Gamma ray astronomy is a difficult science. Like X-ray, ultraviolet, and
infrared astronomy, it requires observatories in space, because
the Earth's atmosphere is opaque to these wavelengths of light.(*) However,
it is unlike most other forms of astronomy in that "imaging" -- focusing
photons using mirrors or lenses to form an image -- is impossible.
Gamma rays are so energetic, they pass straight through any material you
might use to focus them. So to observe gamma rays, you need very specialized
The Compton Gamma Ray Observatory (CGRO) was the second of NASA's
Great Observatory satellites, after the Hubble Space Telescope. As its name
suggests, it was a gamma ray astronomy mission, designed to observe the
most energetic events in the universe. CGRO was named in honor of the
American physicist Arthur Holly Compton, winner of the
1927 Nobel Prize in Physics for his discovery of Compton scattering.
It was ordered by NASA, and TRW was the prime contractor who assembled the
satellite; the (four) scientific payloads were supplied by various
institutions around the world. The satellite was carried into orbit by
the Space Shuttle Atlantis (mission number STS-37) on April 5, 1991,
and flew until it was deliberately de-orbited on June 4, 2000. It was
the largest science satellite ever put into orbit, weighing nearly 35000
pounds (16000 kg). CGRO performed very sensitive and detailed
surveys of the gamma ray sky, and catalogued several
thousand gamma-ray bursts, but it failed
in its key mission to discover the origin of these mysterious bursts.
CGRO had four separate instruments:
- Burst and Transient Source Experiment (BATSE)
- Oriented Scintillation Spectrometer Experiment (OSSE)
- Imaging Compton Telescope (COMPTEL)
- Energetic Gamma Ray Experiment Telescope (EGRET)
Each of these instruments used a slightly different technique to observe
gamma rays, though all telescopes used Compton scattering in one way or
another. They all observed gamma ray photons of different energies, and
were meant for different purposes. I'll describe each briefly below; I know
much more about BATSE (having worked with it in the past) so my descriptions
of the other instruments will be shorter.
BATSE was composed of eight different detectors, fixed at the eight corners
of the spacecraft. BATSE was meant to be an all-sky monitor, to detect gamma
ray bursts coming from
any direction. Each detector was composed of two separate instruments:
a Large Area Detector (LAD), and a Spectroscopy Detector (SD). The LADs
were meant as the workhorses of BATSE, designed mainly as light buckets to
catch gamma ray photons. The SDs were a bit more discriminating, as they
provided a means to actually measure the energy of photons as they came in,
and not just count them. Both detectors used large, thallium-doped
sodium iodide crystals
(I mean large, like a cubic foot) which scintillate when hit
by a gamma ray. The gamma ray was then detected by measuring the flash of
light created by the gamma ray hitting the crystal.
Because the eight BATSE detectors could see gamma rays coming from any
direction, it was thought to be an ideal way of detecting gamma ray bursts.
And it was -- over its lifetime, it catalogued over twenty seven hundred of
them. Unfortunately, BATSE wasn't very good at telling us exactly
where the bursts occurred. You could get a rough idea of where the gamma
rays came from by measuring the intensity of the burst
seen by each detector; a detector pointing right at it would have a stronger
signal than one pointed in a different direction. But BATSE never provided
enough spatial sensitivity to actually locate a burst well enough to
find it with an optical telescope. This wasn't for lack of trying -- CGRO
established the "BACODINE" network, in which preliminary burst coordinates
were sent to astronomers around the world via pager or email, and these
astronomers would then scan that patch of sky for any optical transients.
Unfortunately the BACODINE positions were never precise enough, nor the
alerts fast enough to make an optical detection based on BATSE coordinates.
These coordinates could be combined with observations from other satellites
(particularly the Ulysses solar observatory) to improve the localizations, but this took days to accomplish.
Gamma ray bursts fade on timescales of minutes
so bursts would have long since faded by the time the source locations
could be improved.
