Deep Impact Spacecraft Heads For Comet Hartley 2

The EPOXI mission is actually two new missions in 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, who is principal investigator (PI) for both the overall EPOXI mission and its DIXI component.

The EPOXI mission brings back the Deep Impact partnership between the University of Maryland, NASA's Jet Propulsion Laboratory (JPL) and Ball Aerospace & Technologies Corporation, and adds NASA's Goddard Space Flight Center.

Daughters of Deep Impact

On July 4th 2005, the University of Maryland-led NASA mission Deep Impact made history and world-wide headlines when it smashed a probe into Comet Tempel. The mission yielded a wealth of new cometary information, but the data on Tempel 1 was in many cases startlingly different from that from comet missions Deep Space 1 and Stardust. As a result, rather than revealing the true nature of comets, 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. "We want a close look at Hartley 2 to see if the surprises of Tempel 1 are more common than we thought, or if Tempel 1 really is unusual."

After the completion of Deep Impact, the mission team knew they had a still healthy and flight-proven spacecraft that was capable of traveling to a never-before-visited comet at a fraction of the cost of a newly built and launched mission. In 2006 the A'Hearn-led team began the proposal process that eventually became EPOXI.

Trajectory of a dual mission

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 journey's EPOCh leg

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. Their gravity will pull on the transit planets, altering their orbits and the timing of their transits.

"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," said Deming.

Are we there yet?

In June of 2008, the extended mission will end its EPOCh portion and transition to a long, quiet journey to comet Hartley 2. The total trip -- measured from its December 31, 2007 flyby of Earth to its closest encounter with the comet on October 11, 2010 -- will be roughly 1.6 billion miles or some 18 times the distance from the Earth to the sun. It will take the spacecraft three trips around the sun before it can intercept the comet, which at that time will be at a distance of some 12.4 million miles from Earth.

At the nearest point of its flyby of Hartley 2, the spacecraft will be some 550 miles from the comet. Deep Impact does not have another probe, so Hartley 2 will not get hit, but the close-up view will allow the spacecraft's two telescopes to closely observe surface features of the comet while its infrared spectrometer maps the composition of any outbursts of gas from the surface.

Comet science goals for this phase of the mission are to:

• Search for and, if found, produce maps of outbursts of gas from the surface of comet Hartley 2. Track the outburst as the comet rotates. Correlate outbursts with surface features. Such outbursts were observed during the spacecraft's flyby of comet Tempel 1.
• Obtain infrared spectral maps of gasses in the innermost portion of the coma. The coma is the cloud of gas and dust that surrounds the comet. Investigate the distribution of dust and gas in the coma.
• Search for frozen volatiles (SUCH AS?) on the surface of the comet. Water ice, for example, was discovered when the flyby explored Tempel 1.
• Produce broad band images of the comet that will establish limits on the size of the nucleus. Produce a model of its shape.
• Map the brightness and color variations of the surface. Locate landscape features that indicate the processes by which the comet was formed. Compare the distribution of crater sizes with the distribution of the size of craters on other comets, asteroids and planetary satellites.
• Map the temperature of the surface to assess how readily heat is transmitted to the interior and the flow of subsurface volatiles, such as water vapor, to the surface.

For A'Hearn and his DIXI team the most rewarding time will come after the flyby, as they turn the raw data into new insights on the structure and formation of comets and their place in the history of our solar system.