Drag-Free Flying

Drag-free control is a term that was coined from missions in the 1970's. Back then the biggest force on low-orbiting spacecraft was from atmospheric drag. On the LISA Pathfinder spacecraft, the isolated test masses of the LISA Technology Package (LTP) inside the spacecraft are shielded from the atmosphere, so their orbit is unaffected by the drag. When the spacecraft is forced to follow the test masses, it too orbits as if it were not subject to atmospheric drag.

LISA Pathfinder will fly far beyond the atmosphere, so atmospheric drag is not a concern, but other smaller noise forces are. One such "noise" is the fluctuating force from the Sun. However, the technique developed thirty years ago to deal with disturbances on the fringes of Earth's atmosphere helps equally against the gentler disturbances of deep space. For example, when the spacecraft is moving toward the left edge of a test mass (as detected and communicated by the LTP), thrusters will fire to move it in the opposite direction. In this way, the trajectory of the spacecraft is forced to follow that of the test masses.

Disturbance reduction technology shields the floating mass inside the spacecraft from outside forces, thus providing the quiet environment needed to detect passing gravitational waves.

The positions of the spacecraft and the test masses are locked together, but without physical contact between them. Keeping their separation fixed means that the gravitational disturbance from the spacecraft acting on the test masses is also fixed. Fixed separation reduces disturbances to the test masses that would otherwise be caused by spacecraft gravity. Drag-free control also reduces the disturbances from other spacecraft imperfections, such as one side being hotter than the other.