA, among others. Proposed for launch in 2020, this project aims to slam a “kinetic impactor” into the half-mile wide asteroid Didymos, and see what happens. If all goes well, the collision could change the rock’s speed by “a fraction of a percent”.

If all else fails, there’s always the nuclear option (yes, we're back to Bruce Willis in "Armageddon"). National Geographic reports that an underground atomic explosion could be 20 times more effective than a surface one, and that tentative plans exist for a two-prong attack: one spacecraft impacts to create a crater, and a second follows behind with the bomb.

But such a plan is far from a sure bet, since every asteroid is different. As Megan Syal, a researcher at the Lawrence Livermore National Laboratory told National Geographic, “There's really a lot of variables for an asteroid in a given scenario, including the composition, the porosity, the strength of the material, the way it sustains damage, the shape, its rotational state, and its internal structure.”

The good news is, extinction-level strikes are a once in a million-years event, so NASA isn’t expecting to need these strategies any time soon. The number of known asteroids grows every month, and both NASA and the White House are starting to invest real money in finding solutions. An asteroid researcher at the Southwest Research Institute in San Antonio told NBC that things are looking up: "There was a time where we didn't have a program to look for objects, and it was done privately. You had guys like [astronomer Eugene] Shoemaker driving out every month to Palomar Observatory to look for them," Bottke says. "Now we have $50 million annually to look for them. Now we're getting serious science missions to look at these."