Worried advocates of Mars exploration are waiting for the other shoe to drop in the wake of NASA’s cancellation of the Mars Telecommunications Orbiter — and with good reason. The same budgetary pressures that drove that decision likely will force agency officials to seriously consider postponing a nuclear-powered Mars rover mission that was supposed to rely on the telecom orbiter to relay back to Earth the large, steady stream of data it would produce.

NASA faces a lot of tough money decisions that are certain to mean delays or outright cancellations affecting worthwhile programs. Given some of the other priorities on its plate, NASA might decide that it cannot afford full funding of the Mars Science Laboratory right now and stretch out the program to put its launch off for a few more years. Those priorities — re-fixing the space shuttle, re-furbishing the Hubble Space Telescope, completing the international space station and gearing up for human missions to the Moon by 2018 — are going to make it extremely difficult for NASA to pay for everything that different constituencies would like to see funded in aeronautics, astronomy, Earth science and exploration of other planets.

Under such circumstances, NASA had little choice but to cancel the Mars Telecommunications Orbiter. With the missions that absolutely had to have it for data relay now on hold — sample return missions for example — pressing ahead with the $500 million spacecraft could not be justified.

Should NASA proceed with the Mars Science Laboratory, currently scheduled to launch in December 2009 and arrive at the red planet the following October, plans that call for using the Mars Reconnaissance Orbiter as the data-relay craft. The orbiter is slated to launch Aug. 10 and begin science operations in November 2006.

That plan is perfectly sound, but NASA’s back-up idea is anything but.

If the Mars Reconnaissance Orbiter is for any reason unavailable for the Mars Science Laboratory, NASA would rely on the Mars Global Surveyor and Mars Odyssey orbiters, which were launched in 1996 and 2001, respectively. NASA officials insist that both spacecraft have sufficient fuel to continue operating into early next decade, but to count on them with the science return from a billion-dollar rover at stake seems shaky at best, reckless at worst.

If the Mars Reconnaissance Orbiter launch goes smoothly, putting a healthy spacecraft on a good trajectory, NASA could continue work on the Mars Science Laboratory at the current pace. But things can go wrong at any time, and the orbiter faces another critical juncture upon arrival at Mars in the form of an aerobraking maneuver that will put it into martian orbit. NASA therefore would be wise not to go full-throttle on rover development until the relay craft is safely in orbit around Mars.

If the Mars Reconnaissance Orbiter is lost or encounters serious problems, NASA probably would have little choice but to delay the Mars Science Laboratory. Relying on the Mars Global Surveyor or Mars Odyssey for telecommunications is too risky, and building an alternative relay probably is not in the budgetary cards.

The larger question is whether NASA should decide now to stretch out the rover mission. NASA has major bills coming due between now and the space shuttle’s planned retirement in 2010, a period that coincides with the prime spending years for the Mars Science Laboratory.

First there is the space shuttle. With the U.S. Congress still having not seen the full bill for returning the shuttle fleet to flight following the 2003 Columbia accident, NASA is facing a new investigation and possible redesign work to eliminate the foam-shedding problem that cropped up again during Space Shuttle Discovery’s July 26 liftoff. And NASA still does not have a handle on how much more costly the shuttle will be to operate in the post-Columbia environment.

Then, of course, there is the hardware needed to return astronauts to the Moon. NASA expects to spend $10 billion developing the Crew Exploration Vehicle and the shuttle-derived rocket it would launch atop and wants both systems ready for space station missions by 2011. A heavy-lift rocket is not needed until later, but is expected to cost at least $5 billion to develop, not including long-overdue infrastructure investments needed at NASA’s Kennedy Space Center, Fla.

Unless NASA drops all other science activities for the next five years, or gets a huge budget increase, it is difficult to see how the agency can squeeze in the Mars Science Laboratory during that time frame given its estimated $900 million price tag.

Deferring the launch would by no means constitute an abandonment of Mars exploration. Despite some high-profile setbacks, NASA has had a robust program over the past decade, and this will continue with the Mars Reconnaissance Orbiter, the 2007 Phoenix Mars Scout lander and a yet-to-be-selected Mars Scout mission that would launch in 2011. Although none is as tantalizing to scientists as a two-year mission of a high-power rover, they will continue to add to the growing knowledge base of Mars.

Human exploration of the red planet is a long way off. The Moon is the next destination for human explorers and activities that support that goal, including building a new crew transport and launching robotic lunar precursor missions, should take precedence over a flagship-class Mars mission.

If NASA can somehow handle the Mars Science Laboratory along with everything else it is trying to do over the next five years, that’s great. But if something must be scaled back during this crucial period, the Mars Science Laboratory is a logical choice.