Backdropped against the blackness of space, the STS-122 external fuel tank (ET) begins its relative separation from the space shuttle Atlantis. The fan-shaped bright area is the result of ET venting after orbiter separation. What happens in this nominal occurrence is that the residual cryogenics in the tank heat up to some extent and the pressure increases, popping the relief valve. The residual gases spray out of the tank and are quite noticeable with the light reflection.
Image Credit: NASA