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November 16, 2008

Docking with a Star

Astronauts make it look so easy. All the shuttle has to do is loop around in big circles on that orbital "racetrack" until they "catch up" with the International Space Station (ISS). Then they do a fancy dance including a few flips for the ISS cameras (to photograph the shuttle's belly), and let their airlocks kiss each other hello. This process is called rendezvous and docking.

But it is anything but simple or easy--especially when the space shuttle's Ku-band radar system is on the fritz. The antenna is having trouble staying locked onto the tracking data relay satellites that handle high-speed communications and two-way television. The ground can take over some of the functions, but the system may not be reliable for use during rendezvous. A work-around procedure using small telescopes called "star trackers" was used once before (on STS-92 in 2000) when there was a radar failure, and the astronauts are trained to use this procedure.

Using the star trackers for this tricky rendezvous is the equivalent of an airplane pilot finding Houston at night using the North Star and the Moon to find the direction, radio with the airport tower to decide when to descend, and then looking out the window to spot the runway lights.

However, the ISS does not hold still like an airport, and does not have any "runway" lights. It is zipping around the Earth at about 17,000 mph. And though NASA has called the ISS "a new star on the horizon" in promotional literature, it is obviously not a star. Like the Moon, it shines by reflected sunlight. (This is why those of us on the Earth's surface can only see it pass overhead when it is near sunrise or sunset and the arrays reflect the light).

The shuttle's star trackers are designed to look for stars. They use triangulation to determine what direction the orbiter is pointing. The plan is to use the star trackers to "sight" on the ISS (and some other guide stars) to determine its direction. This sighting will obviously have to occur when the ISS is in sunlight and at the right angle for the shuttle to see it. But a direct look at the sun will blind the star trackers, and the orbiter must be maneuvered so one of the telescopes is looking at a very tiny spot in space.

The orbiter will have to execute a very precise maneuver to get into the right "attitude" for the star tracker to pick up the ISS "star." The position in Mission Control in charge of that tricky little calculation is called "Pointing." They work in the support room of the Flight Activities Officer (a position I held during the early shuttle program).

But the star trackers don't provide any distance or speed information. It is almost impossible to judge speed and distances in space by looking out the window because there is no horizon or stable landmark. (I think the astronauts have a hand-held device they can use once they are close enough to see the ISS.) Fortunately, Mission Control can track both vehicles very precisely and provide additional navigational help.

When I stop and think about all the calculations and communications and piloting requirements for a successful rendezvous in space, I can't help but be a little in awe of the astronauts and ground team who make it look so easy to dock with a star.

Marianne Dyson
NSS member

Posted by m_dyson at November 16, 2008 02:17 PM



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