The spacecraft system provides the support structure and
environment necessary for the telescope and the science
instruments to work as an observatory.
Thrusters
In order to provide motion to the observatory, Chandra has two
different sets of thrusters: one for propulsion and the other
for momentum unloading. The propulsion thrusters were used
immediately after launch to help propel Chandra into its final
orbit, which is elliptical and very high in altitude.
The
momentum unloading thrusters are periodically used to apply
torques to Chandra and, thereby, lower the accumulated momentum
in its reaction wheels, which are used to control Chandra's
attitude.
To control the temperatures of critical components, Chandra's
thermal control system consists of a cooling radiator,
insulators, heaters and thermostats. It is particularly
important that the temperature near the X-ray mirrors be well
controlled to keep the mirror in focus. The temperature in many
parts of the spacecraft is continually monitored and reported
back to mission control.
Chandra's electrical power comes from its solar arrays. This
energy is then stored in three banks of batteries and
distributed in a carefully regulated manner to the Observatory
by the electrical power system. The solar arrays generate
approximately two kilowatts of power for the heaters, science
instruments, computers, transmitters, etc.
Locking On and Staying Steady
Sunshade Door
Aspect Camera
Located at the front of the spacecraft where radiation enters
the telescope, the Sunshade door is one of the most basic and
important elements of the spacecraft system. The Sunshade door
remained closed until Chandra achieved pointing control in
orbit. Now that it is opened, it shadows the entrance of the
telescope to allow it to point as close as 45 degrees to the
Sun.
The pointing control and aspect of determination system has
gyros, an aspect camera, Earth and Sun sensors, and reaction
wheels to monitor and control to very high accuracy where the
telescope is pointing at any given moment. It is as if you could
locate the bulls-eye on a target one kilometer (0.6 miles) away
to the precision of three millimeters--about the size of a
pinhead. This system can also put the Observatory into various
levels of inactive, quiet states known as "safe modes" of
operation during emergencies.
Calling Home
Low Gain Antennae
The communications, control, and data management system is the
nerve center of the Observatory. It keeps track of the position
of the spacecraft in its orbit, monitors the spacecraft sensors,
receives and processes commands from the ground for the
operation of the Observatory, and stores and processes the data
from the instrument so that it can be transmitted to the
ground.
Chandra has two low gain antennae, either one of which may be
used for two-way communications with Chandra's Operations
Control Center (OCC). All ground commands to and from Chandra
along with telemetry data -- sent through a set of three NASA
ground stations constituting its Deep Space Network -- are
routed through one of these antennae, typically about once every
eight hours.
The data are transmitted from the Deep Space Network stations to
the Jet Propulsion Laboratory and from there to the Operations
Control Center at the Chandra X-ray
Center (CXC) in Cambridge,
MA. There the data are processed and made available to
scientists, and eventually put in public archives.
