Astronomy

Earth

Ring-grazing

orbits

Grand Finale

orbits

Final orbit

Ring and planet

occultations

Titan’s orbit

Earth

Ring-

grazing

orbits

Grand Finale orbits

Final orbit

Noon local

time

30 ASTRONOMY • SEPTEMBER 2017

one with Earth (August 1999), and a final

gravity assist from Jupiter (December

2000), during which the craft imaged the

king of worlds and several of its moons. At

mission’s end, f lybys of Titan will carry the

spacecraft to its final encounter with

Saturn’s atmosphere.

Leading up to that end, the craft has

reconnoitered moons during dozens of

encounters, studied the magnetosphere of

Saturn, and scrutinized its clouds. During

the final orbits, however, the rings are tak-

ing center stage. It’s in these last orbits that

the mission is trickiest, but promises the

greatest return. Cassini Project Scientist

Linda Spilker of NASA’s Jet Propulsion

Laboratory explains, “Starting in

November [2016], we began what we call

— very appropriately — ring-grazing

orbits, where Cassini’s periapse [the point

lowest in its orbit] was pulled into the clos-

est point ever to the planet.”

By April, the spacecraft had completed

20 pole-passing circuits. Then Cassini

changed its path, now passing inside

Saturn’s extensive ring system, threading

the needle between rings and planet.

The dramatic orbital change was again

completed with the help of Titan’s gravity.

Flight engineers commanded Cassini to f ly

by the massive moon at just the right angle

to kick its orbital path out of the ring plane.

The new trajectory sent Cassini high above

the planet’s north pole, falling inward to

just outside of Saturn’s F ring. Once Cassini

passed by Titan on April 22, f light

engineers didn’t need any additional burns

to change the orbit. Its eventual entry into

Saturn’s atmosphere was preordained.

But controllers may nudge the space-

craft with small maneuvers to “dip our toe

in the atmosphere” Spilker says. The last

five orbits bring Cassini close enough to

directly sample in situ the composition of

Saturn’s atmosphere. Like her colleagues,

Porco looks forward to the insights it will

provide. Onboard instruments “will take

measurements all the way down,” she says.

“It’s going to be a new day. It’s a lifetime of

firsts when you’re a planetary explorer.”

Still making discoveries

Porco has been with Cassini since its

inception. Before that, she served as part of

The Cassini spacecraft, including the Huygens

probe destined to punch through Titan’s

atmosphere to touch down on its surface, sits

awaiting launch atop a Titan IVB rocket in 1997.

During launch, a payload fairing, or nose cone,

protected the craft. NASA

Enceladus spews plumes of icy water from its

south pole, as seen by Cassini in January 2013.

These plumes are part of the case for Enceladus’

subsurface ocean, which could also host

hydrothermal vents capable of providing heat

and nutrients necessary for life.

Cassini’s Grand Finale began April 22, 2017.

It was skimming the outer edges of Saturn’s

rings, then a final flyby of Titan altered the

trajectory of the spacecraft, plunging it through

the never-before-explored 1,500-mile-wide

(2,400km) gap between planet and rings

at speeds up to 78,000 mph (125,500 km/h). After

22 full orbits, Cassini’s final partial orbit will take

it into Saturn’s atmosphere at a point near noon

(local time). Final loss of signal is estimated to

occur at 5:08 A.M. PDT September 15.

The final orbits

“Propellers” are disturbances in

the rings caused by the presence

of an often-unseen moon. This

propeller in the A ring, nicknamed

“Santos-Dumont,” had been

tracked via less-detailed images

for the past 10 years before

receiving a close-up on February

21, 2017. Close imaging by Cassini

allows researchers to better grasp

the effects of the propeller on the

broader structure of the ring. NASA/

JPL-CALTECH/SPACE SCIENCE INSTITUTE

ASTRONOMY

: ROEN KELLY

NASA/JPL-CALTECH/SPACE SCIENCE INSTITUTE