Stabilising space rocks
Asteroid mining could soon be a reality, but how could we capture one of these lucrative space rocks? LEWIS DARTNELL was reading… Formation of multiple landers for asteroid detumbling by Michael CF Bazzocchi and M Reza Emami. Read it online at doi.org/10
Asteroids present something of a double-edged sword for us Earthlings. On the one hand, some could present a grave danger to life on Earth, if they’re on a collision course to impact. On the other, many asteroids promise huge riches if we can successfully launch missions to mine them; they can provide a source of precious metals or volatile compounds that could be refined into rocket fuel to drive further space exploration.
In both of these cases, mission controllers would probably need to first stabilise the topsyturvy spin of the asteroid – to ‘detumble’ it – before attempting to shift its orbit. If you try to fire a large rocket motor on an asteroid to push it off its collision trajectory with Earth before you’ve first stopped it tumbling madly then it can be very hard to control effectively.
The best way to attempt to stabilise the tumble of an asteroid would be to land a formation of rocket thrusters onto its surface, and have them fire in a carefully coordinated sequence to slow and eventually stop the spinning in three dimensions. The problem is that smaller asteroids aren’t conveniently spherical bodies. They’re shaped more like knobbly potatoes, or even worse, like two blobs stuck together. Understanding how to effectively detumble such a complex – and often rapidly rotating – shape is very difficult. Where would be the best place to land the different thrusters onto the surface? What pattern should they fire in to stabilise the asteroid as quickly or fuel-efficiently as possible?
These are exactly the questions that Michael Bazzocchi and M Reza Emami at the Institute for Aerospace Studies, University of Toronto, have been investigating. They’ve developed a computer model that simulates the action of thrusters landed onto the surface of any given asteroid. Mathematicians have already proved that the minimum number of fixed thrusters needed to provide full attitude control of a tumbling body is four. Bazzocchi and Emami have therefore considered a scenario where a mothership rendezvous with an asteroid, deploys four thrusters to its surface to bring its spin under control, before a fifth, large rocket is landed on the stabilised asteroid to shunt it into a new orbit.
Once it’s been fed the data on the asteroid’s shape and its specific mode of rotation in three dimensions (perhaps determined by telescope light curves or radar mapping), Bazzocchi and Emami’s code first calculates the inertial properties of the oddly shaped body. It then determines the optimum landing placement of the small thrusters for detumbling the asteroid, as well as the best control system for ensuring that the complicated pattern of thruster firing stabilises the spin as quickly as possible.
When they demonstrated their code with a test case – a simulated asteroid with a mass of around 250 tonnes and rotation rates similar to small near-Earth asteroids – they found that the asteroid’s tumble could be successfully stabilised by their optimised thruster configuration in only about 15 hours. This is substantially less than the year or so currently required to redirect the asteroid onto a new orbit.
With the growing commercial interest in exploiting asteroid resources, this is exactly the sort of mission architecture that might become commonplace for space miners in the near future.
“The asteroid’s tumble could be stabilised by their optimised thruster Configuration in only about 15 hours”