Astronomy

26 ASTRONOMY • JANUARY 2018

On September 15, the Cassini
spacecraft took its last bow. It
grazed Saturn’s atmosphere,
burned up due to friction, and
became part of the planet it
had spent 13 years exploring.
The craft arrived at the
ringed planet in summer 2004
after a seven-year journey.
Since then, Cassini has revealed
not just the planet itself in
more detail than ever before,
but also has shown Saturn’s
moons as a diverse bunch, with
a few possibly capable of sup-
porting life. “We’ve changed
how you look at the Saturn
system,” says Linda Spilker,
Cassini’s project scientist. “I’m
just so proud of Cassini and all
its wonderful discoveries.”
The end of this historic mis-
sion went precisely as planned.
The primary mission took four
years, but after two mission
extensions, the spacecraft was
running out of fuel. The
Cassini team decided to make
the final few months memo-
rable via a mission plan they
named the Grand Finale.
On April 22, as Cassini f lew
by Saturn’s largest moon,
Titan, the moon’s mass

tweaked the spacecraft’s orbit.

As a result, four days later

Cassini dove between Saturn’s

inner rings and its upper atmo-

sphere. After that close f lyby,

the spacecraft followed a six-

day elliptical orbit to its far-

thest point before swinging

back through that gap again

May 2. In total, Cassini passed

between Saturn’s inner rings

and its atmosphere 22 times

before finally diving into the

ringed planet’s upper atmo-

sphere and disintegrating.

During each close crossing,

Cassini was between about

1,000 miles and 2,400 miles

(1,700 km and 3,900 km) above

the planet’s cloud tops. The

spacecraft’s imaging camera

continued to take incredible

photographs, while its other 11

instruments also recorded

data. The craft was traveling

too fast at each orbit’s closest

approach — some 75,000 mph

— to snap photos at those pre-

cise moments, but the camera

system could focus well enough

to take images at other times in

the orbit. It captured atmo-

spheric vortices, the great

hexagon at the planet’s north

pole, and features in the rings.

Crossing inside the rings also

meant that Cassini could mea-

sure precisely the mass of the

rings and even sample a few

ring particles, something never

done before. And by f lying so

close to the planet, the space-

craft’s magnetometer was able

to take detailed measurements

of the strength and direction of

the planet’s magnetic field.

Scientists can use that informa-

tion to learn about the rotation

and size of Saturn’s core, plus

determine how fast the cloud

tops are moving with respect to

the core.

After those 22 orbits, Cassini

completed a final half orbit.

On the night of September 14

and into the early hours of

September 15, scientists and

engineers gathered at NASA’s

Jet Propulsion Laboratory and

held a vigil. “I think of Cassini

almost like a person,” says

Spilker. On September 15,

Cassini plunged into Saturn’s

atmosphere; the final signals

arrived at 4:55 A.M. local time.

And then the Cassini mission

team said goodbye to a friend

of nearly 20 years.

Cassini’s kiss goodbye

During its plunge

into Saturn on

September 15, the

Cassini spacecraft

kept its antenna

pointed at Earth

until 4:55 A.M. PDT.

Shortly after its

signal was lost, the

spacecraft ended its

mission as a meteor

streaking across

Saturn’s sky. NASA/

JPL-CALTECH

In one of the last few photos taken

by Cassini, the moon Enceladus

sinks behind Saturn September 13.

NASA/JPL-CALTECH/SPACE SCIENCE INSTITUTE

Cassini imaged its impact site in

monochrome just hours before its

13-year mission at Saturn ended.

This is the last image taken by the

spacecraft’s cameras, recorded

at 19:59 UTC (spacecraft time)

September 14.

NASA/JPL-CALTECH/SPACE SCIENCE INSTITUTE