SAN FRANCISCO - As NASA scientists continue to
report startling discoveries made during the Cassini
spacecraft's initial tour of Saturn, a plan is being drafted that would extend
the mission through 2017.
Funding for
the Cassini program is scheduled to end Sept.
30, 2010. However, the spacecraft remains in good shape and could
continue to return valuable data for years to come, scientists say, provided
NASA approves the necessary funding to extend
Cassini's tour. Mission officials are preparing to
present their case for a seven-year extension to NASA headquarters next month.
"The
things that is magic about seven more years is that Saturn will reach its
northern hemisphere's summer solstice," said Robert Mitchell, Cassini program manager at the Jet Propulsion Laboratory in
Pasadena, Calif.
"When we arrived in June 2004, it was a little ways past the southern
hemisphere's summer solstice. If we could go seven more years, we would see
nearly half of Saturn's orbit."
By monitoring
Saturn during half of its 29-year trip around the sun, scientists hope to study
the effect of seasonal
changes on Saturn, its rings, and its moons Titan and Enceladus.
Saturn's orbit is tilted 27 degrees relative to its equator. Just as it does on
Earth, that tilt creates distinct seasons for different areas of the planet.
If the Cassini mission continues beyond 2010, scientists want to
observe seasonal changes around Titan's
lakes of methane and Enceladus' south pole where vents spew ice crystals and water vapor,
according to Carolyn Porco, Cassini
imaging team leader for the Space Science Institute in Boulder,
Colo.
"Cassini has taken up residence in this magnificent
planetary system," Porco said. "We've
discovered a tremendous amount. We want to stay and observe for as long as
possible."
The current
NASA budget includes approximately
$80 million a year for Cassini's operations and
science. That money pays for an engineering team to operate the 12- instrument
spacecraft, a navigation team to keep it on course and 125 U.S.
scientists associated with the program. In addition, 130 European scientists
participate in Cassini research.
The Cassini-Huygens mission was developed cooperatively by
NASA, the European Space Agency and the Italian Space Agency. NASA funding does
not pay the salaries of the foreign scientists, but their work adds to the
overall cost of the mission because Cassini teams
design flights and instrument plans to cover their targets of investigation,
Mitchell said.
Any Cassini mission that extends beyond 2010 would include
fewer scientific investigations and operate at a much lower cost than the
current program. Initially, program officials hoped to plan an extended mission
that would cost $40 million per year, or half the current funding level.
"We concluded that wasn't quite enough to operate the spacecraft, keep it
on track and conduct worthwhile science," Mitchell said.
As a result,
proposals for the extended mission will include a range of funding levels with
information on the scientific investigations that could be performed at each
level. "There is no single mission laid out," Mitchell said. "We
are looking at options."
Cassini
scientists are scheduled to gather in late January at the Jet Propulsion
Laboratory to select the tour trajectories they would like the spacecraft to
follow if the mission is extended beyond 2010. Those plans will be presented to
NASA headquarters for review in mid-February, Mitchell said.
In addition to
citing the scientific goals that could be attained by extending Cassini's orbit of Saturn, the report to NASA headquarters
will provide a detailed description of the health of the spacecraft, which was
launched in 1997. Overall, the spacecraft appears to be healthy. Some
engineering subsystems show signs of wear and tear, but the spacecraft
operations team has developed a set of rules designed to treat those subsystems
carefully, Mitchell said.
Cassini
uses two types of fuel, hydrazine, which is used for small navigation
maneuvers, and a standard bipropellant for larger maneuvers. While there is
ample hydrazine to continue the mission through 2017, the Cassini
navigation team has to be more conservative with its bipropellant. "We
will design future tours knowing how much propellant is available for velocity
change," Mitchell said.
Recent tours
throughout the Saturn system have presented intriguing findings, such as
evidence of what appear to be cryovolcanoes spewing
extremely cold liquid into the atmosphere of Titan. Scientists also presented
high-resolution images at the American Geophysical Union meeting in San
Francisco in December showing distinct changes over time
in the appearance of the south pole of Enceladus.
Those changes may indicate tectonic
movement on Enceladus similar to the tectonic activity found on Earth.
advertisement
