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| RISKS VS. RETURNS Is the Cassini Mission safe? October 21, 1997 |
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Return to this forum's introduction.
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 used to power Cassini? What other energy sources could have been selected to speed up the mission? How can NASA risk public safety for planetary exploration?
Archie Hinman of Juneau, AK asks: When is the Cassini satellite supposed to swing back near earth? How close will the fly-by be and what are the possible causes of a failure that would release radioactive material into the Earth's atmosphere?
Steven Aftergood of the Federation of American Scientists:
The Earth flyby will occur in August 1999 at an altitude of about 500 miles. NASA has demonstrated an ability to conduct such flybys to an accuracy of within 1-5 miles.
In order for a release of radioactive material into the atmosphere to occur, the trajectory of the spacecraft would have to be altered to place it on an Earth-intercept course. This could occur, for example, through an inadvertent or incorrect firing of the spacecraft's thrusters. But such an inadvertent firing would have to occur in a very particular way, since most incorrect firings would take the spacecraft away from the Earth, not towards it. Furthermore, communication with the spacecraft would have to be lost, so as to prevent course correction. This sequence of events is considered extremely unlikely.
Dr. Gary Bennett, former NASA scientist, responds:
Cassini will fly by Earth in August 1999 to pick up the needed extra energy so the spacecraft can reach Saturn. Cassini is a large spacecraft, mostly because it requires a lot of propellant (over 3,000 kilograms or over 6,600 pounds) to brake the spacecraft so it can go into orbit around Saturn. (Braking will require the main engine system to fire for 94 minutes to slow down Cassini so it can become a satellite of Saturn.) We do not have a launch vehicle large enough to send Cassini directly to Saturn so it must make use of planetary flybys to gain enough propulsive energy to reach Saturn. This technique employing other planets to assist spacecraft has been used successfully on a number of missions, including Mariner Venus Mercury, Pioneer 11, Voyager 1, Voyager 2, Galileo, and Ulysses. NASA and the Jet Propulsion Laboratory have shown over 60 times that they can pass by distant planets and moons at the correct distance.Cassini will use four planetary flybys to mach Saturn: two of Venus (April 1998 and June 1999). one of Earth (August 1999) and one of Jupiter (December 2000). Even with these gravitational assists, Cassini will take about seven years to reach Saturn, arriving in July 2004.
The Earth flyby will be done at a distance of about 1,100 kilometers (660 miles). At all times during the approach to Earth the velocity vector (the direction in which the Cassini spacecraft is moving) will be kept pointed away from Earth so that even if some incredible event occurred that caused JPL to lose contact with Cassini, it would still miss Earth. To reenter Earth's atmosphere would require a failure that placed the spacecraft on a reentry trajectory and made the spacecraft non-commandable. In response to this postulated accident, NASA and JPL have thoughtfully developed a very purposeful (but science sacrificing) design that will keep the likelihood of such a failure at less than one in one million.
To ensure a low failure probability, NASA and JPL have raised the fly-by altitude and biased the spacecraft away from Earth. Additional reviews and checks will be made on all computer software uploaded to Cassini. In this process NASA and JPL have sacrificed 10 flybys of Titan. My own opinion is that they're being too pessimistic with their failure probability. Given the previous successes with other spacecraft in going through the gravitational gates even years after launch, this is the least risky phase of the mission.
Next: What damage would an accident cause?
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