The Compton Observatory is one of NASA's four Great Observatories, with the mission of observing the sky in the light of gamma-rays. Gamma-rays are photons with greater energies than X-rays. The gamma-ray sky is variable and dynamic. Compton has observed many new sources with its four instruments: the Burst and Transient Source Experiment (BATSE), the Compton Telescope (COMPTEL), the Energetic Transient Experiment (EGRET) and the Oriented Scintillation Spectrometer (OSSE).

2) What is the current plan for the Compton Observatory?

During a press conference on March 24th, the NASA Associate Administrator for Space Science, Dr. Edward Weiler, announced that public safety required NASA to cause CGRO to reentry the Earth's atmosphere. This process would begin on May 29th, with reentry occurring on June 3rd. Some parts of the spacecraft will fall into uninhabited regions of the Pacific. While debris would be scattered over a region of 16 by 962 miles, debris are expected to impact only 62 square yards (52 square meters).

3) Why did NASA decide to reentry the spacecraft for safety reasons?

The Associate Administrator stated that if another gyroscope failed, the spacecraft would pose an unacceptable risk to human life.

"You might ask, what is the probability that we lose that gyro and go out of control or out of, not have the capability to bring down CGRO. There is a 10% chance that we could lose that gyro over the next three years. If CGRO reenters on its own, without control, there is a 1 in 1000 chance of a human fatality. That's fatality. That would be the case where we didn't do anything and let it come down by itself."

4) What is the risk of a human fatality from continuing the mission of the Compton Gamma-Ray Observatory?

The increased risk to human life from another gyroscope failure (assuming that the shuttle option is not used) is due to the 10% probability of a gyroscope failure, which would force the use of the zero-gyroscope reentry mode. Since the two-gyroscope reentry mode has a fatality risk of 1 in 29 million, while the zero-gyroscope mode has a risk of 1 in 4.1 million, the increased risk is 1 in 48 million.

Another risk is from a double failure of a critical subsystem. Because all critical subsystems are redundant, two failures of the same subsystem would have to occur for control of the spacecraft to be lost. Because of the redundancy, the casualty risk is about 1 in several million.

These risk probabilities are the result of calculations by NASA engineers.

5) How does the risk of continuing the CGRO mission compare with other risks?

The current worst-case estimate of the risk to any human life is one in several million. Many NASA activities have greater risks to human life, for example, NASA Safety Standard 1740.14 specifies the maximum permitted fatality risk from the reentry of a spacecraft to be 1 in 10,000. In the US, motor vehicle travel has a fatality rate of 1.7 per 100 million miles traveled (1997 data from the National Highway Traffic Safety Administration), so one person traveling a few tens of miles causes a casualty risk comparable to the risk of flying CGRO summed over the entire human population. In short, continuing the CGRO mission is extremely safe.

6) Why have different casualty probability numbers been given?

At the press conference Dr. Weiler stated the risk, after the failure of another gyroscope, as 1 in 1000. This is the risk for an uncontrolled reentry. However, NASA engineers have a plan for a controlled reentry even if all of the gyroscopes fail. Dr. Weiler confirmed the risk from this option as 1 in 4.1 million. We do not know why NASA has focused on the 1 in 1000 risk when a much safer alternative has been developed.

7) What would happen if another gyroscope failed?

The spacecraft would still be controllable and scientific operations could continue, possibly with some degradation. The spacecraft could be reentered in a controlled manner, even if both of the remaining gyroscopes fail. The safety level of a zero-gyroscope reentry is excellent, with a fatality risk of 1 part in 4.1 million.

8) What would happen if any breakdown caused the spacecraft to become uncontrollable?

A complete breakdown is very unlikely because all critical subsystems are redundant and because the gyroscopes are not a critical subsystem. If a total breakdown were to occur, the spacecraft would stabilize in a gravity gradient mode. It would remain in orbit for at least 3.5 years, allowing time for a shuttle repair or retrieval mission. If a shuttle mission were not performed, the spacecraft would eventually reenter in an uncontrolled manner causing debris to impact an estimated 62 square yards of the Earth's surface somewhere between latitudes 28 degrees South and North. The estimated casualty probability would be 1 in 1000.

9) Why did a gyroscope fail? What is the probability that another gyroscope will fail?

One of CGRO's gyroscope began showing anomalous behavior such as increased motor current in February 1999. On 19 December 1999 that gyroscope failed, probably due to bearing failure. The gyroscopes used on CGRO are very reliable and the probability of another failure during the next 3 years is estimated to be less than 10%.

10) Could the Space Shuttle be used to repair or retrieve CGRO?

This option has not been thoroughly explored. There are some astronaut safety issues, but these could probably be managed. The CGRO spacecraft, but not the science instruments, was designed for astronaut servicing.

11) Was the decision to reentry CGRO made because of the cost of operating CGRO?

No. NASA recognizes the continuing very high scientific return of CGRO and the Associate Administrator stated that "Believe me, cost was not, was not, I repeat, was not a variable in this decision because it only costs 3 or 4 million dollars a year."

12) What are some of the important discoveries made by CGRO?

Observations by the Burst and Transient Source Experiment (BATSE) showed that even faint gamma-ray bursts are isotropic and homogeneous, demonstrating that GRBs originate at very large distances. The first observation, in January 1999, of optical emission during the burst phase of a gamma-ray burst was made possible by a location provided by the BATSE instrument on CGRO. Recent analysis of observations made with the EGRET instrument demonstrated the existence of a population of gamma-ray sources in Gould's Belt. EGRET and COMPTEL have also observed variable Active Galactic Nuclei. OSSE discovered a cloud of positrons near the center of our galaxy.

13) If mission of CGRO were to be extended, how likely are additional discoveries?

It is difficult to predict the undiscovered. The gamma-ray sky is variable and unpredictable, so new sources or phenomena are likely. Every gamma-ray burst is unique and the less common bright events are scientifically valuable. Important discoveries have been made in the recent past---this record is likely to continue. The sun is now entering solar maximum, so solar observations are valuable. Because of the recent accident to the High Energy Solar Spectroscopic Imager (HESSI) spacecraft, no other instrument will be able to conduct high-energy solar observations for about a year. Observations by CGRO support numerous other instruments and telescopes by discovering transient sources and providing gamma-ray burst locations.

14) What other space missions and ground-based observatories are assisted by CGRO observations?

Space-based missions assisted by CGRO include: HST, RXTE, Chandra, ASCA, BeppoSAX, XMM, SWIFT. Future space missions that will benefit include: HETE-2, INTEGRAL and SWIFT. Ground-based observatories basing observations on CGRO results include: APT, ETC, GROCSE, LOTIS, Milagro, ROTSE and Whipple.

15) How do scientists feel about the current plan to reentry CGRO on June 3rd?

They believe that the risks of continuing to fly CGRO are extremely low, that the planned reentry is unnecessary and that the intentional destruction of the Compton observatory would be a tragedy.

16) What can I do to help avert this tragedy?

Write the Director of NASA, Dan Goldin, your Congressman and the Chairman of the House Science Committee, Representative James Sensenbrenner, Jr. and request that an Independent Review Committee be formed to determine the true risks of continuing to fly CGRO and to balance those risks with the scientific return.

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