"Deep Impact" Will Scout for Earthlike Planets

"Since Deep Impact will be able to stare at these stars for long periods, we can observe multiple transits and compare the timing to see if there are any hidden worlds."

~ Drake Deming, Goddard scientist

NASA has given the green light to fly the Deep Impact spacecraft to Comet Hartley 2 on a two-part extended mission known as EPOXI. The spacecraft will fly by Earth on New Year's Eve at the beginning of a more than two-and-a-half-year journey to the comet.

The EPOXI mission is actually two new missions rolled into one. During the first six months of the journey to Hartley 2, the Extrasolar Planet Observations and Characterization (EPOCh) mission will use the larger of the two telescopes on the Deep Impact spacecraft to search for Earth-sized planets around five stars selected as likely candidates for such planets. Upon arriving at the comet the Deep Impact eXtended Investigation (DIXI) will conduct an extended flyby of Hartley 2 using all three of the spacecraft's instruments (two telescopes with digital color cameras and an infrared spectrometer).

"It's exciting that we can send the Deep Impact spacecraft on a new mission that combines two totally independent science investigations, both of which can help us better understand how solar systems form and evolve," said Deep Impact leader and University of Maryland astronomer Michael A'Hearn.

Previous comet mission have not revealed the true nature of comets as hoped. In fact, the sometimes conflicting data from these three missions has left scientists questioning most of what they thought they knew about these fascinating, and potentially dangerous, objects; and longing for new data from other comets.

"One of the great surprises of comet explorations has been the wide diversity among the different cometary surfaces imaged to date," said A'Hearn.

When the Deep Impact/EPOXI spacecraft passes by Earth on December 31, 2007, it will use the pull of our planet's gravity to direct and speed itself toward comet Hartley 2. In doing this, the spacecraft is aimed toward an encounter with comet Hartley 2 at a time when tracking stations in two different locations on Earth can "see" the spacecraft to receive data from it and send commands to it. In late December 2007, the spacecraft's instruments will be recalibrated using the Moon as a target.

Hartley 2 was not the original destination of the new mission. It was selected in October following the surprising realization that despite tremendous efforts by many observatories and observers, the scientists could not reliably identify their first choice, comet Boethin, and its orbit in time to plan the mission flyby of Earth. The team then recommended to NASA that it be allowed to fly to the backup target, comet Hartley 2.

"Hartley 2 is scientifically just as interesting as comet Boethin since both have relatively small, active nuclei," said A'Hearn. "As we have worked the details of the comet Hartley 2 encounter, we are confident that the observations will turn out to be even better than Boethin."

The first part of the Deep Impact extended mission -- the search for alien worlds -- will begin in late January as the spacecraft cruises toward Hartley 2. More than 200 alien (extrasolar) planets have been discovered to date. Most of these are detected indirectly, by the gravitational pull they exert on their parent star. Directly observing extrasolar planets by detecting the light reflected from them is very difficult, because a star's brilliance obscures light coming from any planets orbiting it. However, sometimes the orbit of an extrasolar world is aligned so that it eclipses its star as seen from Earth. In these rare cases, light from that planet can be seen directly.

"When the planet appears next to its star, your telescope captures their combined light. When the planet passes behind its star, your telescope only sees light from the star. By subtracting light from just the star from the combined light, you are left with light from the planet,” said Goddard scientist Drake Deming, who heads EPOCh and is deputy principal investigator for EPOXI. "We can analyze this light to discover what the atmospheres of these planets are like."

Planets as small as three Earth masses can be detected in this way. EPOCh will also observe the Earth in visible and infrared wavelengths to allow comparisons with future discoveries of Earth-like planets around other stars.

The mission will observe five nearby stars with "transiting extrasolar planets," so named because the planet transits, or passes in front of, its star. The planets were discovered earlier and are giant planets with massive atmospheres, like Jupiter in our solar system. They orbit their stars much closer than Earth does the sun, so they are hot and belong to the class of extrasolar planets nicknamed "Hot Jupiters."

However, these giant planets may not be alone. If there are other worlds around these stars, they might also transit the star and be discovered by the spacecraft. Even if they don't transit, Deep Impact could find them indirectly. The team is hopeful that this exciting two-fold mission could identify other habitable worlds besides Earth.

Posted by Rebecca Sato

*Adapted from University of Maryland, College Park news release.