The future may not be empty for a pair of NASA space probes that are wrapping up their primary missions this year, as the agency plans to formally announce the availability of the platforms this fall to gather proposals from the academic and science communities.

An artist's concept shows the Deep Impact projectile hitting the comet while the mothership flies past. Credit: NASA

The Deep Impact and Stardust projects are both part of NASA's Discovery program, which aims to develop high-performance and ambitious missions for lower costs than earlier robotic space explorers. Expenses are capped at several hundred million dollars for prime missions, and teams are urged to be ready to launch within 36 months from the official start of the project.

A formal Announcement of Opportunity is expected to be released this fall, though the date could be pushed into the winter if a Congressional inquiry into last year's solicitation is not completed in time. Details of the inquiry could not be obtained, other than it covered the "guidelines followed and decisions made" in 2004's Announcement of Opportunity, according to Discovery program scientist Susan Niebur.

Proposals will be due 120 days after the announcement is released, and a decision will be made in the months following the receipt, Niebur said.

Missions starting from scratch are to be conducted for a cost to NASA of less than $450 million and must be launched by the end of May 2013. Missions of opportunity - like those possible for Deep Impact and Stardust - are constrained to a cost of $35 million to the government.

"The evaluation criteria will be similar to criteria used in previous Discovery program Announcements of Opportunity, that is, scientific merit, scientific implementation merit and feasibility, and technical merit and feasibility," Niebur said in response to written questions.

"The last criterion will have to include the appropriateness of the proposed investigation for the available hardware and consumables," she said.

Deep Impact's thrusters completed a trajectory correction burn July 20 that put the craft in a storage orbit, followed on August 9 by the placement of the spacecraft into deep sleep by ground controllers. Its course will bring it toward Earth for a speedy fly-by on December 31, 2007.

"This maneuver will keep the spacecraft in the vicinity of the inner planets, thereby making the task of tracking and communicating with it easier," said Andy Dantzler, Director of the Solar System Division of NASA's Science Mission Directorate.

"The spacecraft is being offered as is. Proposers must include mission management and spacecraft operations in the total proposed funding."

Stardust is on the final leg of its trip back to Earth after streaking past comet Wild 2 in early 2004. During the high-speed encounter, Stardust collected microscopic samples of the comet's dusty environment to be returned to Earth in a small capsule for study by scientists in laboratories on the ground. Its sensitive aerogel collector plates were also exposed to space for several months en route to Wild 2 to retrieve interplanetary dust samples.

An illustration of the Stardust spacecraft. Credit: NASA

The material will parachute to the ground in Utah on January 15 of next year, while the main spacecraft will continue in its solar orbit after releasing the return capsule. A divert burn will allow Stardust to miss Earth, and officials estimate around 44 pounds of propellant will remain inside its on-board reservoirs.

"Proposals to reuse the Stardust spacecraft are also welcome," Niebur said. "It is anticipated that the consumables available on Stardust will make this more challenging, however, NASA is willing to consider proposals that are able to propose within the stated constraints."

Stardust has been in space - and thus using propellant reserves - since 1999 while covering a total distance of over three billion miles, while Deep Impact was recently launched in January for the comparatively short trip to Tempel 1.

The propulsion system aboard Stardust is relatively modest compared to many deep space missions. The monopropellant design uses just hydrazine given the low-energy trajectory Stardust's primary mission took. Deep Impact's thrusters are also fueled by hydrazine and were designed to provide for a mission total of just over 600 feet per second of velocity change.

Estimates indicate that about 143 pounds of Stardust's hydrazine fuel will have been used by January, leaving about a quarter of the craft's original lot of propellant left in the tank. Almost 35 pounds of fuel currently remains aboard Deep Impact - enough for a change in velocity of around 213 feet per second.

Stardust carries a small group of instruments that includes a cometary and interstellar dust analyzer, a dust flux monitoring device that detects particles around the craft, and an optical navigation camera.

"Deep Impact and Stardust were engineered first and foremost to be able to achieve their prime missions, interaction with comets Tempel 1 and Wild 2. Although these primary missions have been the top priority, each has some capacity remaining that we are willing to make available to the space science community," explained Niebur.

Other missions under the Discovery program umbrella have included the NEAR-Shoemaker asteroid orbiter, 1997's historic Mars Pathfinder mission, the Lunar Prospector, the Genesis solar wind catcher, the failed CONTOUR cometary mission, and the MESSENGER probe currently en route to orbit the planet Mercury. Two more flights are on the books for launch next year and in 2008.

Deep Impact achieved its objective on July 4, when a 816-pound copper-laden impactor earlier released from a mothership smacked into comet Tempel 1 at over six miles per second.

Though the collision destroyed the impactor, the mothership completed a close fly-by of the comet to observe the affects of the impact in great scientific detail. Its high-resolution camera captured impressive images of the impact and the debris ejected away from the comet's surface. The instrument contains an 11.8-inch aperture telescope, an infrared spectrometer, and a multi-spectral camera. The fly-by craft also carries a medium-resolution camera as a backup.

Ball Aerospace and Technologies Corporation of Colorado, the prime contractor for Deep Impact, built the instruments.

The high-resolution camera traces its heritage to work done by Ball on the Wide Field Camera 3, which was developed for the Hubble Space Telescope but is stranded on the ground due to the questionable possibility of a future space shuttle servicing mission to Hubble. Deep Impact's imager is sensitive enough to resolve a car from a distance equal to the width of the state of Colorado.

About the size of a typical sports utility vehicle - and weighing 1,430 pounds at launch - the fly-by spacecraft could use its optical and science suite to study other valuable targets in the solar system.

One such possible destination is Boethin, which is a Jupiter family comet that orbits the Sun about every 11 years. Deep Impact could use its unique cameras and spectrometer as it flies past to add its characteristics to the ever-growing database of closely observed comets. The Earth fly-by in late 2007 could be used as a gravity assist maneuver to slingshot Deep Impact toward Boethin by around the end of 2008.

The mission extension to comet Boethin comes from a proposal being planned by the University of Maryland, which was the institution responsible for science operations and analysis for this summer's impact at Tempel 1.

Scientists have also long been advocating a similar mission to create craters on an asteroid, dubbed Deep Interior.

"It would be helpful to have a similar type of mission go to an asteroid and compare the results," said Deep Impact Co-investigator Lucy McFadden of the University of Maryland. "That mission is Deep Interior. It has been proposed in the past to NASA's Discovery program, but has not been accepted."

Deep Interior would use radio reflection data to determine the structure of the asteroid, which scientists have long hypothesized as widely varying from dense rock to "rubble piles" with potential internal voids. It could also carry with it explosives to be released on the asteroid to blast 100-foot craters in the surface for further studies. The spacecraft could visit several near-Earth objects during its mission.

"Would we want to do the same thing to a different type of comet? I don't want to answer that question until we have analyzed the results from this comet. It depends on what picture we piece together from observing beneath this comet's surface," McFadden said.

Comets and asteroids are on the radar screen of scientists because they are ancient relics left over from the earliest period of the formation of the solar system. It is believed that material remaining from the early solar system could still be harbored inside the icy comets, while asteroids - essentially mini-planets - give researchers clues about how planets formed.