Three years after Sea Launch spiralled towards bankruptcy following the explosion of its Zenit 3SL rocket, the satellite launch provider is preparing to restart operations.
Both of its land and sea launches use the Zenit rocket. But the more expensive sea-based missions allow a much greater payload - up to 6,100kg (13,440lb) against about 3,600kg - because the sea platform can be positioned exactly on the equator, at a point about 2,250km (1,215nm) south of Hawaii.
At that point the force of the Earth's rotation gives maximum assistance to a rocket, which can lift more payload in exchange for the fuel load it must carry when launched from more northerly or southerly points.
Here's a diagram of how Sea Launch's launch works, as well as an explanation below.