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

Sky at Night Magazine - - BULLETIN -

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”

At­tach­ing four rock­ets to an as­ter­oid can stop it spin­ning but a fifth might be needed to set it on a new course

LEWIS DART­NELL is an astro­bi­ol­ogy re­searcher at the Uni­ver­sity of West­min­ster and the au­thor of The Knowl­edge: How to Re­build our World from Scratch (www.the-knowl­edge.org)

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