Tri­umphant tour of Saturn comes to an end

The Washington Post - - FRONT PAGE - BY SARAH KA­PLAN More at wash­ing­ton­post.com/ news/ speak­ing-of-sci­ence

NASA said good­bye to its Cassini space­craft Friday, send­ing it crash­ing into Saturn af­ter 13 years of or­bit­ing and ob­serv­ing the planet. Sci­en­tists on the project said its demise was “like the loss of a friend.” Im­ages from Cassini, clock­wise from top: A view of Saturn re­veals faint, pre­vi­ously un­known rings; a high-res­o­lu­tion im­age of the planet’s B Ring; a 1,250-mile-wide storm at the north pole; clouds in the north­ern hemi­sphere.

pasadena, calif. — NASA sci­en­tists re­ceived their last mes­sage from the Cassini space­craft as it plunged into Saturn early Friday morn­ing. Those fi­nal bits of data sig­naled the end of one of the most suc­cess­ful plan­e­tary­science mis­sions in his­tory.

“The sig­nal from the space­craft is gone and within the next 45 sec­onds, so will be the space­craft,” pro­gram man­ager Earl Maize re­ported from mis­sion con­trol at NASA’s Jet Propul­sion Lab­o­ra­tory, just af­ter 4:55 a.m. lo­cal time. “This has been an in­cred­i­ble mis­sion, an in­cred­i­ble space­craft, and you’re all an in­cred­i­ble team.”

One of the last pieces of data cap­tured by Cassini was an in­frared im­age of its fi­nal des­ti­na­tion: a spot on Saturn’s dark side just north of the planet’s equa­tor where the space­craft dis­in­te­grated shortly af­ter los­ing con­tact with Earth.

Cassini was the first probe to or­bit Saturn. Built and op­er­ated at the JPL, it was launched in 1997 and in­serted into or­bit in 2004. It re­vealed the struc­ture of Saturn’s rings and, by de­liv­er­ing the Huy­gens probe to the moon Ti­tan, ex­e­cuted the first land­ing of a space­craft in the outer so­lar sys­tem. It also ex­posed two moons — Ti­tan, a land of meth­ane lakes, and Ence­ladus, which has jets of wa­ter stream­ing from its south­ern pole — as prime tar­gets in the search for life be­yond Earth.

It’s pre­cisely be­cause of its suc­cesses that Cassini had to die. Once the space­craft ran out of fuel, NASA would not risk let­ting it re­main aloft, where it might be knocked into Ti­tan or Ence­ladus. In April, Cassini be­gan 22 close-in or­bits that took it be­tween and be­hind Saturn’s rings. Ear­lier this week, NASA flew Cassini past Ti­tan one last time, tak­ing ad­van­tage of the moon’s grav­i­ta­tional pull to sling­shot the space­craft to­ward Saturn.

That “good­bye kiss” set Cassini on its fi­nal, fa­tal course. Just af­ter 3:30 a.m. Cal­i­for­nia time Friday, Cassini en­tered Saturn’s at­mo­sphere, plum­met­ing at a pace of about 77,000 mph. For a few min­utes, the space­craft’s thrusters fought to keep its high-gain an­tenna pointed to­ward Earth, so it could con­tinue to send back data from this un­charted ter­ri­tory.

Dur­ing its last mo­ments, the space­craft’s in­stru­ments sam­pled the mol­e­cules in the planet’s at­mo­sphere — in­for­ma­tion that sci­en­tists will use to un­der­stand Saturn’s for­ma­tion and com­po­si­tion. NASA was able to main­tain a link with the space­craft 30 sec­onds longer than the team had ex­pected.

“Those last few sec­onds were our first taste of the at­mo­sphere of Saturn,” said JPL Di­rec­tor Mike Watkins. “Who knows how many PhD the­ses are in that data?”

Be­cause Saturn is so dis­tant, Cassini’s fi­nal sig­nals didn’t reach Earth un­til 83 min­utes af­ter the space­craft was gone.

That last com­mu­ni­ca­tion was dis­played as a green spike of data on a screen above mis­sion con­trol. The spike shrank, then flick­ered, then flat­lined.

“We call loss of sig­nal,” space­craft op­er­a­tions man­ager Julie Web­ster said at 4:55 a.m.

There was si­lence at mis­sion con­trol. And then Maize spoke: “I’m go­ing to call this the end of mis­sion. Project man­ager, off the net.”

The room burst into ap­plause. Maize strode over to Web­ster and gave her a hug.

“I’m al­most with­out words,” Web­ster said at a news con­fer­ence an hour later. She has worked on the Cassini mis­sion for more than two decades and is among the few peo­ple at NASA who ven­tured in­side the craft as it was be­ing built.

“A per­fect space­craft,” she said, her voice al­most crack­ing with emo­tion. “It did ex­actly what it was sup­posed to do.”

“Even bet­ter,” chimed in project sci­en­tist Linda Spilker.

“Even bet­ter,” Web­ster agreed. “Ex­actly as it al­ways did.”

