Some of the best photographs of the stunning, banded rings of Saturn were made available thanks to one craft, densely packed with sensors.
While that craft, Cassini, is known best for its photographs and the information it sent about Saturn’s moons Titan and Enceladus, its own engineers are most proud of the craft’s resilience. Its temperature remained astonishingly consistent in transit from Venus to Saturn thanks to its thermal blanketing. Spacecraft operations team manager Julie Webster spoke to PDD about what made Cassini special.
Cassini was part of a joint mission with Huygens, an European Space Agency probe assembled by Aerospatiale (since renamed Alcatel) in France. The mission is an international cooperative effort, comprised of NASA, ESA, and the Italian space agency Agenzia Spaziale Italiana (ASI) as well as other European and American industrial and academic entities. It launched in 1997 in order to study Saturn’s entire system, including its rings, magnetosphere, and moons.
Along with photographing Saturn’s rings and revealing the details of their rocky structure and patterns, Cassini has discovered that Saturn’s moon Enceladus has hydrothermal activity, like that of volcanoes deep underwater on Earth.
In the United States, the mission is managed under the umbrella of NASA’s Office of Space Science by the Jet Propulsion Laboratory and NASA headquarters. Lockheed Martin built the launch vehicle, upper stage, propulsion module, and radioisotope thermoelectric generators. In total, more than 5,000 people worked on the project.
Marc Rayman, the chief engineer and mission director for the Dawn spacecraft, said that Cassini was one of his favorite craft. (When asked, Julie Webster similarly complimented Dawn.)
“It conducted an extensive and spectacular exploration of the whole Saturn system, the planet, the atmosphere and its many moons,” Rayman said. “Each of the moons themselves are fascinating places. Cassini has just been producing a wealth of absolutely spectacular results for more than a decade, orbiting what I think many people would say is among the most beautiful planets.”
However, Webster said there were other aspects that made Cassini famous to her.
Most of the hardware on Cassini was the same as that used on the two Voyager craft, which launched in 1977 and traveled to Jupiter and Saturn before heading for extrasolar space. Galileo, the probe sent to Jupiter after the Voyagers, also taught engineers about what changes needed to be made on Cassini.
When Voyager flew past Saturn’s moon Titan, cameras couldn’t see through the thick atmosphere. “It kind of looks like Los Angeles smog,” Webster said.
Because of this, the Cassini team knew that the craft’s visible light cameras needed to carry filters in order to see to the surface. Infrared and ultraviolent cameras would also be pointed at the moon, allowing it to capture unprecedented images of Titan’s surface. Cassini carries four optical sensing instruments mounted on its remote sensing pallet.
Galileo also taught spacecraft engineers a lot about what techniques were most effective when it came to thermal blanketing, Webster said, which lead to development of Cassini’s unique thermal system.
“It doesn’t get any press,” Webster said. “It doesn’t ever get touted as a big thing. But I am particularly proud of that because it was a unique design. It actually used the waste heat of the nuclear batteries to keep things warm…It’s amazing.”
The thermal blankets are made of mylar alternating with lacy scrim, layered over and over from four to more than 20 times. Webster said that her team learned a lot from observing Galileo about how to place Cassini’s heat sources and how to use heat in the most efficient way.
She also worked on Magellan, the probe which mapped Venus, and, she said, “fought too much heat the whole time.”
“Galileo fought not enough heat, and they were always having to move the heat around with the power, turning this heater on and this heater off. On Cassini we have never had to turn an instrument off for thermal or power purposes. It’s really an elegant design.”
“Engineers are very incremental,” Webster said, noting that spacecraft engineers especially are notorious for using tried-and-true methods instead of experimenting with new ones. There were some new developments in Cassini’s computer systems, though. Application-Specific Integrated Circuits (ASIC) allowed for a much smaller footprint than traditional chips, and a solid-state power switch increased component lifetime and efficiency. The solid-state switch is a great leap forward even compared to some relay switching NASA used in subsequent spacecraft, Webster said, but it was more expensive.
For all of its interior elegance, Cassini is a heavy vehicle. It weighs in at 5,500 kilograms, of which 3,000 was monomethyl hydrazine and nitrogen tetroxide propellant at launch. Some of the mass comes from the 12 instruments it carries, as well as the European probe Huygens, which was dropped on Titan.
An average space mission today will carry four to six instruments, Webster said. Cassini had to carry a lot more.
“It was designed big, it was designed to last,” she said. “All of the engineering systems, anything that has electronics associated with it, is redundant.”
Other components on Cassini came from a variety of sources, as befits a joint NASA-ESA project. The Huygens probe was built by the ESA, the magnetometer was built at the Jet Propulsion Laboratory but flown by Imperial College in London, the cosmic dust analyzer was built in Germany. The visual cameras were flown out of University of Arizona, the ultraviolet spectrometer and other instruments monitored by other universities.
And all of it just keeps going. Cassini returned to the moon Rhea this month to photograph its icy surface, bringing people one of the highest-resolution color images of the moon ever seen. Images with a resolution of 300 meters per pixel were captured from a distance that ranged from 57,900 to 51,700 kilometers away.
“I’m proud of the fact that we’ll probably last 20 years, and that was a big deal,” said Webster. The Cassini project was only mandated to last until 2008, but everything on board the craft was still functioning well at that time. Therefore, the scientists and navigators designed a new mission, extending Cassini’s travels out to 2017.
Webster said that the original development of Cassini took place in “a unique spot in time with the maximum amount of money, time and resources and experience.” The mission was well-funded, although a partner craft for Cassini was supposed to have been built, but then canceled in 1992 due to, Webster said, “just money and politics.”
“We were designed by guys that had basically had 30 to 40 years – people who had gone through all the Mariners, who had gone through the 70s with Voyager and Viking and Galileo.”
All of that experience lead to the creation of a tough, determined set of eyes in the sky.
On Sept. 15, 2017, Cassini will be out of fuel, and will fall into Saturn. On the way, it will be able to take images of Saturn’s cloud tops and atmosphere, using the end of its life to transmit what Webster called “incredibly unique science.” Cassini was a pioneer from beginning to end.
Spacecraft of the Week is a feature showing the engineering and design that went into creating the vehicles that explore our universe. Illustration by Larry Corby.
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