The Voyager Interstellar Mission sounds like something out of Star Trek, but it is in fact the extended mission of the doughty spacecraft that taught us so much about the outer Solar System. An extended mission can be just as valuable, and sometimes more so, than the original — think about the continuing adventures of our Mars rovers, working well beyond their projected timelines. In Voyager’s case, we’re learning much about how the Solar System behaves as it moves through the interstellar medium, and about the heliopause, where the Sun’s solar winds effectively lose their dominance over the winds from other stars.

Now the Deep Impact spacecraft, which provided such spectacular scenes of Comet Tempel 1, will acquire an extended mission of its own, and in two parts. The one that catches my eye is called Extrasolar Planet Observation and Characterization (EPOCh), which will turn the spacecraft loose on the study of several nearby bright stars already known to have gas giant planets around them. The collected data should tell us more about the planets and their atmospheres as they transit between Deep Impact and the stars they orbit. We may also learn whether some of these planets have moons or rings, with the possibility of finding smaller, hitherto undetected planets in orbit around the same stars.

At the end of 2006, Emily Lakdawalla discussed EPOCh on the Planetary Society Weblog with principal investigator Drake Deming (NASA GSFC), an e-mail conversation that yielded this description of the mission:

“EPOCh will do photometry of giant planets transiting several nearby bright stars (it’s like Kepler in that respect, but Kepler is locked-in to a particular field in Cygnus, and won’t look at nearby bright transiting systems). We exploit the fact that the Deep Impact telescope is out of focus (allows us to get better photometry). We are sensitive to terrestrial planets via their perturbations on the transit times of the giant planets (which we measure very precisely).”

Out of focus? It turns out that Deep Impact’s high-resolution camera does indeed have a flaw, but it’s one that may prove advantageous to the mission, as Deming goes on to explain:

“We are doing very precise photometry, measuring the brightnesses of the stars. As the giant planets pass in front of them (“transit” the stars) we will see the dip in the star’s light. This dip lasts for several hours, but we want to time its occurrence very precisely (to about 1 second accuracy). It’s those changes in the time of the giant planet transits that will indicate the presence of terrestrial planets. That means we have to measure the stellar brightness very accurately, so that the curve of brightness versus time is very “smooth”, i.e. has high signal-to-noise ratio, and we can find the center time accurately. But to get high signal to noise, we have to collect lots of photons from the star. That’s where the defocus helps. Each pixel of the CCD has a limited capacity to collect photons before it saturates. With a defocused image, we have about 75 pixels collecting light for us, so we can collect lots of photons in each exposure without saturating, and that gives us the high signal-to-noise ratio that we need.”

This interesting search will occur as Deep Impact proceeds toward the second goal of its extended mission, a flyby of the comet Boethin planned for December 5, 2008. Both investigations are intriguing ways to stretch existing resources, and they’re complemented by an extended mission for the Stardust spacecraft, which will revisit Comet Tempel 1. Deep Impact’s encounter with Tempel 1 was on July 4, 2005; Stardust, now flying as the New Exploration of Tempel 1 (NExT) mission, is to arrive on February 14, 2011. Encounters with Tempel 1 seem irresistibly drawn to holidays.

When you’ve got a budget as tight as NASA’s, it’s important to find ways to stretch your dollars, and according to the agency’s Alan Stern (who knows something about dollar-stretching through his work as principal investigator for the New Horizons mission), these extended missions will accomplish their work for about 15 percent of the cost of a new mission built from scratch. Thus do two seasoned spacecraft acquire new targets that had previously been unplanned, including an extrasolar study that could prove productive indeed, as EPOCh’s sensitivity should exceed that of existing ground and space-based observatories.

Addendum: Dr. Deming just responded to my note asking about the target list for EPOCh, saying that his team was still evaluating the best candidates. He also said the observations would involve three stars, all relatively nearby, and all, of course, orbited by transiting giant planets.