Eyes on the prize

A space­craft with robot vi­sion

Popular Mechanics (South Africa) - - Contents - – SARAH SCHLEIDER, NASA GOD­DARD SPACE FLI GHT CEN­TRE

NASA’S ORI­GINS, SPEC­TRAL IN­TER­PRE­TA­TION, RE­SOURCE IDEN­TI­FI­CA­TION, SE­CU­RITY-REGOLITH EX­PLORER (OSIRIS-REX) launched on 8 Septem­ber 2016, and is trav­el­ling to a near-earth as­ter­oid known as Bennu, to har­vest a sam­ple of sur­face ma­te­rial, and re­turn it to Earth for study. A trio of cam­eras will cap­ture it all.

The OSIRIS-REX Cam­era Suite, or OCAMS, con­sists of three cam­eras. Poly­cam is a high-res­o­lu­tion cam­era that will ac­quire the first images of Bennu and per­form an ini­tial map­ping of the as­ter­oid. Map­cam is a medium-res­o­lu­tion cam­era that will map the as­ter­oid in colour and search for satel­lites and dust plumes. Sam­cam will doc­u­ment the sam­pling process. The space­craft will store the images cap­tured by OCAMS and send them to the OSIRIS-REX team ev­ery few days.

Sci­en­tists de­signed the cam­era suite to be func­tion­ally re­dun­dant, mean­ing that if one of the cam­eras fails dur­ing the mis­sion, the other two cam­eras can stand in.

“When you have a crit­i­cal mis­sion like this, you want re­dun­dancy,” said Chris­tian d’aubigny, OCAMS deputy in­stru­ment sci­en­tist at the Univer­sity of Ari­zona, Tuc­son. “The cam­eras have some amount of over­lap in their ca­pa­bil­i­ties. They’re not ex­act copies of each other, but if one fails, they can still get the job done.”

The first cam­era to see Bennu is called Poly­cam. Sim­i­lar to a poly­math – a hu­man that is skilled at do­ing sev­eral dif­fer­ent things – Poly­cam can per­form a wide range of op­ti­cal tasks.

Once Poly­cam per­forms an ini­tial map­ping of the as­ter­oid, sci­en­tists will use the cam­era to iden­tify a site where the space­craft might col­lect a sam­ple of Bennu’s sur­face that is as free of haz­ards as pos­si­ble, such as boul­ders and dra­matic slopes.

“Al­ready, at about three kilo­me­tres, we’re di­vid­ing the sur­face of the as­ter­oid into ‘go’ and ‘no go’ places,” said Bashar Rizk, OCAMS in­stru­ment sci­en­tist at the Univer­sity of Ari­zona. “If a place is cov­ered with haz­ards, we’re just not go­ing to go there be­cause we don’t want to risk dam­ag­ing the space­craft.”

The se­cond cam­era to get a glimpse of Bennu is called Map­cam. This cam­era has a wider field of view than Poly­cam and is equipped with a num­ber of colour fil­ters in its wheel to help the sci­en­tists iden­tify lo­ca­tions on the as­ter­oid where spe­cific

min­er­als of in­ter­est may be present, par­tic­u­larly those that may have once been in con­tact with liq­uid wa­ter.

Map­cam will also look for satel­lites and dust plumes, which may present a hazard to the space­craft. There are a num­ber of sus­pected mech­a­nisms for plume for­ma­tion, such as sub­li­ma­tion, in which a frozen sub­stance tran­si­tions di­rectly to a gas with­out first pass­ing through the liq­uid phase, and elec­tro­mag­netic lev­i­ta­tion due to elec­tri­cal charg­ing from so­lar wind as the as­ter­oid gets closer to the sun.

“As­ter­oids are ex­posed to a lot of so­lar ra­di­a­tion be­cause they have no at­mos­phere,” Rizk said. “They’re just mer­ci­lessly tor­tured by the sun ev­ery time they go around it.”

Due to a lack of wa­ter on the sur­face, the sci­en­tists pre­dict that Bennu’s regolith – a layer of loose ma­te­rial, in­clud­ing dust, soil and bro­ken rock – is very dry, sim­i­lar to the sur­face of the moon. The sur­face ma­te­rial can eas­ily stick to things, in­creas­ing the risk of con­tam­i­nat­ing the OSIRISREX space­craft dur­ing sam­pling.

Dust con­tam­i­na­tion is of par­tic­u­lar con­cern to the space­craft’s third cam­era; Sam­cam. This cam­era is a low-res­o­lu­tion, wide-an­gle cam­era de­signed to get up close and per­sonal with the as­ter­oid to doc­u­ment the sam­pling ac­qui­si­tion. When it comes time to re­trieve a sam­ple, Sam­cam must be able to re­tain its func­tion­al­ity for up to three at­tempts.

To com­bat this po­ten­tial threat, the team at the Univer­sity of Ari­zona fur­nished Sam­cam with mul­ti­ple copies of the same fil­ter, which are placed in front of the cam­era op­tics to act as a cover. The fil­ters help en­sure that the cam­era has a dust-free, un­ob­structed view­ing of the sam­pling event in case sam­pling needs to be re­peated.

The team also had to ac­count for ra­di­a­tion from gamma rays and X-rays when de­sign­ing OCAMS. Sci­en­tists housed the cam­eras in a suit of ar­mour made from solid ti­ta­nium and alu­minium. Th­ese ma­te­ri­als can block the ra­di­a­tion OSIRISREX will en­counter dur­ing the mis­sion. The lenses are made of ma­te­ri­als, such as sil­i­con diox­ide, that are ra­di­a­tion re­sis­tant, as well as a num­ber of other types of glass that are in­fused with cerium, which pre­vents the glass from turn­ing opaque when ex­posed to high lev­els of ra­di­a­tion.

“We tried to think of ev­ery­thing that the space­craft might be sub­jected to and ac­count for that,” Rizk said. “It’s a mul­ti­step process of sim­u­la­tions, test­ing and de­sign to en­sure that the cam­eras work prop­erly and that we get the best images we can.”

OSIRIS-REX’S eyes are a crit­i­cal part of re­triev­ing an as­ter­oid sam­ple, which will help sci­en­tists in­ves­ti­gate how plan­ets formed and how life be­gan, as well as im­prove our un­der­stand­ing of as­ter­oids that might im­pact Earth.

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