Chandra Observatory Launch Lights Up the Night Sky
X-ray astronomy moved into the 21st century today with the launch of the
Chandra X-ray Observatory (CXO). Shortly before 12:30 a.m. EDT, the Space
Shuttle Columbia blasted off from the Kennedy Space Center in Florida.
Under the command of Lt. Colonel Eileen Collins, the shuttle will begin to
deploy CXO later today in their 5 day mission.
|Chandra X-ray Observatory will deploy from the |
payload bay of Space Shuttle Columbia
As spectacular a sight as it is to see a shuttle launch in the middle of
the night, beauty is not the reason for the schedule. It all has to do with
physics, primarily with a subsection called orbital mechanics. The field of
orbital mechanics began long ago when Johannes Kepler discovered that
planets moved in regular, elliptical paths and Isaac Newton discovered that
gravity is universal. In other words, if you want to achieve a certain
orbit, you have to launch at a certain time!
One of the driving considerations for selecting the CXO orbit was the
lifetime of the orbit. The same forces that make the Earth's tides rise and
fall will slowly change the orbit over time. NASA's baseline mission for
Chandra is 5 years; the spacecraft has resources to last 10 years.
An additional factor in determining the best orbit is that Chandra has
strict power requirements, so it must be oriented to expose its solar
arrays properly during maneuvers to its final orbit. Furthermore, the upper
stages have a horizon sensor that must look in a certain direction. These
two bits of spacecraft geometry are what finally set Chandra's launch time
relative to the stars, not human work schedules.
Engineers at TRW calculated the best orbit to fulfill all of Chandra's
constraints. The orbit will last for about 30 years (eventually the
satellite will return to Earth in a blaze of glory, burning up as it falls
back into the Earth's atmosphere). The orbit will start out ranging from
10,000 to 140,000 km above the Earth (6,200 x 86,800 mi), almost a third of
the way to the Moon at its the farthest distance. The orbit will be
inclined 28.45 degrees to the equator, the same as the latitude of the
launch site, Kennedy Space Center. Chandra's orbit has a period of 64.2
hours, with about 15 hours of that inside the Van Allen Radiation Belts,
forcing operators to shut down the science instruments while the
Observatory is below 24,000 mi altitude. Essential chores like repointing
the Observatory and other functions that would disrupt science observations
will be scheduled for the passes through the radiation belts.
How does all this lead to a launch at around midnight? Think of Chandra's
final orbit as an ellipse moving with the Earth around the Sun. Even as the
Earth spins, the ellipse stays pointed in the same direction. Launch and
all the intermediate steps to get to the final orbit all lay within this
The question then becomes...when does the Kennedy Space Center rotate into a
proper position on the ellipse's plane so that CXO could wind up in the
right place? Working through the mathematical equations, engineers
determined that for a launch on July 20, 1999, the proper alignment
occurred at 12:36 a.m. EDT. For every day of delay, the proper alignment
for Chandra's launch moved up by 4 minutes.
Once the 50,000 kg satellite leaves the payload bay, it still has a long
way to go to reach its final orbit. The boost from the shuttle orbit to
Chandra's final orbit will begin when the Inertial Upper Stage (IUS)
booster deploys at 7 hours, 17 minutes after launch, and its two stages
will fire about an hour later. Chandra's Integral Propulsion System (IPS)
will make two burns at perigee and three at apogee to raise and finalize