Space-geek Satur­na­lia

As the Cassini mis­sion to Saturn ends, its lead imag­ing sci­en­tist re­calls her fa­vorite pho­to­graphs and mo­ments

Newsweek - - NEWS - BY STAV ZIV @stavziv

A SPEC­TAC­U­LAR SPACE

ex­plo­ration mis­sion will con­clude with a dra­matic death. The Cassini space­craft will self-de­struct by plung­ing into Saturn’s at­mos­phere, ul­ti­mately burn­ing up and dis­in­te­grat­ing. The planned mid-septem­ber dive will be the fi­nal farewell for a nearly three­decade-long col­lab­o­ra­tion be­tween NASA, the Euro­pean Space Agency and the Ital­ian Space Agency. It’s been good while it lasted, Saturn.

The Cassini space­craft launched aboard a Ti­tan Ivb/centaur rocket from Cape Canaveral in Florida on Oc­to­ber 15, 1997, and spent seven years en route to its tar­get, Saturn. It en­tered or­bit around the ringed planet in 2004 for what was in­tended as a four-year mis­sion, but was twice ex­tended for a to­tal run of 13 years, or nearly 20 if you count the jour­ney there.

Cassini com­pleted the first in-depth re­con­nais­sance of Saturn, its moons and its rings. When the mis­sion dropped the Huy­gens probe on Ti­tan, it was the first to land on the moon of a planet other than Earth. There it dis­cov­ered rain, rivers, lakes and seas. Cassini also found the first ev­i­dence of ex­trater­res­trial hy­dro­ther­mal ac­tiv­ity on the moon Ence­ladus, where it also ob­served erupt­ing gey­sers. Its de­tailed ob­ser­va­tions of Saturn’s rings could help sci­en­tists un­der­stand how the plan­ets in our so­lar sys­tem formed.

On the eve of the mis­sion’s ex­pi­ra­tion, Newsweek spoke with Carolyn Porco—the leader of the imag­ing science team on the Cassini mis­sion, who worked as an imag­ing sci­en­tist on the Voy­ager mis­sions in the 1980s—about the stun­ning rev­e­la­tions and vis­ual record of the his­toric flight.

What was it like when you were prepar­ing for the Cassini mis­sion?

I was just com­ing off the Voy­ager project, which was the best mis­sion that’s ever been con­ducted. It had been such a rush to be part of such a his­toric voy­age, like we were all just plan­e­tary ex­plor­ers on Mag­el­lan’s ship. Then it came to an end, and we just wanted to do it again.

How did Cassini build on Voy­ager?

I was hop­ing we would an­swer many of the ques­tions that we were left with af­ter the Voy­ager en­coun­ters with Saturn. Voy­ager at each planet [it vis­ited Jupiter, Saturn, Uranus and Nep­tune] was just a breath­less rush, just a fleet­ing glimpse, maybe a week or two of high-res­o­lu­tion views of what was in the plan­e­tary sys­tem. There were so many things we just got a few looks at, and we won­dered, “What does that mean?”

There were some struc­tures in the rings that hadn’t been pre­dicted be­fore Voy­ager got there, and they were spec­tac­u­lar to see. We spent the in­ter­ven­ing 23 years try­ing to make sense of it on a the­o­ret­i­cal ba­sis. But re­ally we knew [we needed] a closer look and more time there. Then there was Ti­tan—voy­ager never got to see down to the sur­face of Ti­tan be­cause the ca­pa­bil­i­ties of the cam­era, the imag­ing sys­tem, were ex­actly mis­matched to the spec­tral ab­sorb­ing char­ac­ter­is­tics of the at­mos­phere. We were an­tic­i­pat­ing fi­nally be­ing able to ex­plore the sur­face of Ti­tan with Cassini. We knew how to al­ter our ob­serv­ing strate­gies so that we’d be able to see down to the sur­face.

What were your hopes for the Cassini mis­sion?

I was look­ing for­ward to the spec­tac­u­lar stuff I knew we were go­ing to get, be­cause I knew what the ca­pa­bil­i­ties of our cam­era sys­tem were com­pared to what we had car­ried on Voy­ager. I was ex­pect­ing that we would come away with a far richer story about what the Saturn sys­tem was all about. My un­of­fi­cial hope was that we would blow peo­ple away with the vis­ual record of our trav­els around the Saturn sys­tem, like the ul­ti­mate in­ter­plan­e­tary trav­el­ogue. I wanted to give peo­ple a sense of rid­ing along with us. No one

“YOU’VE BEEN CHANGED BE­CAUSE YOU KNOW THAT HU­MAN­ITY JUST DID SOME­THING THAT HAD NEVER BE­FORE BEEN DONE IN HU­MAN HIS­TORY.”

re­ally had ever done this. In the Voy­ager days, no one spent any time try­ing to fig­ure out how to make the im­ages art­ful or beau­ti­ful.

We also took se­quences of mov­ing phe­nom­ena so that we could, in a sense, turn our cam­eras into a video cam­era. We took im­ages of the rings chang­ing per­spec­tive when we were in or­bit around Saturn. We made movies of the moons go­ing around the planet, moons oc­cult­ing other moons, shad­ows of the moons mov­ing across the rings, chang­ing cloud pat­terns, light­ning strikes. We just turned it into a doc­u­men­tary. I think peo­ple re­ally felt a sense of be­ing there. It’s been a feast, an in­tel­lec­tual and vis­ual feast.

What did the Cassini mis­sion see in the rings of Saturn?

