Rosetta spacecraft set for unprecedented close study of a comet

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After 10 years and 4 bil­lion miles, the Euro­pean Space Agency’s Ro­setta space­craft will ar­rive at its des­ti­na­tion to­day for the first ex­tended, close ex­ami­na­tion of a comet.

The last in a se­ries of 10 thruster fir­ings over the past few months will slow Ro­setta to the pace of a per­son walk­ing, about 2 mph rel­a­tive to the speed of its tar­get, Comet 67P/​Churyu­mov-Gera­si­m­enko, at a dis­tance of about 60 miles.

Pho­to­graphs have al­ready re­vealed a sur­pris­ingly ir­regu­lar shape for the 2 1/​2-mile-wide comet, pos­si­bly an amal­ga­ma­tion of two icy bod­ies or a re­sult of un­even weath­er­ing dur­ing pre­vi­ous fly­bys. From a dis­tance, the blurry blob ini­tially looked some­what like a rub­ber duck. As the de­tails came into the fo­cus, it now more re­sem­bles a knob of gin­ger fly­ing through space.

Con­tin­u­ing a trend of an­thro­po­mor­phiz­ing, the Ro­setta mis­sion man­ag­ers tweeted a pho­to­graph of the comet Mon­day with the com­ment “Do you think I got #67P’s good side yes­ter­day?”

Over the com­ing months, Ro­setta and its comet, called C-G for short, will plunge to­gether to­ward the sun.

“The key thing is we’re ren­dez­vous­ing and es­cort­ing right in along­side the comet for an ex­tended pe­riod, for over a year,” said Mat­thew Tay­lor, the mis­sion’s proj­ect sci­en­tist.

They are 334 mil­lion miles from the sun (more than three times as far out as Earth), trav­el­ing at 34,400 mph.

Comets, made of ice, dust and rock, are fro­zen left­overs from the for­ma­tion of the so­lar sys­tem. Ro­setta is named af­ter the Ro­setta Stone, the en­graved block that was cru­cial in de­ci­pher­ing Egyp­tian hi­ero­glyph­ics, and sci­en­tists hope that the space­craft’s ob­ser­va­tions will of­fer im­por­tant clues to how the so­lar sys­tem came to­gether 4 1/​2 bil­lion years ago.

In June, the space­craft mea­sured the flow of wa­ter va­por stream­ing off the comet at a rate of about two cups a sec­ond, which would fill an Olym­pic-size swim­ming pool in about 100 days. As the comet ac­cel­er­ates to­ward the sun, its sur­face will warm, and the trickle will grow to a tor­rent of hun­dreds of pounds a sec­ond, form­ing the long tail char­ac­ter­is­tic of comets.

Mea­sure­ments in July put the sur­face tem­per­a­ture at mi­nus 94 de­grees Fahren­heit (mi­nus 70 Cel­sius). That was warm enough to in­di­cate that the sur­face was not ex­clu­sively ice, and that some parts were dusty and darker, ab­sorb­ing more heat from the sun.

Ro­setta is car­ry­ing a small 62-pound lander, named Philae af­ter the is­land in the Nile where the Ro­setta Stone was found. In No­vem­ber, Philae is to leave the space­craft, set down on the comet and har­poon it­self to the sur­face. That will be the first time a space­craft has gen­tly landed on a comet.

De­signed to op­er­ate through 2015, Ro­setta and Philae will make ob­ser­va­tions as the comet makes its near­est ap­proach to the sun a lit­tle more than a year from now, at 115 mil­lion miles, still out­side of the or­bit of Earth. The comet will re­main too dim to be seen by the na­ked eye.

Other mis­sions to comets have made brief fly­bys, be­gin­ning with the In­ter­na­tional Sun-Earth Ex­plorer-3 in Sep­tem­ber 1985.

The Ro­setta mis­sion, costing 1.3 bil­lion eu­ros ($1.7 bil­lion), will pro­vide a much lon­ger, much closer look at one comet. In­stead of tak­ing a brief snap­shot, Ro­setta will ob­serve the comet go­ing from qui­es­cent to ac­tive, and then will make be­fore-and-af­ter com­par­i­sons. Launched in March 2004, the space­craft fol­lowed a cir­cu­itous route through the so­lar sys­tem, us­ing fly­bys of the Earth and Mars to fling it­self into the same or­bital path as Comet C-G.


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