In a move coinciding with heightened concern over the plutonium-powered generator aboard the Cassini mission to Saturn, two Democratic members of California's congressional delegation are asking the US space agency to delay the craft's October launch.
"I ask that you consider postponing the Cassini launch until alternative power sources are thoroughly researched and an independent safety review is completed," Representative Lynn Woolsey wrote in a letter sent to NASA's associate administrator for space sciences, Wesley T. Huntress Jr.
Representative Ron Dellums plans to send a similar letter of concern to NASA next week, spokesman Charles Stevenson said.
At issue is the route Cassini will take to reach Saturn and the energy source to power it. The probe will take a circuitous route to the sixth planet, looping first around Venus then back around Earth, using the accelerating force of gravity to gain sufficient velocity to make the trip. The "gravity-assist swingby" will bring the probe within 500 miles of Earth.
Critics' concern centers on what NASA calls the remote possibilities of a launch accident or inadvertent re-entry during the swingby - both events that could cause the 72 pounds of plutonium aboard Cassini to be scattered over a wide area.
"What we're talking about here is human error," said Dr. William Rothman, director of the Marin County Peace Conversion, a county government commission that persuaded Woolsey to write the letter to NASA. "This particular mission is a terrible danger."
Physicians for Social Responsibility and other anti-nuclear activist groups criticize the plutonium generator as unsafe and have blasted NASA for underestimating the risks of the launch. Protests are scheduled for later this month in San Francisco, Washington, DC, the United Nations in New York, and at Cape Canaveral, Florida.
But NASA's Jet Propulsion Laboratory in Pasadena, California, which is heading up the mission, maintains that the craft is safe. In an environmental impact statement released in June, the agency said Cassini is designed so there is "less than a one-in-a-million chance of an inadvertent atmospheric re-entry accident during the Earth's swingby."
Officials from the JPL could not be reached for comment, but in a letter to Marin County Peace Conversion's Rothman, spokeswoman Mary Beth Murrill wrote that the chances of a crash back to Earth are slim, although "we have not said it is impossible for vaporization of the plutonium aboard Cassini to occur."
JPL further said in the statement that a re-entry to Earth would only be possible if a system failure changes the craft's velocity at exactly the right magnitude and direction. And, even if Cassini were to crash back to Earth, the plutonium is encased in a ceramic material that would break up in big chunks much like pottery, minimizing contamination.
Tom Roth, a spokesman for Woolsey said NASA has not responded to her letter, first sent in July, and that the representative had no plans to take further steps at this time.
But the mission may be postponed anyway. Earlier this week, NASA announced that a two-inch tear in the foil covering the insulation of a probe escorting the Cassini craft could delay the mission. NASA officials said at a news conference that the tear was "insignificant" and that the probe would launch. To catch the swingby orbits, Cassini must launch during the window of opportunity between 6 October and 4 November.
For Cassini opponents, the possibility of a delay is good news.
"Saturn's been there a long time," said Rothman. "In my personal point of view, it wouldn't matter to me if they ever went to Saturn."