In the case of the moon Pan, we got to see that there were scal­lops on the edges of the Encke Gap. We didn’t ex­pect that any dis­tur­bance cre­ated by a moon like Pan would go all the way— 360 de­grees—around the ring. We thought it was just lo­cal­ized to the moon, and it turns out we were wrong.

Where the ring par­ti­cles are shoved to­gether be­cause of per­tur­ba­tions by moons, the par­ti­cles

have to go some­where. They all go up­wards, per­pen­dic­u­lar to the ring plane. At the outer edge of the B ring—the place that has the strong­est res­o­nance with a moon, the moon Mi­mas—the ring par­ti­cles are shoved to­gether, and they go more than 2 miles up per­pen­dic­u­lar to the ring. That’s like the stuff of science fic­tion. I wish they could put that in a movie: You’re fly­ing along in a shut­tle­craft, and you come upon a wall 2 miles high.

We had this fan­tas­tic op­por­tu­nity to wit­ness the north­ern spring equinox. The sun was hang­ing above the equator of Saturn and, there­fore, right above the rings. This is a ge­om­e­try where any­thing pro­trud­ing above or be­low the rings is go­ing to cast long shad­ows. We did a lot of imag­ing of the rings dur­ing that time, and we just found in­cred­i­ble things.

We also found that there are “moon­lets” em­bed­ded in the rings. That was a ma­jor discovery. We’ve been able to watch them move and or­bit, but they also mi­grate across the rings. And we be­lieve they are do­ing ex­actly what we be­lieve the plan­ets did when they were form­ing out of the disk of de­bris that was the so­lar neb­ula. This is how the plan­ets changed their po­si­tion when they started to form. All of this has given us great in­sight into what hap­pens in a disk of de­bris.

What have been some of the stand­out mo­ments?

We had to fly by Jupiter for a grav­ity as­sist, so that we could get across the abyss be­tween Jupiter and Saturn in only three years. When the staff called me into our lab to say we just got our first im­age of Jupiter, I nearly fainted. We were very far away, but it was so in­cred­i­bly de­tailed. And I fi­nally un­der­stood ex­actly what kind of cam­era we had and how great it was go­ing to be by the time we got to Saturn. It re­ally just hit me be­tween the eyes, like, “Wow!” I re­mem­ber that mo­ment vividly.

Then there was the Huy­gens land­ing. When the first pic­tures were re­leased to the pub­lic, it was a look at the sur­face of Ti­tan that we could not get from or­bit. We had al­ready been in or­bit about six and a half months, and our im­ages of Ti­tan were very hard to read, very hazy. We couldn’t get a re­ally firm grasp on whether we were see­ing any­thing that was carved by a liq­uid or looked like a liq­uid. And then came the Huy­gens land­ing, and it was an un­am­bigu­ous drainage pat­tern carved in the sur­face. It was as­ton­ish­ing.

“I THINK PEO­PLE ARE GO­ING TO BE LOOK­ING AT CASSINI IM­AGES 100 YEARS FROM NOW.”

Then, of course, the lan­der reached the sur­face. We had landed some­thing of our own mak­ing in the outer so­lar sys­tem. You’ve been changed be­cause you know that hu­man­ity just did some­thing that had never be­fore been done in hu­man his­tory. The whole so­lar sys­tem at that point be­came to me a very much smaller place. And then dis­cov­er­ing a lake at the south pole of Ti­tan and the gey­sers com­ing off the south pole of Ence­ladus—there have been so many mo­ments.

Is there more anal­y­sis that can be done on the im­ages from Cassini?

Of course. We have pro­duced hun­dreds of thou­sands of im­ages and are nowhere near through look­ing at them all. We were look­ing at Voy­ager im­ages for 23 years be­fore we got Cassini im­ages. So I think peo­ple are go­ing to be look­ing at Cassini im­ages 100 years from now. It’s a body of work on the most phe­nomeno­log­i­cally rich plan­e­tary sys­tem that we have near us, and peo­ple will be min­ing it for many years to come.

Were there any mo­ments where you felt any frus­tra­tion?

The chal­lenges in this mis­sion were all po­lit­i­cal. We didn’t get ad­e­quate fund­ing un­til we were about one and a half years away from Saturn, so we were in a mad rush in those last months.

What’s next for you?

I’m hop­ing for some rest and re­lax­ation and then get­ting back to... I don’t even re­mem­ber what a nor­mal life is. And get­ting the op­por­tu­nity as soon as pos­si­ble to go back to Ence­ladus and see whether this moon is home to a sec­ond ge­n­e­sis of life.

I also hope we learn to use what we have found out about our so­lar sys­tem from mis­sions like Cassini and what we’ve found about just how in­cred­i­bly unique our own planet is to pro­tect it and its bio­sphere. I hope we do that with all our might, be­cause there’s no other planet like it for light years around. It’s unique, and it’s our home planet, and we need to take care of it, or we will be doomed.

+ Left, from top: NASA sci­en­tists, in­clud­ing Carl Sagan, dis­cuss im­ages from the Voy­ager 2 probe; prep­ping the Cassini space­craft at Kennedy Space Cen­ter in 1997; Porco dur­ing a press tour in 2017.

+ Af­ter its launch at Cape Canaveral, Florida, in Oc­to­ber 1997, Cassini trav­eled 2.2 bil­lion miles to reach Saturn in June 2004.

+ Left: Cassini helped re­veal the makeup of Phoebe, the tiny moon of Saturn that ap­pears to have an ice-rich body coated with a thin layer of some dark ma­te­rial. Far left: Ligeia Mare is the sec­ond-largest known body of liq­uid on Ti­tan; bot­tom, left: Saturn’s rings, from the in­side and out­side.

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