Here’s How to Build a Gravity Tractor, Save Mankind From an Asteroid Apocalypse

We don't have to die like the dinosaurs, because we have opposable thumbs and computers and satellites and stuff.

Airbus Defence and Space

Thanks to Hollywood, the idea that an asteroid could bring about the end of the world — at least as we know it — doesn’t seem as preposterous. And the film industry may be doing us a favor on that score. NASA says there is nothing to fear, but an asteroid killed off the dinosaurs and other space rocks likely caused other mass extinctions as well. There is a reason that Brian May, Queen guitarist and astrophysicist moonlighter, runs around the world trying to tell everyone about the threat posed by the estimated million-plus asteroids. He gets it.

We need to protect ourselves.

If the asteroid threat is real, we need to start thinking about how we’re going to defend the planet. Detection and tracking technology is not enough. We need to develop systems to alter the trajectory of a near Earth object. We need an extraterrestrial guard dog.

Meet the gravity tractor: a theoretical spacecraft designed to deflect an oncoming object in space — without incurring any physical contact whatsoever — by using a gravitational field of its own. Make no mistake: it’s a crazy idea. And yet it’s one that scientists from NASA and elsewhere have continued to look into because, well, maybe it could work.

The concept of a gravity tractor was first put forth in a 2005 paper written by Edward Lu and Stanley Love at NASA’s Johnson Space Center. Given the fact that an asteroid that’s just about 650 feet could still bring about widespread death and destruction, the pair argued that it was necessary to begin considering realistic ways we can protect the planet. Any approach that involved direct contact with the object was rife with risks. So Lu and Love chose to think bigger — if objects in space exhibit gravity, perhaps we could send something up in space that could apply its own gravitational force towards an oncoming asteroid and nudge out of the way.

Essentially, the gravity tractor tows an asteroid out of the way by hovering close by using its own slight gravitational pull to push an object into a different trajectory. The vehicle’s thruster would have to maintain a stable direction that they neither push the tractor into the asteroid nor do they cause the tractor to come out of orbit.

Now, if you know anything about gravity, you’re probably well aware that a spacecraft that could be feasibly built here on the planet would still exhibit only a slight gravitational pull relative to other objects in space. In fact this pull would be so minute that a spacecraft would have to spend several years to significantly force an asteroid out of the way of Earth.

And that’s exactly what Lu and Love suggest. You would have to send the gravity tractor out to the oncoming asteroid a few decades beforehand in order to tow the death rock far away enough. But as long as you do this before the asteroid passes a threshold distance, it would be more than enough to turn a hit into a miss. If need be, you could send multiple gravity tractors out to an object, which would help increase the odds the mission is successful. Slow and steady wins the race.

There are two big considerations to making a gravity tractor work. One is to make sure we can find a near-Earth object soon enough before it pummels us and turns the world into a barren wasteland for a few hundred millennia. NASA actually seems to have that problem somewhat under control. One of the main objectives behind the agency’s Planetary Defense Office is to identify and track near-Earth objects around the solar system and verify if they could potentially hit Earth, and approximate when that would be.

The key to that work is the Near-Earth Object Wide-field Survey Explorer (NEOWISE) spacecraft, whose job is to look at comets and asteroids within Earth’s neck of the woods and figure out where they’re going. In just over two years’ worth of data collection, NEOWISE has successfully characterized 439 different objects and taken measurements of the 19,000 objects in total.

As of now, the range for tracking flybys of big objects is anywhere from a few months to several years, but NASA is working to shift both limits of that range as far back as possible to give us enough time to plan for an impact.

Conceptual art for the IKAROS spaceprobe with solar sail in flight.

Andrzej Mirecki

The second problem is propulsion. We are nowhere near the development of a propulsion system that could allow for interstellar travel. For a spacecraft to travel side-by-side an asteroid for several years, it’s going to need a renewable form of propulsion that doesn’t rely on a finite resource like fuel.

Experimental architect Rachel Armstrong told Inverse last December that the most sustainable propulsion method would be a solar sail, which uses solar winds to push the spacecraft forward. We’ve already started building and testing solar sails. Unfortunately, the sail’s thin material could be spread to bits in seconds by tiny dust and debris coming off an asteroid.

Instead, we’re going to have to think more radically. Some kind of fusion technology might work — perhaps based off of lasers or electron beams that push a ship forward. Maybe the rumors around the faster-than-lightspeed EmDrive are actually true, and NASA will announce the breakthrough of a lifetime in just a few months’ time.

The Asteroid Redirect Mission vehicle could test the gravity tractor planetary defense technique on a hazardous-size asteroid.


Propulsion aside, there are some other things NASA is doing that could be related to testing the viability of a gravity tractor. The purpose behind the agency’s upcoming Asteroid Redirect Mission — in which a robotic probe will land on a near-Earth asteroid, pick up a boulder, and take it to lunar orbit — are not totally clear, but NASA has expressed that one of the mission’s alternative objectives would be to test certain planetary defense techniques.

That includes the concept behind a gravity tractor. Whatever robotic spacecraft NASA sends over could be driven in such a way to test how well it could influence the movement and rotation of a near-Earth asteroid without touching it.

If an asteroid ever threatens the survival of humanity in the distant future, it’s quite probable the gravity tractor will be our way of avoiding total annihilation. It’s not as epic as watching Bruce Willis and Ben Affleck set off a nuclear bomb on a space rock, but then again, nothing ever will be. (Thanks again, Michael Bay.)

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