Australian Sky & Telescope - June 2018

32 AUSTRALIAN SKY & TELESCOPE July 2018

speedpassesseveralkilometresfromtheasteroidandacouple

months in an initial 1.5-km-high ‘practice’ orbit, Osiris-REX

willbeginthedetailedsurveyandreconnaissancephaseof

its mission in late February 2019. Its three-camera suite will

maptheasteroid,providedetailedcontextforthesamplesite,

andmonitortheprogressofthesampleacquisitionitself.

Meanwhile,alaseraltimeterwillproduceafinelydetailed

topographic map, while three different spectrometers map the

mineral,organicandthermallayoftheland.

TheTouch-And-Go(TAG)sampleacquisitionstrategyis

designed with both spacecraft safety and preservation of the

sample’s pristineness in mind. The microgravity environment

makesextendedspacecraftcontactwiththesurfacearisky

venture,especiallygiventhatOsiris-REXisaboutthesizeof

awalk-inclosetandcouldeasilytipover.So,muchliketheir

Japanese colleagues, the Osiris-REX team has opted for a slow

descent, brief touchdown, and get-out-of-there plan of action.

This plan centres on the Touch-And-Go Sample

Acquisition Mechanism (TAGSAM), an articulated arm

that extends a few metres below the main structure of the

spacecraft. At the business end of the arm is a large-dinner-

plate-sizeannulusoffinemetalmesh,whichmakesthe

instrument look like the combination of a metal detector

andavacuum.Osiris-REXwilledgeuptoBennuandplant

TAGSAM on the surface. As soon as the spacecraft senses

propercontact,theinstrumentwillspewoutastreamof

pure nitrogen gas, kicking up loose material and blowing it

upintotheTAGSAMhead,therebytrappingsmallpebblesup

to2cminsize.Asabackup,engineersdesignedtheTAGSAM

contactpadstoallowsomeregolithparticlestostickthereas

well.Althoughthisstrategywillpickupregoliththat’sbeen

exposedtospaceweathering,itdoesavoidthecomplicated

multi-impactapproachHayabusa2willuse.

Withinsecondsofcollectingitssample,plannedforJuly

2020,Osiris-REXwillbeatacomparativelyhasty0.7-metre-

per-secondretreatfromtheasteroid.Oncethemissionteam

has verified that the required 60 grams or more of

carbonaceous regolith has been collected, the

spacecraft will stow TAGSAM and its regolith

sampleinitssamplereturncapsule.Osiris-

REXwillthenbeginaslowdriftawayfrom

Bennubeforeinitiatingitsreturn-to-

EarthburninMarch2021.Thesample

impact will travel up the horn and into one of three separate

sample containers inside the return capsule. The edge of

thesamplinghornisalsofoldedbackundertocreatean

innerrimdesignedtotrapmaterialthatgetsstuckinthere

whenthehorntouchestheasteroid,asabackupincasethe

projectile sampling mechanism fails to fire (as happened on

theoriginalHayabusamission).There’snotimetolinger:Just

onesecondafterthissequence,thespacecraftwillboostitself

awayfromtheasteroidtoavoidtippingover.

Theentiredescent,sampleandascentsequencewillbe

repeated,uptoatotalofthreetimes,fillingallthreesample

containers with about 100 milligrams of regolith particles.

Thefinalgrab-and-gowillbespecial,though.Touncover

materialunalteredbyspace-weatheringeffects,Hayabusa2

willfirstshootanexplosiveimpactortowardtheasteroid,

acceleratingacopperprojectilefastenoughtoexcavatea

craterafewmetreswide.Thefreshlyexposedsubsurface

materialwillserveasthesamplesite.Asmallcamera

subsatellite will watch the cratering process unfold from

aboutakilometrewayinspace,takinganimageeverysecond

forlaterdownlinktoEarthwhilethemothershiphides

behindRyugu.Oncethingsaresafe,themothershipwill

againdescend,sample,anddartaway.

Onceallthreesampleshavebeenacquired,thesample

containerwillbeshuttightinsidethereturncapsulewithan

aluminiumsealthatwillprotectanyvolatilesinthematerial

from being vaporised by exposure to space’s vacuum.

InDecember2019,Hayabusa2willdepartRyuguforits

year-longjourneybacktoEarth.Justafewhoursfromhome

in December 2020, the return capsule will separate from

themainspacecraftbeforebeginningits12-km/sreentry

throughtheatmosphere,endingwithaparachute-assisted

landingattheWoomeraTestRangeinAustralia.

Notacarboncopy

In contrast to Hayabusa 2’s adventurous multi-vehicle

choreographyandblast-and-gosampling,theOsiris-REX

missionteamchoseamoreconservative,single-spacecraft

methodology. Having roared off its

CapeCanaverallaunchpadon

September8,2016,thetwo-

tonnespacecraftbegins

itsapproachphaseto

BennuinAugust2018,

duringwhichtimethe

operations and science

teamswillbegina

search for small moons

and any damaging dust

that might orbit the

asteroid, examine the

surface, and refine their

modelofBennu’sshape.

Afterafewtentativelow- RYUGU AND ITOKAWA: JAXA, BENNU: OSIRIS-REX TEAM / NASA / GSFC

Size of spacecraft

WSIZE AT A GLANCE Ryugu is

nearly twice as large as Bennu, and

both are more rotund than kidney-

bean Itokawa. The spacecraft icon

shows the approximate size of

Osiris-REX and Hayabusa 2.

ASTEROID PLUNDERING