This was somewhat of an embarrassment for NASA as they
had hoped to discover the origins of gamma ray bursts with this satellite.
It was even more of an embarrassment when a small Dutch-Italian satellite
called BeppoSAX finally managed to catch a gamma ray burst in the act and
localize it in 1997. They are now believed to be massive hypernovae or
the merger of two neutron stars or black holes at
cosmological distances (millions or billions of light years away).
The official energy range of BATSE was 20 keV to 1 GeV, but in reality the
low energy boundary of the SDs could be dropped to less than 7 keV by changing
the gain on the photomultiplier. They could also be used for imaging, but
this requires a bit of trickery. Since the satellite orbits the Earth, some
sources which lie within the plane of the orbit are occasionally
occulted by the Earth. If you measure the
change in the total gamma ray flux as the target gets eclipsed
by the Earth, you have a good estimate as to how many gamma
rays per second were coming from that target. In principle, you could do
this for just about any source on the sky, but in reality, only the strongest
sources produced enough flux to be measurable in this way. For example,
I was involved in a project to measure a five-year-long light curve of
Scorpius X-1 using BATSE. It was difficult to do, but it produced
BATSE was designed and built by NASA's Marshall Space Flight Center,
along with the University of Alabama (Huntsville), and the Universities
Space Research Association.
OSSE was a set of four detectors, each able to point in a different direction.
Unlike BATSE, the OSSE detectors could be slewed on their own to point at
individual targets. Also unlike BATSE, they had a much narrower field
of view -- a few degrees rather than half a steradian -- provided
by tungsten collimators. The detectors are otherwise similar
to BATSE, with the use of doped sodium iodide and cesium iodide crystals as
the scintillators. The detector design is called a phoswich -- which
simply means you have two different crystals attached to one another, and you
the incoming photons by measuring how the incoming gamma ray interacts with
each. The detector was able to distinguish gamma ray photons of different
energy depending upon the pulse of light observed when the crystal
scintillated, and thus OSSE was able to perform spectroscopy (with an
accuracy of about eight percent in energy).
Because OSSE was able to slew its four detectors in almost any direction and
it had a small field of view, it was primarily used to observe point
sources like neutron stars, x-ray binaries, AGN, and
quasars. The detectors acted like photometers and spectroscopes
simultaneously -- photometers because they measured the light curve,
and spectroscopes because they could actually measure the energies of the
incoming photons. OSSE had an energy range between 50 keV and 10 MeV.
The energy range covers energies common in many nuclear reactions (especially
the electron-positron annihilation emission line at 511 keV). OSSE
concentrated on observing places where we expect to see these nuclear
reactions, particularly supernova remnants, and the spallation-generated
nuclear fusion emission from solar flares. It also mapped
the 511 keV annihilation emission from the Milky Way, including the
OSSE was designed and built by the Naval Research Laboratory of the
United States Navy.
COMPTEL was the core of CGRO, despite the satellite's main mission being
the search for gamma ray bursts. This experiment, along with EGRET, performed
a detailed sky survey of the cosmos at gamma ray wavelengths.
COMPTEL was an odd design, consisting of a liquid at the top of the
instrument, and sodium iodide crystals at the base. Gamma rays passed
through the outer shielding of the telescope, and underwent Compton
scattering in the liquid. The energy released by this first scattering was
then analyzed. However, the gamma rays were scattered by the liquid down into
sodium iodide detectors where they would then scintillate, as in the
BATSE and OSSE detectors. The data from the two interactions was then
combined to determine where the gamma ray came from (to within a
degree or so), and what its energy was.
COMPTEL had a large field of view (about a steradian), but by combining
the data from the two detectors, you could pinpoint the source location of
the gamma ray to within a few degrees. COMPTEL was used to survey the
entire sky in gamma rays with energies between 0.8 and 30 MeV, but it was
also used to point at individual sources once the sky survey was complete.
