The probe was trans­mit­ting data un­til its fi­nal mo­ments

Sky at Night Magazine - - SPACE IN 2017 -

On 15 Septem­ber, af­ter a mission of 20 years, the Cassini or­biter met its end in Saturn’s at­mos­phere. But it did not go qui­etly into the night. In­stead, the space­craft spent its last year per­form­ing some of the most au­da­cious ma­noeu­vres of the mission.

“At the start of the year we were in a set of ‘ring graz­ing’ or­bits, where Cassini came as close as pos­si­ble to Saturn’s rings, just out­side the F Ring,” says Linda Spilker, Cassini’s project sci­en­tist.

Dur­ing these fly­bys Cassini took some of its best im­ages of the planet’s rings and sev­eral moons. Twenty or­bits later, the probe moved to a new tra­jec­tory, and on 26 April it per­formed one of its most dar­ing feats – a dive be­tween the planet and its rings. Un­sure if Cassini could sur­vive the plunge, the team used the probe’s an­tenna as a shield against po­ten­tially dam­ag­ing ice par­ti­cles. It quickly be­came ap­par­ent that any dust within the gap was too small to af­fect Cassini, al­low­ing sci­en­tists to use the space­craft’s full set of in­stru­ments. While pre­vi­ous ob­ser­va­tions had looked at the out­side of the planet, these passes would al­low them to look within Saturn, at its mag­netic and grav­i­ta­tional pro­file. But as the team be­gan to in­ter­pret their data, some­thing was not quite as ex­pected. “We were look­ing at some­thing called the grav­i­ta­tional co­ef­fi­cients, which tell us what Saturn’s in­ter­nal struc­ture is like. As we got closer in those co­ef­fi­cients were so dif­fer­ent to what we pre­dicted that the in­ter­nal struc­ture of Saturn is not what we ex­pected at all,” says Spilker. Not only that, but the planet’s mag­netic field was found to be aligned with Saturn’s axis of ro­ta­tion to within less than 0.06º. “All our mod­els need there to be a larger off­set, be­cause that drives the cur­rent flow in Saturn which main­tains the mag­netic field. Ei­ther the mag­netic field is go­ing to die out or there’s some­thing we don’t un­der­stand shield­ing the real field from us,” says Spilker.

These ring dives put for­ward a new set of ques­tions for plan­e­tary sci­en­tists, but Cassini won’t be on hand to help an­swer them. On 11 Septem­ber, the space­craft took its last grav­i­ta­tional nudge from Ti­tan, send­ing it on a col­li­sion course with Saturn’s at­mos­phere four days later. Even dur­ing this fi­nal de­scent, Cassini was still trans­mit­ting data back to Earth.

“We had a pre­dic­tion for when the sig­nal would drop off but it lasted about 30 sec­onds longer, prob­a­bly be­cause the at­mos­phere was a lit­tle bit less dense than we had es­ti­mated,” says Spilker. “The space­craft did ev­ery­thing we asked it to do.”

At 07:55 EDT (12:55 UT) the Deep Space Network de­tected Cassini’s fi­nal trans­mis­sion. One minute af­ter en­ter­ing Saturn’s at­mos­phere, the space­craft broke apart and fell si­lent. But the sci­en­tific le­gacy of the mission is far from over. With years’ worth of data to look at, Cassini will con­tinue to help astronomers push the bound­aries of plan­e­tary science for decades to come.

Cassini met its end af­ter a 20-year mission, 13 of which it spent or­bit­ing Saturn Cassini’s last look at cloudy Ti­tan be­fore its fa­tal plunge, cap­tured on 13 Septem­ber Cassini burned up in Saturn’s eerily Earth-like blue skies Cassini’s fi­nal shot of Saturn shows its night side, where it would en­ter the at­mos­phere

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