Spacesuit goes overboard for unusual mission

Safety concerns
Dropping SuitSat off the space station isn't as simple as it sounds. The esoteric rules of orbital motion demand that it follow a specific path as it departs from the station. Otherwise, the risk is that it could boomerang back and hit the space station, as a tool dropped during a spacewalk nearly did in 2001.

NASA officials have explained that the SuitSat will be manually shoved directly backwards relative to the station’s immense orbital motion. The two spacewalkers will be standing near the Russian air lock that extends down (toward Earth) from the Russian pressurized modules at one end of the station.

The exact speed at which SuitSat departs will depend on how exactly it is launched, as well as whether it stays clear of the Russian modules as it skims below them. But it probably will be moving between 1 and 2 feet per second. Once it is about 500 to 1,000 feet (150 to 300 meters) out, SuitSat will slow its departure rate, sink into a slightly lower orbit — and begin heading back toward the station.

Fortunately, since SuitSat was thrown into an orbit that has an average speed less than that of the station (by the amount of push the men were able to impart to it), it follows Newton’s Laws and orbits at a slightly lower altitude. This guarantees a miss, but it will still look like a fairly dramatic close call.

Within about another 10 minutes, it will pass beneath the station at a range of 2,000 to 3,000 feet (600 to 900 meters). As it slips downhill into its new orbit, SuitSat will pick up speed and will seem to be traveling in the opposite direction that it originally left the space station.

The only ones likely to see it during this phase are the spacewalkers. The small spacesuit is too close to the giant space station for ground radar to notice it (there is no radar on board the station), and ground observers won’t be able to spot it until it approaches sunset and the ground below is in darkness.

After one 92-minute orbit, SuitSat should wind up between 3 to 5 miles (5 and 8 kilometers) ahead of the space station and continue to move ahead at that rate, according to a NASA trajectory officer. After a full day of free flight, Suitsat will thus be at least 50 miles (80 kilometers) ahead of the station, or about 10 seconds at the speed the objects are orbiting.

SuitSat will never return to the vicinity of the station.  While pulling ahead hour by hour, the separation rate will increase as air drag (which affects the light suit much more than the massive station) drops the suit into an even lower orbit that in turn makes it speed up and pull away even more quickly.

A tricky throw
This separation profile may seem scary, but it’s safer than any alternative. The laws of orbital motion have logical — but unearthly — consequences for departures in any other direction.

Just throwing it straight down, for example, is among the worst choices. Since the speed is imparted crosswise to the object’s original forward motion, it makes practically no change in the total speed — so the resulting orbit will be uncomfortably parallel to the station’s.

With an impulse of 1 foot per second, the jettisoned object would drop for a few minutes and then begin pulling ahead. But with its absolute speed undiminished, it would then begin to coast uphill, and soon would be crossing the station’s orbit, about a mile in front. It would continue to rise, slip backward, then begin falling again — and smash back into the station with the same speed but from the opposite direction it was thrown at.

Throwing straight up has the same results but with a mirror image. And throwing sideways results in the object boomeranging back in half the time the up-down jettison causes.

Throwing it forward results in a higher, slower orbit that sees the object pass overhead and disappear to the rear. But then the long-term effects of air drag cause it to slip into a lower, faster orbit and it begins to overtake the station, threatening to impact with many times the speed it departed with.

So when SuitSat takes its walk into space, there is only one safe path for the space crew to strive for, since they don’t ever want to see it come back. Like the jettisoned spacesuits from Mir, SuitSat will be aimed straight backward. Them’s the rules of the road in Earth orbit.

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