Over the past two years, NASA’s robotic explorers have opened our eyes to the scientific beauty of Mars and our solar system. We have found intriguing evidence for water and just begun to understand the implications for life on Mars and on three outer-planet moons (Europa, Titan and most recently Enceladus).

We have returned pristine samples of a comet that could hold clues to the formation of the solar system and the Earth within it. At this very hour, the Spirit and Opportunity rovers are continuing their journey across the surface of Mars, long past their expected lifetimes, uncovering geologic evidence of Mars’ past water environment and the implications for life on the Earth’s closest sibling. In addition, robotic spacecraft are currently headed towards Mercury and Pluto to explore the extreme environments of our solar system.

Paced by the robotic exploration successes of the past few years, our country has completed the critical first steps of a truly phenomenal exploration adventure. Technology and operational experience developed through our robotic exploration program have made possible realistic attempts to ascertain whether there is life elsewhere in our solar system.

Bold expeditions to explore the oceans of Europa and return samples from Mars are within our grasp. Space telescopes, planned for launch in the next decade, could yield the discovery of Earth-like planets orbiting distant stars. In addition to a dramatic expansion in scientific knowledge, these future robotic explorers could prove the engineering technology required to prepare for eventual human exploration beyond the Earth-Moon system.

Unfortunately, this exciting future is in grave jeopardy. Despite the tremendous achievements of the past few years, NASA’s solar system exploration budget is slated for a $3 billion decrease over the next five years so that we can increase our investment in the space shuttle and international space station.

If this solar system budget decrease is enacted, we will not explore the oceans of Europa, we will not return samples of Mars to the Earth for detailed analysis, and we will not begin to prepare for human exploration of Mars for at least the next several decades. The engineering and science talent that delivered these recent exploration achievements will also be lost.

NASA recently announced that the next shuttle flight has been delayed yet again. While consuming a considerable budget, this high-risk human spaceflight element has flown just once in the past three years. In this same time, NASA’s robotic exploration achievements have garnered worldwide acclaim. Future solar system exploration budgets will be under increasing pressure as NASA’s plans for the shuttle, station and human exploration of the Moon experience further schedule delays.

One alternative is to mothball the shuttle now and accept a hiatus in U.S. human-rated launch capability until NASA’s new Crew Exploration Vehicle is ready for flight. In this case, the United States and our international partners would need to build upon our partnership with Russia for crew rotations and capability enhancements to the s pace station.

In hindsight, this human spaceflight policy change became necessary Feb. 1, 2003, when Columbia failed during re-entry. It is time that we acknowledge that further investment in returning the shuttle to flightworthiness is a poor use of our resources. By mothballing the shuttle now, NASA will be able to accelerate development of the Crew Exploration Vehicle, reduce the gap in U.S. human space launch capability and build toward an expanded human presence in space. Such an approach would also blunt, if not eliminate, the funding cuts presently planned within NASA’s Mars and solar system exploration programs.

The president’s Vision for Space Exploration, defined in January 2004 , spelled out an ambitious plan for extending human presence to the Moon, Mars and beyond. NASA’s response has been to cut the very programs that were to lay the groundwork for human exploration of Mars. Plans for lunar exploration appear to have shifted from use of the Moon as a technology and operations stepping stone to an end-state in itself.

Rather than beginning the reshaping of NASA into an exploration agency properly sized to accomplish the Vision for Space Exploration, NASA’s 2007 budget will significantly reduce the pace of scientific discovery and technological innovation. By 2010, this budget erodes the buying power of NASA’s Mars and outer-planet exploration missions to the point where no flagship missions of discovery can even be contemplated. These missions are not being deferred, they are being outright canceled.

Planetary exploration is a unique symbol of our country’s technological leadership and pioneering spirit. We are fortunate to be part of a society that has the opportunity to expand the reach of humanity from the cradle of our Earth to Mars and throughout the solar system. These challenges inspire our children, build the scientific and engineering literacy of our country, and increase our economic and technological competitiveness.

Now is the time to accelerate, not curtail, the pace and scope of our robotic planetary exploration program. We cannot let our investment in Mars and solar system exploration dwindle as funds are diverted in a futile attempt to salvage existing human spaceflight infrastructure. Bold technology challenges and compelling scientific discoveries are within our grasp.

Robert Braun is currently an associate professor in the Guggenheim School of Aerospace Engineering at the Georgia Institute of Technology. From 1989 to 2003, he worked at the NASA Langley Research Center.