The idea of avoiding asteroid impacts has featured prominently in the public’s mind for decades – especially since the release of Deep Impact and Armageddon.  But is using a nuclear explosion the best way to deal with potentially hazardous space rocks?  Decidedly not.  If given enough time, there is a much more effective (and safer) way to dealing with any object on a collision course with Earth – a gravity tractor.  Now, Dr. Yohannes Ketema from the University of Minnesota has developed a flight pattern that makes this simplest of all asteroid defense mechanisms that much more effective.

Gravity tractors have been around for a while.  They use the gravity of an artificial body to pull the object toward it and slightly changing its trajectory.  Over long periods, this would pull the hazardous object out of the current trajectory into a safer one.  It also has the advantage of not requiring any direct impact or explosion on the surface of the asteroid itself. Since many asteroids are “rubble piles,” such direct kinetic impactors or nuclear explosions would at best knock apart some of the larger parts of the object, but at worst, it would create a multiple of chaotic trajectory objects that could impact the Earth at an even higher speed.

UT video on different techniques to capture an asteroid.

Designed to avoid such outcomes, gravity tractors come in one of four varieties.  The stationary version parks a relatively heavy probe next to an object and slowly pulls it into a different trajectory.  A halo orbit version has the probe slowly circle the object in a pattern designed to push it in a specific direction.  While the first two techniques would use traditional chemical rockets to reach their targets, a solar sail-equipped gravity tractor could slowly move into position to allow the probe to nudge the object out of the way. Finally, a constellation of probes could work together to push an object into a new path.

Dr. Ketema’s work suggests using a modified version of a stationary and halo orbit.  The new orbit is called “restricted Keplerian motion,” which involves moving a probe back a forth on a specific side of the asteroid to try to force it as much as possible in a particular direction. Initially, Dr. Ketema suggested this solution in a paper back in 2017 and recently released a new one that improves upon the orbit by decreasing the weight required in the probe.

Mathematical graph of the restricted Keplerian motion orbit suggested by Dr. Ketema in the paper.
Credit – Yohannes Ketema

To do this, he turned to mathematical optimization.  In optimization problems, there are goals and constraints.  In this case, there was one goal (moving the asteroid out of a hazardous orbit) and three constraints: 1) Don’t directly impact the asteroid, 2) Don’t hit the asteroid with thrusters, 3) Give enough time for the gravity tractor to do its work. Best estimates for that third constraint seem to be around ten years. Such long time horizons show the importance of early detection in asteroid defense strategies.

That time factor is also vital due to the amount of time it would take for a gravity tractor to reach it.  Since the weight of the probe is an essential factor in the tool’s effectiveness, the more fuel that burns up with it (i.e., if the probe had to arrive in place quickly), the less effective it will be at pulling the asteroid off course.

Scott Manley’s video describing gravity tractors.
Credit – Scott Manley YouTube Channel

To test his optimization technique, Dr. Ketema simulated his new style gravity tractor on an existing asteroid – 2007 VK184.  While it will pass near to Earth soon, this asteroid will not hit it.  But by placing a gravity tractor next to it about ten years out, calculations show that it could be moved into an even safer orbit.

Even with this real-life simulation, there are still a few kinks to work out.  First, gravity tractors don’t work well on larger objects since their effectiveness depends entirely on how their size compares to the object they are trying to move.  Luckily, most larger asteroids on unsafe orbits are already closely tracked and don’t appear to be heading for Earth anytime soon.  A more specific problem to the modeling done in the paper is that asteroids don’t have a spherical gravitational field, making it harder to calculate the best orbit to deflect them onto a safer course.

Concept for a possible gravity tractor. https://www.mansbrand.com/did-titan-give-saturn-its-tilt/

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