An advisory group to NASA urges the space agency to re-think its planetary exploration agenda, particularly how best to probe Europa for possible evidence of an ocean and advance Mars science investigations. The group also advises NASA to stay-the-course and fly the now-cancelled New Horizons mission to explore Pluto and Kuiper Belt objects.

In a March 31 letter to NASA officials, the Solar System Exploration Subcommittee (SSES) of the Space Science Advisory Committee (SscAC) reviewed the overall health of the space agency's present and future planetary plans. A copy of the letter was made available to SPACE.com.

In its review, the SSES commends NASA on achieving an Office of Management and Budget (OMB) proposed budget increase for Office of Space Science programs. Cash to fund projects is to increase dramatically by $682 million in fiscal year 2007.

As example, the SSES salutes a new NASA planetary program called New Frontiers, capped at $650 million per fully competed mission with a launch every three years.

The SSES views the new NASA nuclear initiative as welcomed news. Nuclear power and propulsion enables long duration surface activities on planets, complex maneuvering of space probes and faster trip times to the outer solar system.

On the negative side, the SSES letter points out that an orbiter to explore Europa -- a moon of Jupiter -- and the New Horizon/Pluto-Kuiper Belt mission are both cancelled in President Bush's new NASA budget for fiscal year 2003. In addition, all Mars missions for next decade are cancelled pending a review of the program.

The call to cancel the New Horizons mission to Pluto and the Kuiper Belt, as supported by NASA chief, Sean O'Keefe, has been rejected by the SSES. O'Keefe has argued that a Pluto mission is best served by waiting for nuclear propulsion technology to become available.

Doing so would mean a ten-year mission arrival delay, the SSES letter notes.

NASA's Nuclear Systems Initiative "will notbe useful for the New Horizons mission to Pluto and the Kuiper Belt," the SSES letter advises.

The SSES strongly encourages NASA to make New Horizons the first New Frontiers mission, giving that spacecraft mission $122 million in fiscal year 2003 to keep it on track for a 2006 launch.

The Pluto-Kuiper Belt mission "remains the highest outer planets priority of the SSES. It is also urgent," the letter to NASA explains.

Why the hurry?

Due to the alignment of the Sun and Pluto, the distant planet is headed for the deep freeze. By 2029 most of one hemisphere of Pluto will be in "winter night" lasting decades. The atmosphere of Pluto is likely to have snowed out by 2020, reducing the science to be gained from a spacecraft flyby.

"Going to Pluto later than the New Horizons time line means less of Pluto and less of Charon [a moon of Pluto] to study. This point is a simple matter of celestial mechanics," the SSES letter points out.

A mission to Europa remains the highest SSES priority in the outer solar system "after a mission to Pluto and the Kuiper Belt," the advisory group's letter observes.

While a high priority, the SSES suggests that a re-evaluation of a Europa Orbiter mission is warranted. Two factors now call for this reexamination.

"First, the evidence is now very strong for a subsurface water ocean on Europa," the SSES letter says. Data gleaned from NASA's Galileo spacecraft show that Europa behaves like a conducting body. "The only plausible explanation for this conductivity is a salty ocean." This being the case, a re-look at the kind of science to be done at the mysterious moon is needed.

But current estimates now peg a Europa orbiter mission at more than $1 billion.

"The lowest cost missions that are conceivable for Europa, including multiple flybys or very short-lived orbiters, may significantly enhance our knowledge, but are less likely to lead to a quantum leap in our understanding, such as would occur by determining ice thickness and its regional variation. Ambitious missions that would do so may not fit within the current budgetary constraints and the need for a balanced program," observes the SSES advisory group.

"It is incumbent on the scientific community to develop the case for a $1 billion plus mission to Europa, or find a cheaper way to accomplish high priority Europa science," the SSES letter states.

Looking at the future of Mars exploration, the SSES flagged several areas of concern. "The earliest sample return opportunity is now delayed until 2016 and would consume at least two launch opportunities, at least four years of budget, or at least $1.4 billion," the letter explains.

The SSES notes that the soon-to-be-launched Mars Exploration Rovers and the 2009 Mars Mobile Laboratory all exceed the $700 million cost cap. Furthermore, Mars Sample Return Missions are estimated to cost $1.6 billion to $2.8 billion.

There are "serious questions" about the viability of NASA's Mars program after 2009, the SSES warns.

Concern is raised in the letter that the search for current or past life on Mars may not be adequately addressed next decade. That could mean loss of public support for ongoing Mars exploration.

The return of samples from Mars "is the only known way to unequivocally address the life question, although incremental progress towards that goal can be made by orbital and landed missions," the SSES letter states.