The extraordinary success of NASA’s Deep Space 1 mission creates a familiar dilemma for the U.S. space agency — what do you do with a perfectly functioning spacecraft now that its mission is over?

With money being tight, as always, NASA cannot continue to operate every healthy spacecraft until it either runs out of fuel or otherwise breaks down. The answer to this dilemma is to subject proposals to extend missions such as Deep Impact to the rigors of competition against other science projects.

The results of the $333 million Deep Impact mission were everything the project’s scientists could have hoped for. Even if it ends now it should be considered wildly successful.

After separating from its mother ship, the mission’s Impactor probe performed flawlessly as it slammed into comet Tempel 1 at a speed of 10.2 kilometers per second. Even before striking the comet, the Impactor captured revealing close-up images — some taken just seconds before impact. The mother ship, or Flyby spacecraft, also performed superbly with results that surprised mission scientists as it captured images of a long plume of very powdery material — very different from what had been expected — that spewed from the comet for hours after impact.

That mission, of course, is now over. The Impactor’s job ended at the moment it crashed into Comet Tempel 1.

But the Flyby spacecraft and its sensors are fully functional and clearly capable of examining another comet. Engineers at Ball Aerospace and Technologies Corp., which built both the Impactor and the Flyby spacecraft, had anticipated that the Flyby spacecraft would sustain some damage when it flew through the tail of Tempel 1. But after impact they concluded that no appreciable damage occurred to the spacecraft, its antenna, optics or even a single cell on its solar arrays.

With a fully functioning spacecraft, the Deep Impact science team already has another mission in mind: a visit to Comet 85P/Boethin, which was first spotted in 1975 by the Rev. Leo Boethin, of the Philippines. The comet will make two close approaches to Earth and two close approaches to Jupiter in the next 50 years, with the first of those opportunities coming in 2008.

As an interim step, NASA has authorized the Deep Impact team to nudge the Flyby spacecraft into position for a maneuver that would take it past Earth in early 2008. The Earth swingby would allow the spacecraft to be retargeted toward Comet 85P/Boethin, which it would reach by late 2008 if NASA approves the extended mission.

It could be a very worthwhile endeavor . One of the things scientists learned from Deep Impact is that there are major differences in the nuclei of the comets that have been explored up close by spacecraft so far, including comets Borrelly, Halley, Tempel 1 and Wild 2. Tempel 1 did not conform to the popular “dirty-iceball-with-no-impact-craters” model. In addition to the powdery substance found after impact, the images taken by the Deep Impact mission revealed a cratered surface that looked a lot like an asteroid.

Scientists working on the project anticipate that in order to win funding for an extended mission they will probably have to compete for another round of funding under the agency’s Discovery Program. They should have a distinct advantage because what they propose to do would not require the construction and launch of a new spacecraft.

While it would be great if every spacecraft could be kept operating until it ceased functioning, this clearly is not possible . NASA already faces tough decisions about whether to end the missions of the Hubble Space Telescope, the twin Voyager spacecraft, and others such as Ulysses, Polar, Wind, Geotail, the Fast Auroral Snapshot Explorer and the Transition Region and Coronal Explorer.

Through competition and peer review, NASA could subject proposed mission extensions for healthy spacecraft to the same scientific scrutiny used to pick between candidates for new missions in programs like Discovery.

The Deep Impact mission already has made its mark. If it can compete fairly for another assignment, all the better for science.