In a JPL au­di­to­rium min­utes af­ter the end, sci­ence plan­ner Jo Pitesky gazed up at the video from mis­sion con­trol with a slightly stricken look on her face.

“She’s us,” said Pitesky, who has worked on Cassini’s op­er­a­tions team since 2001. “We can’t go there our­selves, so we build a space­craft and load it up with in­stru­ments, and then we put on our hopes and de­sires and we send them there.”

Thanks to its sci­en­tific suc­cesses and stun­ning im­ages, and the sad cir­cum­stances of its demise, Cassini is viewed with deep af­fec­tion by NASA re­searchers and space en­thu­si­asts alike. Some of the fea­tures in Saturn’s rings are named for team mem­bers’ pets; one of the en­gi­neers named his daugh­ter Phoebe, af­ter one of Saturn’s moons.

Many mem­bers of the Cassini team re­fer to the space­craft as “she,” and they as­cribe “her” hu­man traits: cu­rios­ity, in­tel­li­gence, de­ter­mi­na­tion, valor.

“It’s like the loss of a friend,” said Spilker, who has worked on the mis­sion since its in­cep­tion in the late 1980s.

The start of the grand fi­nale in April set off a pe­riod of pub­lic mourn­ing. The non­profit Plan­e­tary So­ci­ety filmed a short op­er­atic trib­ute to the mis­sion. Fans on Twit­ter posted silly car­toons and tear­ful eu­lo­gies. The Cassini Vir­tual Singers — a group of JPL em­ploy­ees who per­form Cassinithemed par­o­dies of pop­u­lar mu­sic — rewrote the lyrics to “Sea­sons of Love,” a bal­lad from the mu­si­cal “Rent.”

“The truths that we learned and the things that we tried,” they crooned at a meet­ing of the project sci­ence group this week. “The fuel that we burned. And the way that she’ll die.”

Trina Ray, a se­nior sci­ence sys­tems en­gi­neer for Cassini and found­ing mem­ber of the singing group, handed out hand­ker­chiefs to her col­leagues so they could mop up their in­evitable tears.

But the mood was all business at mis­sion con­trol early Friday. Con­ver­sa­tions about the space­craft’s sta­tus were con­ducted in the same se­ri­ous tones the flight team has al­ways used. The only dif­fer­ence was a clock dis­played above one of the room’s main mon­i­tors, count­ing down the min­utes un­til the sig­nal from the space­craft was lost.

So many cur­rent and for­mer Cassini team mem­bers flocked to Pasadena for the end of the mis­sion that there wasn’t room for them at JPL. In­stead, a view­ing party was ar­ranged on the cam­pus of nearby Caltech.

In the predawn dim­ness, hun­dreds of bleary-eyed sci­en­tists gath­ered to watch the livestream from mis­sion con­trol. Three jum­botrons had been set up on a lawn out­side an au­di­to­rium; they played a slickly pro­duced NASA video show­ing some of Cassini’s great­est im­ages.

Sean Hsu, a re­searcher at the Univer­sity of Colorado at Boul­der who works on Cassini’s cos­mic dust an­a­lyzer, flew out with his wife and two chil­dren to at­tend.

“It has been a tremen­dous mis­sion to be a part of,” he said. “It has been a lot of new sci­ence, a lot of new data, and sud­denly there will be no more data.”

With the loss of Cassini, the space around Saturn has gone dark. There are no mis­sions in progress to re­turn to the ringed planet.

But Cassini’s rev­e­la­tions at Ti­tan and Ence­ladus in­spired NASA last year to add the moons to its call-out for pro­pos­als for the New Fron­tiers pro­gram — a group of medium-size mis­sions that in­cludes the New Hori­zons flyby of Pluto and the Juno orbiter around Jupiter.

Spilker is part of a New Fron­tiers pro­posal to study Ence­ladus, a tiny body that har­bors a sub­sur­face ocean and boasts jets of wa­ter spout­ing from cracks in its icy sur­face. She called Cassini’s rev­e­la­tions about this moon “one of the most as­ton­ish­ing dis­cov­er­ies for plan­e­tary sci­ence . . . that has re­ally changed our think­ing about where to look for life.”

Spilker would like to re­turn to Saturn and sam­ple the Ence­ladus plumes for large or­ganic mol­e­cules that could be signs of bi­o­log­i­cal ac­tiv­ity. Oth­ers have pro­posed sim­i­lar mis­sions to test for “biosig­na­tures” in Ti­tan’s at­mo­sphere.

If and when a space­craft is sent back to Saturn, it will ar­rive at a place ever so slightly touched by hu­mans. Be­cause NASA chose to end Cassini’s life by plung­ing it into the planet, “its bits and pieces are now one with Saturn it­self,” Spilker said. “So when I look up at Saturn in the fu­ture, I’ll know . . . Cassini is there too.”

PHO­TOS BY NASA/JET PROPUL­SION LAB­O­RA­TORY-CALTECH/SPACE SCI­ENCE IN­STI­TUTE

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