LAUREL, Md. — Scientists are warning of a looming crisis for small-scale planetary exploration missions, with stagnant budgets and fixed costs expected to endure for the foreseeable future.

“This is potentially a time of crisis for planetary exploration,” said John Sommerer, head of the space department at the Johns Hopkins University Applied Physics Laboratory (APL) here. “We have tremendous budgetary pressures in all the space-faring nations.”

He spoke June 21 at APL’s Low-Cost Planetary Missions Conference.

The budget plateau at NASA has scientists competing more fiercely to launch small-scale missions, like those funded by the agency’s Discovery program, even as the cost  of space access is climbing.

John McCarthy, vice president of strategic development for Orbital Science Corp.’s science and technology satellites business, said that a diminishing pool of launch vehicles and rising component costs amount to a near-term “crisis in the planetary science community.”

McCarthy cited the impending retirement of United Launch Alliance’s Delta 2 rocket and the end of the space shuttle program as the primary drivers of pricier launches for science payloads.

While NASA has launched nearly 60 percent of its science missions since 1998 on the Delta 2, United Launch Alliance stopped making the rocket after the Air Force shifted the bulk of its operational satellites to the company’s larger Atlas 5 and Delta 4 rockets.

NASA has launched five spacecraft on the Atlas 5 since 2005 and plans to continue using the rocket at a rate of two launches per year. But with prices projected to jump as much as $100 million after the shuttle program ends, the Atlas 5 is priced out of reach of many of NASA’s smaller-budget science missions.

Space Exploration Technologies’ more affordable Falcon 9 rocket was added to the NASA Launch Services-2 contracting vehicle last September, but the medium-lift rocket is still a couple years away from being certified to launch NASA science satellites.

Orbital also aims to launch NASA satellites that traditionally would have launched on Delta 2. But the Dulles, Va.-based company’s Delta 2-class rocket, the Taurus 2, is still at least several months away from making its debut.

At the APL conference, McCarthy said that launch costs for Taurus 2 would range from $75 million to $95 million, depending on which variation of the rocket the customer wants. Orbital plans to make three versions of the Taurus 2 in the coming years. The first Taurus 2 is slated to launch on its maiden flight in early October.

Successively larger versions of Taurus 2 are scheduled to be introduced in 2013 and 2014.

McCarthy said June 21 that the date for Taurus 2’s inaugural flight has not been changed, despite the premature shutdown of an AJ-26 engine during an acceptance test at Stennis Space Center in Mississippi on June 9.

The AJ-26, a liquid-oxygen/kerosene engine based on the Russian NK-33, is the main engine for Taurus 2. Each rocket uses a pair of the engines.

“We’ll see how this turns out,” McCarthy said of the aborted engine test. “We all know that test failures have a way of putting pressure on schedules.”

Meanwhile, a NASA official, discussing the agency’s Discovery Program, hammered assembled scientists at the APL meeting with a blunt reminder about the opportunity cost of doing space science within a Discovery paradigm.

The Discovery Program’s “requirements are intentionally created to constrain your hopes and dreams,” said Curt Niebur, a program scientist at NASA headquarters in Washington. “Ambitions can become very costly.”

Discovery was started in 1992 as a way for scientists and engineers to complement NASA’s enormous flagship science missions with smaller, competitively selected, cost-capped missions.

NASA began the Discovery program with a meeting in San Juan Capistrano, Calif., in 1992. The meeting attracted 73 mission proposals, of which 14 were selected for further study.

The meeting gave NASA a deep reservoir of possible Discovery missions, and the space agency is still drawing down that supply, Niebur said. Of the 11 proposals so far selected to become Discovery missions, nine have flown successfully, one was lost in Earth orbit after launch, and another is awaiting launch.

In May, NASA selected three finalists for a 2016 launch opportunity. Two of those missions want to use a plutonium-fueled power system provided by NASA at no cost to the mission.

Dan Leone is a SpaceNews staff writer, covering NASA, NOAA and a growing number of entrepreneurial space companies. He earned a bachelor’s degree in public communications from the American University in Washington.