Rendezvous and Docking

Imagine playing a game of catch up with an orbiting space station, traveling 17,500 miles per hour and located 200 nautical miles overhead. This is the great challenge of our Shuttle-Mir "rendezvous" or meeting in space, and begins with our launch from the Kennedy Space Center. Mir is in an orbit inclined 51.7 degrees to the equator and at an altitude of over 200 nautical miles. As Mir passes over Florida, Atlantis will be launched on a northeasterly route to align its orbital plane with that of the Mir station. This requirement to be in the same orbital plane as Mir restricts the "launch window" for the shuttle to approximately 10 minutes. A similar time constraint exists for all Shuttle-Mir docking flights, as well as for satellite retrieval/repair missions like the Hubble Space Telescope servicing flights.

Initially, Atlantis will enter an orbit that is behind Mir and at a much lower altitude. We talk about orbits having a high point, or an apogee, and a low point, or perigee. Mir is located in a roughly circular orbit 210 nautical miles above the Earth's surface (apogee and perigee are about the same). Atlantis' lower altitude early in the mission will allow us to catch-up with Mir: we'll have an apogee of 165 nautical miles and a perigee of just 85 nautical miles to begin the flight. Without going into a discussion of orbital mechanics---how spacecraft behave orbiting a planet---let's just say that the lower an orbit you have, the faster you travel around the Earth. A series of on-orbit burns (OMS engine firings and RCS jet firings) will be completed during the first couple of days that will adjust the height of Atlantis’ orbit and therefore control the speed with which Atlantis approaches Mir. The desired end result is to arrive and dock with Mir over Russian ground communication stations, so that they too can monitor the docking.

[Mir Rendezvous Profile]

During the orbital burns early in the flight, final burn solutions are computed by using ground tracking radars to determine the relative position of the two spacecraft. The flight dynamics officer (FDO) in Mission Control-Houston determines the burn duration and burn type, i.e. whether we'll need to use our large OMS engines for a large burn, or our RCS jets for a smaller one. The graph above shows our target, Mir, at the origin and the Shuttle chasing it from below and behind. As the Shuttle loops closer and closer to the target, we'll raise our perigee (low point of the orbit) to match Mir's circular orbit. As Atlantis approaches Mir (within 3 hours of docking), the on-board sensors on the shuttle, both the star tracker and the range radar, are used to gather information to determine relative position of the two spacecraft. This information is then used by the onboard shuttle computers to compute burn solutions.

[Post-Ti Approach to Mir]

A critical point in the rendezvous is the Ti burn, shown in the graph above, which is a one orbit transfer to the target. Following this burn, four small "midcourse correction" burns are performed to fine tune our trajectory to intercept Mir. Approximately 1 hour prior to docking, the commander will begin flying from the aft flight station on Atlantis, and he'll position us to approach Mir on the radius vector or R-Bar. The R-Bar is an imaginary line that extends from Mir to the center of the earth. By approaching the station from below, a natural braking effect takes place, slowing the shuttle’s approach to Mir. This natural braking is an important feature of the R-bar approach, since it allows the Shuttle to approach the station without having to fire thrusters in the direction of Mir in order to slow down. Solar arrays and other sensitive space structures don't do well with repeated pluming from RCS jets.

Jim will fly the final approach visually, using camera views and the Crew Optical Alignment Site or COAS (shown above, as seen on STS-79), positioned in an overhead window of the flight deck. He'll also use data from the radar, a laser tracking device called TCS, and range information from a hand-held laser to guide the approach. This data will be processed on a pair of laptop computers being monitored by Mike and Scott. Vladimir is our hand-held laser man, and Jean-Loup will make sure the whole thing gets filmed and photographed for posterity. Wendy will probably help out with the communications to Mir as we near the station. Atlantis will stop the approach, or "station keep", twice during the final approach. At 170 feet, the crew will check to ensure Mir is in the proper docking attitude and that all systems are go for the docking. At 30 feet, the crew will ensure that the docking port on shuttle is correctly aligned with the docking port on Mir. Once the crew is happy with the alignment of the two spacecraft, the final approach to docking will commence.

The shuttle will approach Mir at a rate of 0.1 feet per second while maintaining a 3 inch approach corridor. When the shuttle and station are within 2 inches of each other, the shuttle will fire its thrusters to increase the approach speed. This ensures that the latching mechanism on the docking systems will engage upon contact. Jean-Loup and Scott will control the docking system, enabling a hard dock with Mir and pressurizing the vestibule between the Shuttle's external airlock and Mir's Docking Module. Once completed, the real work of transfer begins...

At the completion of our docked mission, Pilot Mike Bloomfield will be at the controls on the aft flight deck of Atlantis. He will back the Shuttle down the R-bar, careful to stay within an 8-degree corridor, so that an experimental sensor can acquire data as we separate from Mir (the European Proximity Operations Sensor, which bounces lasers off of reflectors located on Mir's docking module). Mike will stationkeep at 90, 300 and 600 feet away from Mir, each time for 5 minutes, to allow data collection. After the last stationkeeping at 600 feet, Mike will fly Atlantis back towards Mir and a range of about 170 feet. From this point, the crew will initiate a fly-around of the Mir station, to photo-document damage incurred during the Progress-Spektr collision several weeks ago. Once the fly-around is complete, Mike will perform a small separation burn, and orbital mechanics will then take over --- the distance between Atlantis and Mir will grow rapidly with each passing orbit of earth.

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