Image below: The Centaur upper stage is hoisted at Launch Pad 40 at Cape Canaveral Air Station for mating with the Titan IV rocket that will propel the Cassini spacecraft to Saturn. Credit: NASA.|
By Kristin K. Wilson
(From the Lewis News, October 1997 issue)
The launch of the Cassini spacecraft from Cape Canaveral Air Station, FL, later this month promises to be spectacular. Boosted by a Titan IV rocket and Centaur upper stage, America's largest and most powerful expendable launch vehicle, the spacecraft will lift off amidst billowing clouds of smoke and vapor to one of the most ambitious and far-reaching planetary explorations ever mounted.
Thirty NASA Lewis employees will be in mission control at the Cape to participate in the launch of the $3.4 billion mission that will orbit Saturn, study the planet and its icy rings, and send a European-built probe to the surface of Saturn's largest moon Titan.
Weighing in at 12,500 pounds, Cassini is the heaviest and most complex spacecraft ever to be launched on an interplanetary mission. The spacecraft's size and complexity posed numerous challenges for the NASA Lewis team, whose role in the endeavor over the past eight years has included procuring the launch service from the Air Force, participating with the Air Force in overseeing the design, fabrication and test of the launch vehicle system, performing extensive independent verification and validation activities usually performed on the Titan program by subcontractors to LMA, working with Lockheed Martin to design and develop mission-unique hardware and software, performing and publishing analyses to support the nuclear safety
launch approval process, and by developing launch operations activities unique to the Cassini mission.
Image left: Cassini is lifted for placement onto a transporter that will move it to the launch pad. The launch window for Cassini opens Oct. 15. Credit: NASA.
The spacecraft's electrical power source is three Radioisotope Thermoelectric Generators (RTG) containing about 26 pounds each of plutonium dioxide. Missions carrying nuclear material undergo extensive safety reviews.
"From an engineering standpoint launching RTGs made our integration role quite difficult. In order to meet the requirements of the mission, it was necessary to develop new hardware and software. Some of the more significant items are the extensive modifications of the vehicle's flight termination system, modifications of the payload fairing for RTG access, development of a laser illumination system to permit better viewing of the nighttime launch by the range safety officer, and flight software modifications for contingency operations," said Heinz Wimmer, NASA Lewis' project manager for Cassini.
In terms of the spacecraft's weight, Wimmer explained that the Titan IV/Centaur provides a large performance margin for the Cassini spacecraft. Packed with 1.9 million pounds of propellant, the rocket generates 3.2 million pounds of thrust and is capable of launching payloads up to 13,500 pounds to geosynchronous orbit. But that's still not enough power to send Cassini on a direct path to Saturn.
After launch, when the three Titan IV stages have burned out, the Centaur will separate from the booster vehicle and burn for approximately two minutes, placing the spacecraft in a "park" orbit. After 20 minutes in orbit, the Centaur will fire for the last time and launch Cassini out of Earth orbit and onto its trajectory toward Venus.
Image right: It will take nearly seven years for the Cassini spacecraft to reach Saturn, the sixth planet from the Sun. Credit: NASA
The spacecraft will make its way to Saturn via a seven-year, unpowered coast that will take advantage of gravity-assist flybys at Earth and Jupiter, and two at Venus, imparting enough energy for Cassini to arrive at the planet and accomplish its science objectives.
Cassini is a cooperative endeavor of NASA, the Euoropean Space Agency (ESA), and the Italian Space Agency. The mission, which is managed by NASA's Jet Propulsion Laboratory, Pasadena, CA, will send the Cassini spacecraft equipped with 12 scientific experiments to orbit Saturn for a 4-year period and study the Saturnian system in detail. Cassini will deploy the ESA-built Huygens probe that will parachute into Titan's thick atmosphere carrying six scientific instrument packages.
The mission marks the first time a space probe has attempted to land on the moon of another planet, providing the first direct sampling of the Earth-like atmosphere of Saturn's moon Titan and the first detailed pictures of its previously hidden surface.
Although it is believed to be too cold to support life, haze covered Titan is thought to hold clues to how the primitive Earth evolved into a life bearing planet. The moon has an Earth-like, nitrogen-based atmosphere and a surface that many scientists believe probably features chilled lakes of ethane and methane.
For the NASA Lewis team, whose experience in launch management spans nearly three decades, the Cassini mission is special.
"This is an important international program, and to see it come through to this point and on track to launch successfully is gratifying. We all look forward to a successful launch in October and to seeing Cassini enter Saturn's orbit in 2004 and start returning science data. The Cassini program has been very exciting and rewarding. But it looks now like it will be our last deep space mission," said Wimmer, who was involved in launching Viking missions in the 1970s.
During the Cassini launch, Wimmer and the other members of the NASA Lewis team, including Center Director Donald Campbell, will monitor the flight of the spacecraft through completion of the Centaur separation including contamination and collision avoidance maneuvers. If all aspects of the launch are declared nominal, NASA Lewis' involvement in the mission will end approximately 66 minutes into flight.
"We'll know that we've done our job well when Cassini is on its way to Venus and we get word from JPL that the spacecraft is healthy. Then we can go to the post launch party," Wimmer said.
Editor's Note: The Earth Observing Satellite AM-1 launch scheduled for 1998 will be the last launch managed by NASA Lewis. Per the Agency's Zero Base Review, all launch management responsibility will transferred to Kennedy Space Center.
NASA Glenn Research Center