NASA is discontinuing the Mars Scout line of relatively low-cost missions to the Red Planet, but there is still an opening for resourceful scientists seeking an inexpensive ticket for Mars research.

The Phoenix mission excavated several trenches near its landing site in the Martian northern polar plains. Credit: NASA/JPL-Caltech/University of Arizona/Texas A&M University

The end of the Mars Scout program comes after it fostered two missions, the Phoenix polar lander launched in 2007 and the MAVEN orbiter that will study the Martian atmosphere after its 2013 blastoff.

NASA says it is scrapping the Mars Scout mission line because future Martian probes will be heading to the surface. Most lander missions are more expensive than orbiters, and it would be challenging for a future surface probe to fit within the $485 million cost cap for Scout projects.

Doug McCuistion, NASA's chief Mars program official, said the agency is opening up its Discovery program to robotic Mars missions, beginning with an ongoing competition to select the next solar system exploration mission for launch between 2015 and 2017.

"We're in a phase where surface science is becoming more and more the driving factor at Mars," McCuistion said in a July 14 interview.

Launched in the 1990s, the Discovery program broke onto the public stage with the highly successful Pathfinder rover that landed on Mars in 1997. Pathfinder was the Discovery program's second mission.

But NASA later shut out Mars from Discovery competitions, decreeing proposals could go anywhere in the solar system except Mars or the sun. The agency formed the Mars Scout program to develop budget-minded projects devoted to the Red Planet, while NASA also accelerated more costly and ambitious missions to Mars.

NASA kicked off the competition for the 12th Discovery mission in June and is collecting proposals from science teams through Sept. 3.

Officials plan to choose candidates in April 2011 and announce the final selection in June 2012. Discovery proposals must cost less than $425 million, not including partnerships or launch services.

Phil Christensen, an Arizona State University scientist, said he is involved in several Mars proposals for the next Discovery mission.

"I really don't know how Mars will fare in the Discovery program, but there certainly are a lot of non-Mars concepts," Christensen told Spaceflight Now. "I think if the Mars concepts have the highest science value then they will be very seriously considered, especially since there is currently no other way to get small mission concepts flown to Mars."

Christensen is a prolific Mars researcher and currently works on the Mars Reconnaissance Orbiter, Mars Odyssey, and Spirit and Opportunity rovers. None of his Discovery proposals are surface missions, Christensen said.

"I suspect someone might try to do one, but that will be pretty challenging in the Discovery program," Christensen said.

Artist's concept of the MAVEN spacecraft, the final Scout mission in NASA's plans. Credit: NASA

Peter Smith, the lead scientist on the Phoenix lander, said it is a challenge to fit a worthwhile Mars mission into Discovery's $425 million cost cap.

"Discovery proposals never have enough funding as you like," Smith said in an interview Tuesday. "It will be tough" to conduct a Mars surface mission on a Discovery budget, he said.

Smith, a researcher at the University of Arizona, is part of a proposal to follow-up on Phoenix, which returned data from the northern polar plains of Mars for six months in 2008.

"If we can make the case on cost, hopefully NASA will consider Mars in the Discovery selection," Smith said.

McCuistion agrees, saying economic realities will likely prohibit Mars Scout from returning again.

"Most of the science is on the surface," McCuistion said. "And it's very difficult, if not impossible, in a Scout-sized budget to do National Academy (of Sciences)-class science on the surface. While we didn't plan it that way, I think it came at the appropriate time."

NASA's next mission to the Martian surface is the Curiority rover, a $2.3 billion mission to determine whether the planet was ever habitable.

After MAVEN, the final Scout mission, launches in late 2013, NASA will partner with the European Space Agency for a methane-sniffing orbiter in 2016 and a nearly $3 billion dual-rover landing mission in 2018.

The joint Mars program was formed with an eye toward a sample return mission in the 2020s.

The audacious missions planned for the next decade will also consume a larger share of NASA's Mars budget, limiting funds for more frequent, less expensive Scout-class missions.

Previous unselected Scout proposals included small impactor-type landers, aerial vehicles and balloons, and orbiters.