Equipment for Positioning the Spacecraft
To communicate with a spacecraft up to 400 million kilometers
(249 million miles) away, engineers must know not just where it is, but in
which direction it is pointing. There are three on-board systems to help:
- two star trackers,
one on each side of the spacecraft, identify the direction in which the
spacecraft is pointing by analyzing the patterns of stars through
small telescopes
- three laser gyros
measure the spacecraft's rotation
- two sun sensors that
allow the spacecraft to orient itself with respect to the sun
Small corrections to the spacecraft's orientation can be achieved by
altering the rotation of spinning reaction wheels attached to the
underside of the spacecraft. They correct any jitter that could disturb
observations when the thrusters are fired and also rotate the spacecraft
slowly as it moves round its orbit so that the instruments or antenna are
kept pointing in the right direction.