Above & Be­yond

Air & Space Smithsonian - - Front Page - PAOLO FERRI

Hitch­hiker on a comet


Novem­ber 12—still 9 p.m. the prior night on the U.S. East Coast—when Elsa Mon­tagnon, the night shift flight di­rec­tor of the mis­sion con­trol team, woke me with the news I’d waited more than 10 years to hear: “Phi­lae is GO for land­ing.” Af­ter trav­el­ing 6.5 bil­lion kilo­me­ters from home, the space­craft Rosetta was ready to drop our lan­der Phi­lae onto the sur­face of comet 67P Churyu­mov-gerasi­menko.

Ar­riv­ing at my work­sta­tion at the Euro­pean Space Agency’s space op­er­a­tions cen­ter in Darm­stadt, Ger­many, I had my doubts about the “go” de­ci­sion. Elsa walked me through the de­tails: The Lan­der Con­trol Cen­tre in Cologne had re­ported the fail­ure of a small rocket, re­quired to keep Phi­lae pushed against the sur­face while it an­chored it­self to the comet with two har­poons. In the near-zero-grav­ity en­vi­ron­ment, not hav­ing this rocket was trou­bling, but not part of the go/ no-go cri­te­ria that I’d ap­proved as head of mis­sion op­er­a­tions. Now I was won­der­ing if it should have been, but there was lit­tle time to think about it. It was time to fo­cus on a crit­i­cal tra­jec­tory cor­rec­tion, div­ing Rosetta to­ward the comet and re­duc­ing its ve­loc­ity by 18 cen­time­ters per sec­ond. The ma­neu­ver went per­fectly, but the ra­dio sig­nals telling us it suc­ceeded, trav­el­ing at the speed of light like all our com­mu­ni­ca­tions with Rosetta, took more than 28 min­utes to reach us. Twenty-eight min­utes is a long time to wait.

A lit­tle af­ter 8:30 a.m., our flight dy­nam­ics team had mea­sured the Dop­pler shift of Rosetta’s ra­dio sig­nal and con­firmed the land­ing could pro­ceed. Elsa an­nounced the fi­nal “GO” shortly be­fore hand­ing over op­er­a­tions to the day shift, led by Flight Di­rec­tor An­drea Ac­co­mazzo, at 9 a.m.

Now only an hour re­mained un­til the lan­der sep­a­rated from the mothership. We trans­mit­ted the com­mand to Rosetta that en­abled the lan­der to take con­trol of its sep­a­ra­tion and land­ing ac­tiv­i­ties—another 28-minute jour­ney of a sig­nal across deep space. Now we could only wait and ob­serve.

At 10:02 the teleme­try data in­di­cat­ing the sep­a­ra­tion se­quence had be­gun to reach our con­soles. We had just enough time to note the lan­der cruise locks had been re­leased and all on­board checks had re­ported suc­cess be­fore, at 10:03, the long awaited sig­nal ar­rived: Sep­a­ra­tion!

We had feared that as Phi­lae broke away, the sud­den 100-kilo­gram re­duc­tion in mass might make Rosetta shake vi­o­lently. If the os­cil­la­tions ex­ceeded the thresh­olds we’d de­signed, the space­craft would go into safe mode, shut­ting down all nonessen­tial sys­tems and cor­rect­ing it­self to a safe at­ti­tude. This would in­ter­rupt the mis­sion for sev­eral hours—and make it im­pos­si­ble for us to es­tab­lish a ra­dio link with Phi­lae prior to touch­down. This was one of the most crit­i­cal stages of the en­tire mis­sion: Fail­ure to sep­a­rate would end not just Phi­lae’s land­ing at­tempt, but pos­si­bly Rosetta’s on­go­ing mis­sion to or­bit and ob­serve the comet, too.

But we’d avoided that grim fate, and the mood in the con­trol room was eu­phoric. I jumped to hug An­drea and Stephan Ulamec, the Lan­der Project man­ager. I told Stephan, only half in jest, “The best mo­ment in the mis­sion: We fi­nally got rid of Phi­lae! Now it’s up to your lan­der to do its job.”

