Mission controllers at NASA's Jet Propulsion Laboratory in Pasadena, Calif., have relayed final instructions to their comet-bound spacecraft today, Nov. 3. The new programming will guide NASA's EPOXI mission through its close approach with comet Hartley 2, scheduled for tomorrow, Nov. 4, at about 7 a.m. PDT (10 a.m. EDT).
"The last 1 million kilometers are always the hardest," said Tim Larson, EPOXI project manager from JPL. "But we've prepared thoroughly for this day and are confident that come tomorrow morning, we'll be getting the kind of data and imagery that will keep our scientists busy for months to come."
Today at 7 a.m. PDT (10 a.m. EDT), NASA's EPOXI mission was about 1,064,900 kilometers (661,700 miles) from Hartley 2 and closing at a rate of 12.3 kilometers (7.6 miles) per second. Tomorrow at the same time, the spacecraft will be at its closest approach distance to the comet -- approximately 700 kilometers (435 miles) away from its nucleus. All the while, the spacecraft's two imagers and one infrared instrument will be acquiring data.
"We are really looking forward to this because the comet has shown so many surprises, both in the data from EPOXI and the data from our many collaborators, over the last several months," said EPOXI principal investigator Mike A'Hearn from the University of Maryland, College Park."
When the EPOXI mission spacecraft is 18 hours and 798 thousand kilometers (496 thousand miles) away, it will lock its instruments on the comet and begin its encounter phase data collection. As the distances between man-made machine and mysterious space dirtball closes, the frequency of image-taking will increase, reaching a crescendo in the minutes surrounding approach. All data collected during encounter phase will be loaded into spacecraft memory for later playback.
If all goes as planned, about 50 minutes before closest approach, the spacecraft's onboard autopilot – AutoNav Mode – is expected to go active. When in AutoNav Mode, the spacecraft receives attitude (pointing) instructions from its computer to help keep the comet's nucleus centered in spacecraft's imagers.
"We're using AutoNav Mode because our mission control is 23 million miles away from the spacecraft at time of encounter," said Larson. "Any command we would send to the spacecraft would take 75 seconds to get there. Not the kind of thing you want to do when you're talking about hurtling past a 2.2 kilometer-wide object [1.36 miles] at 27,500 miles per hour [about 44,256 kilometers per hour]." AutoNav Mode works by having the spacecraft's Medium-Resolution Imager look for the brightest light source in the sky (excluding the sun). It then assumes that the bright light source must be the comet's nucleus and adjusts the spacecraft's attitude accordingly to keep its imagers centered.
"This spacecraft's AutoNav worked great during its prime mission in 2005 at comet Tempel 1," said Larson. "While that comet's shape provided one central light source, new data from Arecibo indicate that comet Hartley 2 is more elongated and could have two unique bright spots on the ends. If that is the case, we expect Auto Nav to make a decision on which of the two is brightest and focus on that spot."
The EPOXI team expects to begin receiving imagery from the spacecraft starting about 30 minutes after closest approach. The first images received will be those that were taken when the spacecraft was 18 hours out from its target. They will depict the comet nucleus as little more than a point of light, with the fuzzy coma surrounding it. A few of the close-approach images should be received on the ground one hour after the event occurs.
"Those early images may not be the 'money shot,' but we on the science team will prize them just as well, as they will help us further understand the nature of comets," said A'Hearn. "And when we first see those images surrounding closest approach, we are looking forward to yet another type of nucleus compared to any we have seen up close thus far."
EPOXI is an extended mission that uses the already "in-flight" Deep Impact spacecraft to explore distinct celestial targets of opportunity. The name EPOXI itself is a combination of the names for the two extended mission components: the extrasolar planet observations, called Extrasolar Planet Observations and Characterization (EPOCh), and the flyby of comet Hartley 2, called the Deep Impact Extended Investigation (DIXI). The spacecraft has retained the name "Deep Impact."
NASA's Jet Propulsion Laboratory, Pasadena, Calif., a division of the California Institute of Technology, manages the EPOXI mission for NASA's Science Mission Directorate, Washington. The University of Maryland, College Park, is home to the mission's principal investigator, Michael A'Hearn. Drake Deming of NASA's Goddard Space Flight Center, Greenbelt, Md., is the science lead for the mission's extrasolar planet observations. The spacecraft was built for NASA by Ball Aerospace & Technologies Corp., Boulder, Colo.
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