©2004 IUPUI, all rights reserved. Written by Gregor Novak and Andrew Gavrin, Indiana University-Purdue
University-Indiana. Used by permission.
Early on Wednesday morning, October 15, 1997,
a Titan-4 rocket launched into space the Cassini Probe. If all goes according to plan the spacecraft
will reach its destination, the planet Saturn with its moons, in July 2004. It will then tour the area
for four years, sending back data about the planet and its moons. During the tour the spacecraft will
deploy a small probe and let it parachute to the surface of Saturn's moon Titan. NASA has prepared a virtual tour of the
The Long Cruise
Saturn is ten times farther from the Sun than the Earth (about 900,000,000 miles). The Cassini probe
will take a roundabout way to get to Saturn, traveling 2 billion miles to get there, 6.7 years after
the launch. The spacecraft will fly by the planet Venus twice at 30,660 mph (13 km/s), return and fly
by the Earth, and also visit the planet Jupiter for a flyby on New Year's Eve of 2000. These flybys
are the famous NASA "gravity
assist," techniques designed to "steal" energy from celestial bodies.
Propelling Cassini with a Gravitational Slingshot To fly into space, a spacecraft has to overcome the gravity pull of the Earth. The escape
speed is about 11 km/sec. The best rocket currently available can impart only enough kinetic energy to
the Cassini spacecraft to accelerate it to about 4 km/sec. Cassini is about the size and mass of a bus
(5650 kg). With the limited kinetic energy that the craft receives from the launch rocket, it would be
limited to the planetary environments that NASA visited in the 1960s: Venus, Mars, and Jupiter. Using
"gravity assist," a concept developed in the late 1960s, a visit to all the planets is now possible.
We have to pay an energy price only to reach the first planet; the rest of the trip is free.
How Does It Work? As a spacecraft approaches a planet, the planet's gravitational field is
increasing the spacecraft's speed and is also bending the spacecraft's trajectory. If the approach
is too close, the craft crashes into the planet. If the approach is more distant, the craft will fly by
the planet and either go into orbit around the planet or leave the planet altogether. The energy
gained by the craft is lost by the planet. Since the planet has a huge mass compared to the
spacecraft, the speed decrease experienced by the planet is minuscule. When the Voyager spacecraft
swung by Jupiter, its speed increased by 16 miles per second, whereas Jupiter slowed down by 1 foot per
The Cassini spacecraft will use this gravitational slingshot effect four
times on its trip to Saturn. It will fly by Venus in April 1998 and again in June 1999. After the
second Venus flyby the craft will approach the Earth and fly by in August 1999. It will then have
enough energy to reach Jupiter on December 30, 2000, where the last of the four flybys will propel it
towards Saturn, where it will arrive in July, 2004. It will then insert itself into an orbit around Saturn. The four flybys will
gain the craft an energy equivalent of 75 tons of rocket fuel. Over 99 percent of the Cassini's trip
will be unpowered. Click on the trajectory picture for a large-scale version of the trajectory
and the project timeline.
The Scientific Objectives The Cassini is carrying a suite of scientific instruments
and communication equipment, including the Huygens probe, supplied by the European Space Agency.
During the time of its mission, Cassini will collect and transmit back to Earth an amount of scientific
data equivalent of 800 sets of Encyclopedia Brittanica. Images and data related to the physical
properties of Saturn, its rings, and its satellites will be collected.
While Cassini will collect
data all through its long journey, the heart of the mission is the tour of the Saturn environment. In addition to its
famous rings, Saturn has many interesting satellites. Some flybys of these are planned to examine their
features. Cassini will also orbit Saturn several dozen times, passing through the gaps in the rings.
Nineteen days before its arrival to Saturn, the spacecraft will swing by Saturn's most distant moon, Phoebe.
Titan In November 2004 the Cassini probe will approach Saturn's moon Titan and release the
Huygens probe, which will pass through Titan's atmosphere and eventually land on the surface of the
satellite. The Huygens probe is equipped to collect temperature, pressure, density, and energy balance data. Below Titan's
cloud cover Huygens' camera will take pictures of the Titan landscape. Titan is an intriguing
celestial object, with features similar to those present on the early Earth.
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