Spaceships flying in close formation
Q: Is it possible for two spaceships of different masses to fly in close formation to each
other in Earth orbit? Or does one vehicle have to continually use its thrusters to maintain
position with the other vehicle?
 [NASA] On 13 November 1966, Astronaut Edwin Aldrin, pilot of the Gemini 12 space
flight, took this picture of his spacecraft while standing outside with the hatch open.
A: It depends on what you mean by "close
formation". The spaceships could fly "in
trail", one following the other--as close as
desired. "In that case, no thrusters would
be required to maintain the relative
positions," says Dave Ransom, retired
aerospace engineer and physicist from
Arizona.
In practice, the pilots would occasionally
fire their thrusters to compensate for minute
differences in orbits. Also they must use
their thrusters to compensate for drag on
the spacecraft. The extremely thin
atmosphere at orbital heights increases in
times like now, says Ransom, when solar activity is high and causes Earth's atmosphere to
expand.
On the other hand, one spaceship could fly in a higher orbit than the other. In this case, the
spaceship in the higher orbit must continually use its thrusters.
The higher spaceship must travel at a higher speed than the lower ship since it has farther to go
and, like the end of a pivoting line in a marching band, must go faster to keep up. Once both
craft attain orbit about Earth, they go at constant speeds. So, at first glance, it seems like the
higher ship need only to apply extra thrust when getting into orbit to attain the faster speed.
But that's the rub. It can't attain faster speed at a higher orbit. Orbital mechanics gets in the
way. The faster the orbital velocity for any object orbiting Earth, the lower the orbit, says John
Hodges, British aerospace engineer. That's how Newton's laws of gravity and motion work out.
The only way the higher spaceship can stay even with the lower one, is to continually apply
power in the right direction. But, Ransom points out, the higher-orbit pilot won't actually fire his
thrusters often if his orbit is only tens of feet higher than the other.
Further Surfing:
"Software and Data from David H. Ransom, Jr." Excellent information and links to other
space-related sites
"Orbital Mechanics Calculations for Spacecraft and Satellites", The Orbital Mechanics
Education Network
NASA "Hot Topics" top-notch site
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