Space station sinks to new low — but it’s OK

How the TOPO team does its job
Halfway through the movie "Star Trek II: The Wrath of Khan," Captain Kirk and his crew are groping their way through a space dogfight in a foggy nebula. Science Officer Spock assesses the tactics of their opponent: “He is intelligent but inexperienced — his maneuvers indicate he thinks in two dimensions.” Kirk uses that insight to outfox his foe.

Space is indeed three-dimensional, and navigating through it requires unearthly mental attitudes. For real spaceflight, there are specialized teams in Mission Control in Houston and in Moscow’s equivalent facility. On behalf of the spaceships they are steering, they think, plan and maneuver in three dimensions.

Ainsley Collins, manager of NASA’s team of Trajectory Operations Officers in Houston, recently talked with MSNBC.com about how her group does its job.

• Pick an orbit, but not any orbit: Conflicting factors argue for higher or lower altitudes within a broad range. Too low, and air drag inflicts too strong a force, decaying the orbit much faster than reboost maneuvers can compensate and twisting the station out of its preferred orientation by aerodynamic torques on the vast solar power panels. Too high, and the station skims the lower reaches of the Van Allen radiation belts, especially in the region over the South Atlantic where the zone dips closest to Earth and can expose the crew and equipment to damaging radiation levels during solar flares.

Collins said payload weight is the limiting factor for the next space shuttle flight to the station, scheduled for launch no earlier than April 21. The amount of payload a shuttle can carry diminishes as the destination orbit grows. The rule of thumb is about 100 pounds for every mile: Thus, if a payload of 30,000 pounds can be carried to 240 miles, a payload of 33,000 pounds can be carried to 210 miles — a 10 percent bonus. This year and next, that bonus makes all the difference in designing mission (and station) orbits.

Flying this low has a price: it exposes the station to thicker atmosphere and higher air drag. This is undesirable — especially since current missions are deploying a series of bigger and bigger solar panels, making the station "draggier," Collins said. Fortunately, luck is on NASA's side: The fringes of the atmosphere are thinner when the sun is in the quiet phase of its 11-year activity cycle, reducing the drag effect.

• Think in three dimensions: If the timing and power of each rocket burn were merely a question of altitude — an “up-down” sort of selection — the scheduling problem would be straightforward. But space is three-dimensional, and so are the tightly woven strategic choices facing the station orbit’s managers.

Trajectory planners have to figure out how much an orbital target (the space station) leads the spacecraft that's chasing it (the space shuttle). The shuttle must blast off during the few moments that Earth carries the launch pad through the orbital path (the “plane”) of the target’s orbit. But when that occurs, the target might be anywhere along its track — just ahead, perhaps, or halfway around the planet, or almost catching up from behind.

Depending on how far ahead its target is, a just-launched chasing spaceship maneuvers into a lower orbit (to overtake more quickly) or a higher orbit (to do so more slowly) and thus forces the actual docking time to occur during a desired interval. This may look like a "forward-backward" challenge, but the up-or-down question plays a role in working out the timing.

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Raising or lowering the target’s orbit places it in different relative positions at the moment the chaser spaceship is launched. If you can put the target at an altitude that repeats the same path every two days, planning is much easier. The station is in just such an orbit now, Collins said.

Mission Control must also plan where the station — or more accurately, where a spaceship that is returning to Earth from the station — is flying over the ground. This is the “left-right” challenge, the third dimension to the design problem.

The station cannot itself swerve from side to side – that’s too expensive in terms of fuel. But it can let Earth do the swerving through the expedient of hastening or delaying its arrival over a given spot. This lets Earth’s rotation place the desired ground point directly under the chosen orbital path.

The TOPO designers modify the arrival time by changing the station’s altitude and hence its orbital period. They control all three dimensions by the same reboost maneuver, through varying its exact execution time and force. Making all three factors line up is the “Rubik’s Cube” of orbital flight – especially when considering that the planned burn might (as it sometimes does) not be successfully achieved.

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