Sky at Night Magazine - - DAWN ARRIVES AT CERES -

Be­fore Dawn set sail for Ceres, it spent 14 months or­bit­ing the smaller as­ter­oid Vesta. Vesta mea­sures some 573x557x44­6km (it is markedly oblate) and it has a high-den­sity, dif­fer­en­ti­ated in­te­rior, com­pa­ra­ble to the struc­ture of larger ter­res­trial plan­ets. In fact, some plan­e­tary re­searchers de­scribe Vesta as a pro­to­planet: if there had been more small plan­etes­i­mals around, it could have grown into an ob­ject with a size more sim­i­lar to the Moon.

Dawn was at Vesta be­tween July 2011 an Septem­ber 2012. Its most re­mark­able find­ings are re­lated to the huge im­pact basin at Vesta’s south pole, named Rheasil­via. A dra­matic col­li­sion with an­other large body chipped away part of Vesta’s south­ern hemi­sphere, leav­ing this mas­sive cir­cu­lar scar with a large cen­tral moun­tain be­hind. The im­pact all but shat­tered the as­ter­oid: girdling its equa­tor are gi­ant ridges and trenches that formed in the af­ter­math of the col­li­sion. Im­pact de­bris – recog­nis­able through its min­er­alog­i­cal com­po­si­tion, which is unique to Vesta – was strewn around the in­ner So­lar Sys­tem. In fact, five per cent of all me­te­orites on Earth are ac­tu­ally small frag­ments of Vesta.

In late 2014, plan­e­tary sci­en­tists pub­lished a de­tailed ge­o­log­i­cal map, based on Dawn’s data. Thanks this space probe, Vesta is now the best-known as­ter­oid.

the skies with their tele­scopes in vain, Pi­azzi stum­bled upon the new ‘planet’ by ac­ci­dent. It was the evening of 1 Jan­uary 1801. He named it Ceres, af­ter the Ro­man pa­tron god­dess of Si­cily. A mere 20 years af­ter the dis­cov­ery of Uranus, the So­lar Sys­tem had gained yet an­other planet, bring­ing the num­ber of known plan­ets to eight. How­ever, Ceres didn’t en­joy its plan­e­tary sta­tus for long: it is now known to be the largest body in the as­ter­oid belt, bear­ing mi­nor planet num­ber 1.

De­spite its great dis­tance, be­ing a few hun­dred mil­lion kilo­me­tres away, as­tronomers have suc­ceeded in cal­cu­lat­ing Ceres’s di­am­e­ter. It mea­sures some 975x909km across, while its ro­ta­tional pe­riod is nine hours and 4.5 min­utes. Ceres is 80 per cent larger and 3.6 times more mas­sive than the sec­ond-largest as­ter­oid, Vesta. In fact, it con­tains about one-third of the to­tal mass of the as­ter­oid belt. Us­ing the Hub­ble Space Tele­scope and the Keck Tele­scope, sci­en­tists have dis­cov­ered some bright and dark re­gions on its sur­face; the largest dark ‘spot’ has unof­fi­cially been named Pi­azzi.

The two-or­bit pi­o­neer

The mass of Ceres has been de­rived from its minute grav­i­ta­tional per­tur­ba­tions on other as­ter­oids. From the ob­ject’s known mass and size, its den­sity has been cal­cu­lated at a pretty low 2.1g per cu­bic cen­time­tre, sug­gest­ing a large frac­tion of ice – per­haps as much as 25 per cent, which would mean that Ceres con­tains more wa­ter than the Earth’s oceans, al­beit in frozen form. In­deed, spec­tro­scopic ob­ser­va­tions have re­vealed the ex­is­tence of hy­drated (wa­ter-bear­ing) min­er­als on the dust-cov­ered sur­face. And in Jan­uary 2014,

the Euro­pean infrared Her­schel Space Ob­ser­va­tory dis­cov­ered lo­calised plumes of wa­ter vapour – some­thing that has never be­fore been wit­nessed on any as­ter­oid. Lit­tle won­der, then, that plan­e­tary sci­en­tists are ea­gerly look­ing for­ward to the ar­rival of the Dawn space­craft.

