Reboosting the Compton Gamma-Ray Observatory
The Compton Gamma-Ray Observatory (CGRO) has been in Earth orbit for nearly
six years now. In that time, observations from its 4-instrument complement
have greatly expanded our understanding of the most energetic objects in the
Universe. The EGRET instrument discovered that some quasars were also
powerful sources of gamma radiation; BATSE observations of gamma-ray bursts
have forced astronomers to dramatically revise their ideas on this mysterious
phenomena; OSSE has mapped the distribution of electrons colliding with their
antimatter counterparts near the center of our Milky Way galaxy; and COMPTEL
has mapped the galactic concentration of aluminum, which has given new
insights into the production of the chemical elements in stars. In December
1995, CGRO discovered the
bursting pulsar, while in December 1996
first apparent repetition from a gamma-ray burst source was seen.
The popular perception of space is that it is just empty and once in
orbit, without the drag of the Earth's atmosphere to slow it down, a satellite
can continue forever. Unfortunately, that is not the case. Even at altitudes
of hundreds of miles above the Earth, there's a little bit of the Earth's
atmosphere left. Over time periods of years, even this tiny amount of drag
will bring a spacecraft plunging earthward. On July 11, 1979, the Skylab
space station, launched in 1973, fell to Earth. More recently, some smaller
satellites have made headlines as their orbits decayed and they fell to Earth.
Fortunately, most of these satellites have impacted in the oceans or other
isolated areas far from human habitation.
The time is approaching when CGRO's orbit will have decayed sufficiently
that it would cause a final plunge into the Earth's atmosphere. Current
projections indicate this will happen sometime in 1999 - unless the reboost
motors on the spacecraft are fired to send it back into a higher orbit. If
the spacecraft's orbit can be raised from its current altitude of 440
kilometers up to 520 kilometers, it could remain in orbit until 2009.
Plans are already being made to boost CGRO into a higher orbit in the
Spring of 1997. If all goes well, we can expect another ten years of
CGRO observations, which should yield even greater insights into the
What are the benefits of reboosting the spacecraft?
At present, there are no satellites currently in orbit (or scheduled to
be in orbit soon) that can make some of the observations that CGRO can
perform. In five years of observations, BATSE has already increased our
data on gamma-ray bursts by about a factor of five over what was performed by
the previous 20 years of spacecraft observations. No replacement for a
BATSE-like burst detector (which can view most of the sky all of the time),
has yet been approved for construction and launch. In addition, we are long
overdue for a supernova in our own Galaxy. Some spark-chamber gas for EGRET
has been placed in reserve for just such a possibility. CGRO had been
scheduled to be in orbit back when Supernova 1987A exploded in the nearby
Large Magellanic Cloud, but was not launched until four years later when the
supernova had faded considerably.
The recent failure of the HETE spacecraft missed
the chance to view the recent repeating gamma-ray burst to search for a
counterpart in the ultraviolet. Numerous golden opportunities to expand our
knowledge have been missed because a spacecraft just missed being in orbit on
time. It is to our advantage to keep CGRO operating until its replacement is
actually working in orbit.
On the financial side, by extending the usable lifetime of the spacecraft,
taxpayers get a better return on a $600 million investment.
How does the solar cycle affect the Earth's atmosphere?
We are currently entering a period of increased solar activity (the level
of activity on the Sun oscillates with an 11 year period) where there will
be increased sunspot activity and solar flares. The particles streaming
from the Sun (the solar wind) heat up the atmosphere of the Earth, causing
it to expand. This larger atmosphere results in increased atmospheric drag
on all Earth-orbiting spacecraft.
When CGRO was launched, the Sun was active and as a result the orbit
decayed quickly. After the time of the first reboost in October of 1993,
solar activity had declined and the orbit decayed less swiftly. Now we are
beginning entry into the next cycle of increased solar activity which will
dramatically shorten CGRO's orbital lifetime if the reboost is not
Why don't we just use all the fuel and send CGRO into a really high
Since the spacecraft will eventually fall to Earth, as a precaution,
enough fuel must be kept on board to enable flight controllers to force the
spacecraft to deorbit in a controlled fashion. This way, they can insure
that the spacecraft will impact in the ocean or isolated land away from human
How much of CGRO would survive the fall to Earth?
CGRO is designed to collect gamma-rays and as a result, it loaded with a
number of heavy metallic components. Many of these would survive the
high-temperature reentry through the Earth's atmosphere to impact on the
ground. You wouldn't want to be under one of these heavy components when it
hit. Specific components which might survive: The metal plates used in the
EGRET spark chamber and the tungsten collimators used in OSSE.
Why wasn't CGRO launched into a higher orbit to start with?
CGRO is one of the largest and heaviest satellites ever launched by the
Space Shuttle. In fact, it's size and weight made the Space Shuttle the
only practical way to get CGRO into orbit. As a result, CGRO was limited
to the altitudes which could be reached by the Space Shuttle.
What are the disadvantages of reboosting the spacecraft?
One disadvantage of reboosting the spacecraft is that the instruments
will be switched off during the reboost and no observations can be
performed. Fortunately, this only affects the spacecraft for a few days.
Also, in the higher orbit, the spacecraft will be hit by a larger number of
high-energy particles trapped in the Earth's magnetic field. These
particles will increase the background noise registered in some detectors
(particularly OSSE) which means faint sources will be lost in the noise
unless the duration of the observation is increased.
Has a reboost maneuver been done before?
CGRO was given a boost back in October of 1993. At that time, the orbit had
decayed from its launch altitude of 450 kilometers down to about 340
kilometers. The reboost placed CGRO back up to 450 kilometers.
How will the reboost maneuver be performed?
The reboost maneuver is a complex process which requires a great deal of
planning. Initially, flight controllers will test the instructions for the
reboost on a CGRO simulator to ensure the spacecraft is sent the correct
instructions and that it can be programmed to handle certain emergency
situations. The maneuver must also be performed when no Space Shuttles are
flying, to ensure continuous communication with CGRO during this critical
Since it has been over three years since the reboost engines have been used,
flight controllers will conduct a test burn - firing the thrusters for 90
seconds. They will compare the results of this test against how the
thrusters are expected to perform to determine if any changes need to be
made in the reboost plan.
If the engineering test goes well, about a month later, several thruster
burns will be performed to achieve the desired orbit. First, the thrusters
will be fired again when the spacecraft is in the low part of its orbit
(called the perigee) to raise the apogee (the high point) of the orbit. They
will then evaluate the results of this maneuver. Then, about a month later,
when the spacecraft is at it's new apogee, the thrusters will be fired again
to raise the perigee of the orbit, so the final result is a near-perfect,