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 the 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 high-energy Universe.

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 performed.

Why don't we just use all the fuel and send CGRO into a really high orbit?

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 population.

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 time.

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, circular orbit.