Planetary scientists are developing new ideas for the exploration of the outer solar system, with missions to moons at Jupiter and Saturn some of the most popular proposals.

During a meeting June 9-10 of NASA’s Outer Planets Assessment Group (OPAG), there was strong enthusiasm for proposals to dispatch a spacecraft to Jupiter’s moon, Europa, an ice-covered world that may support an ocean beneath its ice pack that could possibly be teeming with life. Another high-priority target is Saturn’s moon, Titan, which was recently visited by the European Space Agency’s Huygens probe as part of NASA’s Cassini mission to Saturn.

Data collected by the Huygens lander showed Titan to be an intriguing mini-world, deserving of further, intensive scrutiny, scientists said. One concept is to study Titan using a blimp-like vehicle that floats over the moon’s surface.

The Outer Planets Assessment Group met to begin planning, prioritizing and advocating a new exploration agenda for the distant planets.

Exotic places

“We need to understand the giant planets … to find out how the solar system was made. The number one problem is how habitable planets are made,” said Fran Bagenal, chair of the OPAG and a leading space scientist at the University of Colorado here in Boulder.

Bagenal said the exploration of the outer planets can provide knowledge about the elements bound up in the giant planets, particularly Jupiter. Studying the processes involved in planetary formation can help scientists discern the conditions that played a role in shaping life on Earth and, perhaps, elsewhere.

“There are the exotic places that are cool and neat — Titan and Europa — that might be a successful place for life,” Bagenal said.

A wide array of topics is under consideration, including an orbiter for Europa, an orbiter and a rover to explore Titan, a mission to Neptune, and a mission to snag bits of a comet and return them to Earth. In addition, there are proposals for developing new technologies for such things as advanced atmospheric probes.

OPAG was established by NASA in late 2004 to identify scientific priorities and pathways for exploration in the outer solar system. It solicits views from the scientific community and reports its findings to NASA h eadquarters in Washington . While OPAG provides input to NASA, it does not make recommendations.

OPAG is evaluating outer solar system exploration goals, objectives, investigations and required measurements on the basis of the widest possible outreach into the scientific community.

Earlier this month, NASA gave a go-ahead to the Juno mission to conduct an in-depth study of Jupiter, the second mission in NASA’s New Frontiers Program. It is to be ready for launch no later than the end of June 2010.

The first New Frontiers spacecraft — the nuclear-powered New Horizons mission to Pluto and the Kuiper Belt — is headed for a January 2006 departure. While the approval process for launching a payload with a nuclear power source is still ongoing , the spacecraft itself is undergoing final testing for next year’s liftoff.

U.S./European collaboration

NASA Administrator Mike Griffin has already backed a Europa Orbiter in Senate testimony. Accordingly, a team has been funded by NASA to take a quick look at the kind of spacecraft that would be needed, in terms of power, mass, travel time and other items.

For planning purposes, this group is looking at launch dates for a Europa Orbiter in the 2012-2015 range, although the later dates are more likely in terms of funding, according to information distributed at the OPAG meeting.

In parallel with this work, a 10-person team of U.S. and European scientists is being tasked to scope out the requirements of such a mission and potential areas of collaboration. There is interest in shaping the Europa Orbiter mission as an international undertaking — patterned after the highly successful Cassini mission now orbiting Saturn.

Once underway, this study group would provide their findings in eight months time. At the moment, there is no official new start for the Europa Orbiter. However, some at NASA would like to see such a mission — including a version with a lander/impactor — as part of the agency’s fiscal year 2007 budget.

At the OPAG gathering here, there was support for a dedicated look at Europa, one of many moons circling Jupiter. A Europa Orbiter mission has been under consideration at NASA for several years.

Scientists are eager to find out whether an ocean truly exists underneath Europa’s icy facade.

The spacecraft would also scan for organic and inorganic material on Europa — to determine whether it might harbor some form of life and to help set the stage for future exploration there.

Earth-like, but bizarre

Thanks to the work of the Cassini spacecraft and Europe’s Huygens probe, Titan, also cries out for follow-up exploration. It has turned out to be an exotic world with some Earth-like characteristics, but it also exhibits bizarre characteristics.

Ralph Lorenz, a scientist with the Lunar and Planetary Laboratory at the University of Arizona in Tucson, who took part in the OPAG meeting, said Titan appears set to challenge even Mars as a priority exploration target.

Lorenz is an advocate for an airborne platform to explore Titan. It would cover vast distances of Titan’s terrain, flying over possible lakes and seas of liquid methane and ethane. A new vista would be surveyed by the aerial platform every day.

The autonomously controlled robotic airship might even deploy sample devices, even mini-robots lowered by tether onto Titan, Lorenz said. This mission concept and others are detailed in a soon-to-be published paper in Advances in Space Research, he said.

Unimaginable kind of life

“The outer planets program is an opportunity to look for life under the ice in the ocean of Europa. It’s a way to also look for, who knows, some unimaginable kind of life on Titan,” said Jeff Moore, a space scientist at NASA’s Ames Research Center in Mountain View, Calif .

“If we’re going to understand under what conditions life can occur — not only in our solar system but in other nearby planetary systems — we must understand the outer planets,” Moore said .

“We have to be clever and use technology to ask the right questions,” said Bagenal, OPAG’s chair. “And we have to be patient. These missions take a long time … but they pay off.”