Around noon we re­ceived the first teleme­try pack­ets from Phi­lae. As they scrolled across my dis­play, I felt over­come with joy. For the last sev­eral days I’d told my teams over and over, “Your job is to make sure Phi­lae hits the comet and that we re­ceive and main­tain a ra­dio link with it.”

Ev­ery­thing else was out of our con­trol: Would Phi­lae sur­vive the touch­down? What would it find on the sur­face?

We’d done our jobs. Now we could only wait and ob­serve Phi­lae’s five­hour fall to­ward 67P Churyu­movGerasi­menko.

We gazed in won­der as the pic­tures that Rosetta and Phi­lae had taken of each other shortly af­ter sep­a­ra­tion fil­tered back to us from 300 mil­lion miles away—another mov­ing mo­ment. Also a use­ful one, be­cause it con­firmed that Phi­lae’s land­ing gear had de­ployed. But now the mo­ment of truth was fast ap­proach­ing. The con­trol room was nearly silent.

By now we knew the touch­down would come very close to our pre­dic­tion of 5:03 p.m. Ev­ery five to 30 sec­onds, a new packet of teleme­try, sent by Phi­lae and re­layed by Rosetta, would scroll across my mon­i­tor. Wait­ing for each packet to con­firm that Phi­lae was still alive and talk­ing to us be­came a gen­tle tor­ture. At 5:03, An­drea and Stephan shouted con­fir­ma­tion of the touch­down.

I kept my eyes on my dis­play, re­gard­ing each new packet as a won­der­ful gift: Phi­lae was on the sur­face, and data was still flow­ing! Know­ing now that the data would keep com­ing, I stood up and hugged any team mem­ber I could reach: An­drea, Elsa, Stephan, then ev­ery­one in our Flight Con­trol Team. We all had tears in our eyes.

Our ex­cite­ment was short-lived: Soon af­ter touch­down, the land­ing cen­ter in Cologne said it sus­pected the har­poons in­tended to an­chor Phi­lae to the sur­face had not fired. Also, anal­y­sis of the so­lar cells cov­er­ing the small lan­der’s sur­face showed that the cur­rent power out­put was mod­u­lated, in­di­cat­ing Phi­lae was still in mo­tion!

For­tu­nately, Phi­lae’s ra­dio link to Rosetta was still op­er­a­tive. We could see the lan­der work­ing through its ini­tial sci­en­tific mea­sure­ments. In fact, Phi­lae was work­ing so well it took us some time to ac­cept the land­ing cen­ter’s claim about Phi­lae’s har­poons. It was two hours af­ter touch­down, when the power mea­sure­ments be­came more sta­ble, be­fore we were con­vinced the cen­ter was cor­rect: Phi­lae had bounced on the sur­face be­fore set­tling in its fi­nal po­si­tion.

Twenty min­utes later, the sig­nal stopped. This could mean two things: Ei­ther Rosetta was al­ready be­low the lo­cal hori­zon (pos­si­bly be­hind a rock or a cliff) or Phi­lae had stopped func­tion­ing.

This was the first mo­ment af­ter land­ing when I be­gan to fear Phi­lae’s mis­sion could be al­ready over.

But at ap­prox­i­mately 7 a.m., Phi­lae was back, hap­pily talk­ing to us and de­liv­er­ing the mea­sure­ments it col­lected dur­ing its “night.”

The lan­der sur­vived 63 hours, 54 of them on the sur­face, as ex­pected. I spent those 54 hours on the sur­face with it, sleep­ing 10 hours in three nights, feel­ing the same emo­tions at each new con­tact. Phi­lae man­aged to com­plete its planned tasks: It passed ra­dio waves through 67P for Rosetta to pick up. It an­a­lyzed the dust and gas around the comet, and mea­sured the comet’s elec­tri­cal and ther­mal prop­er­ties. It drilled into its sur­face to at­tempt col­lec­tion of a sam­ple.

I was at home when Phi­lae fi­nally went to sleep. When I heard the news, I fell asleep too.

Rosetta’s OSIRIS cam­era took these im­ages of Phi­lae’s de­scent to the icy sur­face of comet 67P. Inset: A teleme­try feed re­ports Phi­lae’s sep­a­ra­tion from Rosetta.

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