|RISKS VS. RETURNS|
Is the Cassini Mission safe?
October 21, 1997
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Questions answered in this forum:
What is the Cassini mission all about? What could cause an accident when Cassini swings by Earth? What damage would an accident cause? Why was plutonium selected to power Cassini? What other energy sources could have been used to speed up the mission? How can NASA risk public safety for planetary exploration?
G. Hermanowycz of Stratford, CT asks:
Disregarding the worst possible case disaster from a possible accident, what manner, possible today, would allow the Cassini space probe to reach Saturn quicker than the installed propulsion device?
The thought is, if a payload of 72 lbs. of highly toxic plutonium is launched and will propel the probe to Saturn over the next X years, would additional plutonium or a more advanced propulsion device (requiring perhaps an even more hazardous substance) get the probe there in X years?
I think that in the scheme of things, too many people are looking at the short sighted view of "if something goes wrong, we'll be in big trouble" rather than "we need to learn more about our local universe provided the end relatively justifies the means."
Steven Aftergood of the Federation of American Scientists responds:
Cassini uses chemical rockets (thrusters) for propulsion. The plutonium power supplies are not used for propulsion, but only to provide electricity (and thermal heat).
In the near term-- but not today-- it may be possible to use electric propulsion to go on a direct trajectory to Saturn, instead of the roundabout path that Cassini will take using its chemical thrusters. Electric propulsion could be driven by solar energy, at least in the inner solar system, and maybe further.
But for long-duration missions beyond our solar system into interstellar space-- which I hope may take place in my lifetime-- a nuclear reactor would almost certainly be needed, both for power and propulsion. Although this would not be welcomed by people who are categorically opposed to nuclear power, a uranium-fueled reactor would arguably be safer to launch than a plutonium power source, since a reactor that has not operated is not highly radioactive.
Dr. John Gofman of the University of California at Berkeley responds:This is part of a very common misunderstanding that has permeated the debate about Cassini . THE PLUTONIUM HAS NOTHING TO DO WITH PROPELLING THE SPACE PROBE TO SATURN. Propulsion is definitely not part of plutonium's mission. The plutonium is there to power instruments involved in the measurements to be made and to handle the communication equipment and also to provide some heat for the instrumentation. The total is only about 800 watts of power required in total, but it must be absolutely reliable power, and that is a feature which makes the RTGs attractive to the engineers of the mission.
Dr. Gary Bennett, former NASA scientist, responds:Cassini is being propelled to Saturn by a combination of chemical propulsion and gravitational assists from planetary flybys which is the only available way to do it. The main "push" came on 15 October 1997 when the Centaur upper stage fired its two liquid-hydrogen/liquid-oxygen engines and sent Cassini on its spiraling trajectory to Saturn.
The plutonium-238 radioisotope is not involved in propelling Cassini to Saturn - it is there to produce heat that is converted into electricity to power the spacecraft.
There are more advanced propulsion systems which could (if developed) have shortened the travel time. For example, electric propulsion (such as ion thrusters or "ion engines") can provide a low but steady, long-term thrust that is predicted to move spacecraft quicker over the long haul than the short burst from chemical engines. Electric propulsion, however, requires even more electrical power than Cassini has. One way to use electric propulsion might be to use a solar-powered electric propulsion stage (assuming we had a launch vehicle that could put it and Cassini in space, which we don't have) to propel Cassini part of the way until the sunlight becomes too dim and then jettison the stage.
Another, better way, would be to use nuclear power to keep the electric thrusters operating all the way to Saturn. NASA plans to use solar-powered electric propulsion as the primary mode of propulsion for its experimental New Millennium Deep Spa I spacecraft which is scheduled to be launched next year. If successful (as ground tests indicate it will be) this experiment will give spacecraft designers confidence to use electric propulsion on future missions in the inner Solar System.
In the 1960s, the U.S. did extensive work on nuclear thermal propulsion ("nuclear rocket") in which a nuclear reactor heats the hydrogen fuel. Such a system was shown to be at least twice as efficient in using propellant as the best chemical thrusters. However, it would probably take decades to qualify such a system. Even more exotic systems, such as antimatter propulsion, may make 21st century missions move even faster. But for now, given the state of our propulsion technology and the size of our launch vehicles, using chemical propulsion and gravitational assists is the only available way to send Cassini to Saturn.
Next: How can NASA risk public safety for planetary exploration?