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Encountering a Comet
by Kevin Gilliland, Attitude Control Subsystems, LMSS

The year began with Stardust-NExT spacecraft flying by Earth in a maneuver called a "gravity assist." On January 15, the spacecraft came within 9,200 km (5,700 miles) of California as it flew by. Many interplanetary missions use an Earth gravity assist, or EGA, to change their orbit by using the Earth's gravity to accelerate the spacecraft. Our January flyby was designed to set Stardust-NExT on course to a Tempel-1 encounter in February 2011.

Changing the orbit using EGA saves fuel, since it's the Earth's gravity, not thruster firing, that changes our course. To get the desired change, however, requires a precise path by Earth. The mission operations team had to plan for every scenario that could put us off course, and execute required corrections. Months of planning and testing were required to ensure that the flyby was successful. The spacecraft would be safe, but, if the spacecraft were off course as it passed, it would not reach the comet. All went according to plan, and our Earth flyby set us on our way to comet Tempel 1. Our focus now is on the encounter in February 2011. 

Red/green stereo anaglyph of Comet Wild 2  
Red/green stereo anaglyph of Comet Wild 2. Credit: NASA  

The spacecraft team faces challenges we didn't have in 2004 for our Wild-2 encounter. One challenge on this trip is fuel. The spacecraft was launched with more fuel than we expected to need, so, we did not have to closely monitor daily usage.  Now, more than three years beyond the original mission and after billions of miles of space travel, we must find ways to conserve. Small thruster firings are used to keep the spacecraft's antenna pointed to Earth, and every thruster firing uses fuel. We have been modeling and testing to improve the efficiency of the thruster control, to react more slowly and fire less frequently while still pointing as required. After we had completed testing, we sent commands to the spacecraft to modify the spacecraft thruster controller.

Another challenge will be improving the performance of our trajectory correction maneuvers (TCMs).  As with our fuel conservation, we've had to improve the way we execute these correction burns. The most significant correction of the mission is coming early next year, when we will adjust our arrival time. We will command a burn to either speed up or slow down our orbit to time our arrival when we will have the best view of the comet nucleus. Like a planet, the Tempel-1 nucleus rotates, and we'd like to time our closest approach for the most interesting features on the surface.

Imagine driving from LA to Times Square to watch the ball drop on New Year's Eve. If, in Kansas, you realize you'll be an hour late, a small change in speed, say 2 miles per hour, will get you back on schedule. If, however, you were only 10 miles away, and an hour late, you would be too close to make a correction. So, about one year before we reach
Tempel-1, we will execute a TCM to adjust our arrival time. Firing the thrusters in the direction we're flying will speed us up to arrive earlier. Firing in the opposite direction will slow us down. TCMs are most efficient at a distance (like the speed adjustment in Kansas), although the burn must be precise. Our limited fuel requires us to make a few very precise and very efficient adjustments. Since the new mission began, we've been able to improve TCMs to give exactly the required change.

Artist rendition of Stardust-NExT spacecraft approaching comet  
Artist rendition of Stardust-NExT spacecraft approaching comet  

Another challenge is the size of the team. Our Tempel-1 Encounter mission operations team will be about 10 engineers. By comparison, the 2004 Wild-2 Encounter team was about three times as many people. Most of the engineers also work on other projects during down time. One of the challenges in preparing for Tempel-1 is working together efficiently, with each of us busy with other tasks. Aside from our regular formal meetings, spending a few minutes with another engineer to stay on top of the next activity is very important.

As the encounter draws closer, members of the science and flight team will conduct a series of spacecraft tests to ensure we are ready for a successful encounter.