Launched on 27 Septem­ber 2007, Dawn flew past Mars on 17 Fe­bru­ary 2009 and en­tered or­bit around Vesta on 15 July 2011. Af­ter study­ing this in­trigu­ing ‘pro­to­planet’ for over a year (see ‘Dawn at Vesta’, page 43) it de­parted, set­ting off for Ceres. It’s the first time in the his­tory of space ex­plo­ration that one craft will or­bit two sep­a­rate ce­les­tial bod­ies dur­ing its mission – a feat made pos­si­ble by Dawn’s ver­sa­tile ion propul­sion en­gine, which uses the minute thrust of elec­tri­cally ac­cel­er­ated xenon atoms to nav­i­gate the craft through in­ter­plan­e­tary space.

Tucked be­tween the huge so­lar ar­rays – which have a to­tal span of al­most 20m – is the Dawn space­craft it­self. It is al­most as large as two queen-size beds on top of each other and has a 1.5m high­gain an­tenna for ra­dio com­mu­ni­ca­tion with Earth. A Ger­man-built cam­era will im­age Ceres in seven dif­fer­ent colour bands, while the Ital­ian vis­i­ble and infrared map­ping spec­trom­e­ter stud­ies the com­po­si­tion and ther­mal prop­er­ties of the dwarf planet’s sur­face. An Amer­i­can gamma-ray and neu­tron de­tec­tor will de­ter­mine the rel­a­tive abun­dances of chem­i­cal el­e­ments. Fi­nally, us­ing space­craft teleme­try, as­tronomers will learn about Ceres’s ro­ta­tional prop­er­ties.

Ceres the reser­voir

On 6 March, Dawn will be cap­tured in Ceres’s grav­i­ta­tional field. But it won’t start or­bit­ing the dwarf planet right away. First, the space­craft will fly out to a dis­tance of some 75,000km, then turn around us­ing its ion en­gine and fi­nally ma­noeu­vre it­self into a po­lar or­bit around 23 April. In Novem­ber 2015, the space­craft will be moved into a low-altitude map­ping or­bit, at a mere 375km above the sur­face. The orig­i­nal or­bit in­ser­tion plan was much sim­pler, but mission con­trollers had to re­design the pro­ce­dure from scratch af­ter an en­er­getic cos­mic ray caused tem­po­rary havoc to the en­gine’s elec­tron­ics on 14 Septem­ber. Since ion en­gines work so slowly, the glitch had a huge im­pact on Dawn’s thrust timeline.

Dawn’s ex­plo­ration of Ceres will yield valu­able in­sights into the early evo­lu­tion­ary his­tory of the ter­res­trial planet. Will Pi­azzi’s planet in­deed turn out to be the largest wa­ter reser­voir in the in­ner re­gions of the So­lar Sys­tem? And will the deu­terium (heavy hy­dro­gen) abun­dance of this wa­ter match the value in the Earth’s oceans? If so, it would sug­gest that our planet’s wa­ter could have been de­liv­ered by as­ter­oids, not by comets – af­ter all, mea­sure­ments by the Euro­pean Space Agency’s Rosetta mission have shown that the deu­terium abun­dance in comet 67P/Churyu­movGerasi­menko is three times as high as the ter­res­trial value.

Al­ter­na­tively, if Ceres also has a high deu­terium frac­tion, this could im­ply that it was born in the far reaches of the So­lar Sys­tem, way be­yond Nep­tune’s or­bit, and that it mi­grated into the as­ter­oid belt by grav­i­ta­tional in­ter­ac­tions of the plan­ets some four bil­lion years ago – a sce­nario sug­gested by plan­e­tary sci­en­tist Bill McKin­non of Wash­ing­ton Uni­ver­sity. If that turns out to be the case, Ceres would be the first Kuiper Belt Ob­ject to be stud­ied up close – a few months be­fore New Hori­zon’s ar­rival at Pluto.

Ceres (inset) was quickly found to be a large mem­ber of the As­ter­oid Belt; in fact, it ac­counts for one-thrid of the Belt’s mass


The huge im­pact that cre­ated this basin also blew chunks of Vesta into space

Dawn’s cam­eras were able to cap­ture stunning close-up images of Vesta dur­ing its flyby

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