NASA’s Cassini Spacecraft At Saturn’s Doorstep

Saturn is now a day away for the Cassini spacecraft, a seasoned
traveler that began its journey nearly seven years ago.

On June 30 at 7:36 p.m. Pacific Time (10:36 p.m. EDT), Cassini will
begin executing a series of commands to enter orbit around the ringed
planet. The spacecraft will fire its main engine for a crucial 96
minutes to slow down and be captured in orbit about Saturn.

Besides launch, orbit insertion is the next most critical part of the
mission. “Everything has to go just right. The burn must occur for
all 96 minutes, the turns must occur at the right time, the computers
must keep the sequence going even in the event something unexpected
should happen,” said Robert T. Mitchell, program manager for the
Cassini-Huygens mission at NASA’s Jet Propulsion Laboratory, Pasadena,
Calif. “The spacecraft has been programmed to continue even in the
event of an emergency. With a one-way light time of 1 hour and 24
minutes, we had to teach the spacecraft to take care of itself. We
don’t want Cassini to call home if a problem arises, we want it to
keep going. That is precisely what we’ve told the spacecraft: Don’t
stop, keep going until you’ve put in all 96 minutes of burn,” he said.

During the orbit insertion, Cassini will fly closer to Saturn than at
any other time during the spacecraft’s planned four-year tour of
Saturn. This provides an unprecedented opportunity to study the planet
and rings at close range. It will pass approximately 20,000 kilometers
(12,427 miles) above Saturn’s cloud tops, closer than any other
spacecraft in history. It will also be flying about 10 times closer to
the rings than at any other point in the mission

Cassini carries 12 instruments that will study the planet, rings and
moons in extensive detail. Riding aboard Cassini is a second
spacecraft, the Huygens probe, built by the European Space Agency. It
carries half a dozen instruments that will study Titan, Saturn’s
largest moon, a prime target for both Cassini and the Huygens probe.
Titan is the only moon in the solar system to have a dense atmosphere
and resembles the early Earth in deep freeze.

“In a sense, Cassini and the Huygens probe are like time machines that
will take us back to examine a world we’ve never seen before, a world
that may resemble what our own world was like 4.5 billion years ago,”
said Dr. Jean-Pierre Lebreton of the European Space Agency, who is
mission manager and project scientist for the Huygens probe.

Eighty-five minutes before the engine burn, Cassini will rotate to
point its main antenna dish forward. The Italian-built antenna, 4
meters (13 feet) in diameter, will offer shielding against dust
particles the spacecraft may hit as it crosses a gap in the rings. The
spacecraft will continue transmitting a monotone “carrier” signal with
a secondary antenna for tracking from Earth. Cassini will pass twice
through a known gap between the F and G rings, first while ascending
shortly before the burn, then while descending shortly after the burn.

The engine burn will slow the spacecraft by 626 meters per second
(1,400 miles per hour). Five science instruments will be on during the
burn, and others will be used shortly after the engine cuts off. The
magnetometer will measure the strength and direction of the magnetic
field to understand the physics of Saturn’s magnetic dynamics.
Lightning may also be detected. Another instrument will provide a
record of the dust hits as the spacecraft flies through the ring
plane. These observations may tell scientists the size of these tiny
particles and the thickness of that ring region. The remote sensing
instruments will assess the rings’ composition, temperature, and
structure. Then the spacecraft will be oriented for the outbound ring
plane crossing. After crossing the ring plane in the descending mode,
Cassini will look back at the sunlit face of the rings to take more
data before turning to Earth to transmit its data.

“Should something happen during the burn, the science sequence will
stop,” said Dr. Dennis Matson, project scientist for the
Cassini-Huygens mission at JPL. “We are prepared to live with this
outcome. Getting into orbit is the priority. Getting the science is
extra credit.”

The Cassini-Huygens mission is a cooperative project of NASA, the
European Space Agency and the Italian Space Agency. The Jet Propulsion
Laboratory, a division of the California Institute of Technology in
Pasadena, manages the Cassini-Huygens mission for NASA’s Office of
Space Science, Washington, D.C. JPL designed, developed and assembled
the Cassini orbiter.
For the latest images and more information about the Cassini-Huygens
mission, visit http://www.nasa.gov/cassini