COMPTEL's energy range was also well-suited to observe
high-energy solar flares, so it conducted many observations of
the Sun as well.
Like BATSE, COMPTEL was fixed to the spacecraft, and so pointing COMPTEL
meant reorienting the entire spacecraft. In fact, most spacecraft
reorientations were made to service COMPTEL or EGRET observations, and
observations with the other instruments were scheduled around this. On a few
occasions, a gamma-ray burst would occur in the field of view of COMPTEL,
and thus these bursts were slightly better localized than those detected
by BATSE alone. (But it still wasn't good enough to determine their exact
location. Oh well.)
COMPTEL was an international collaboration, led by the Max Planck
Institute in Garching, Germany, along with the Space Research Organization
of the Netherlands, the University of New Hampshire, and the
European Space Agency.
EGRET was the final experiment on board, and it observed the highest energy
photons in CGRO's spectral range -- 30 MeV to 30 GeV. It utilized a
combination of detectors, like COMPTEL. The upper stage of the detector was
a spark chamber -- a wire-mesh chamber filled with an inert gas. The chamber
was designed to convert the highest-energy gamma rays into
a cascade of electrons, positrons, and lower-energy
gamma rays. At the top of the chamber, the gamma ray produces an
electron-positron pair, which ionize the inert gas. These ions then result
in a spark being generated in the wire mesh. You can determine the direction the initial gamma ray came from by following the trail of the
sparks. At the bottom of the detector, a sodium iodide detector
catches the remaining gamma rays from the cascade, to measure their total
energy. Because it could measure a photon's path and
energy, EGRET could generate images and spectra
Because EGRET was designed to observe the highest-energy photons, its purpose
was to study the most energetic phenomena in the universe. These include
processes which occur near the surfaces of neutron stars, and near the
event horizons of black holes, thus giving us a view of these extreme forms
of stellar matter. On larger scales, supermassive black holes lying at the
centers of quasars, Markarian galaxies, and other AGN
also create incredibly energetic gamma rays -- some
billions of times more energetic than even EGRET could detect! EGRET let us
measure the spectra of these objects at gamma ray energies with very good precision, which in turn helps us build models of how such energetic photons are created.
EGRET was also a multinational collaboration, involving NASA's Goddard
Space Flight Center, Stanford University, the Max Planck Institute,
Hampden-Sydney College (Virginia), and the University of Heidelberg.
The CGRO mission was considered a success, and deservedly so. Although it
never discovered the source of gamma ray bursts, CGRO provided a us with
a wonderful new view of the heavens. BATSE generated
a nearly complete catalog of gamma ray bursts over the course of its
operation. The structure of the BATSE light curves told us
much about the physics of these events, and the distribution of gamma
ray bursts nearly evenly on the sky strongly suggested a cosmological
origin. The EGRET and COMPTEL telescopes produced very sensitive maps
of the universe at gamma ray energies, much better than those that had
previously been done. The EGRET map showed that much of the high energy
gamma radiation in our sky comes not from point sources or explosions like
supernovae, but is a diffuse emission arising from the interaction of
high energy cosmic rays with the interstellar medium. EGRET
also provided us with a view of the very highest energy gamma rays, allowing
us to view some of the most energetic events in the universe, like
the centers of active galaxies. In fact, it discovered a new class of
super-powerful AGN, the gamma-ray emitting blazars.
COMPTEL generated a map just like
EGRET, but at lower energies. This map also provided us with a clearer view
of the gamma ray sky, particularly of gamma rays generated in our own galaxy.
For example, COMPTEL mapped the distribution of the radioactive isotope Aluminum 26 in the Milky Way, helping us understand how matter ejected from
the galaxy. Finally, OSSE detected the presence of an antimatter source in
the center of the Milky Way, strongly suggesting the presence of a
supermassive black hole there. These discoveries, and many others, greatly
expanded our understanding of the high-energy universe.
However, there was one more disappointment with CGRO besides the gamma ray
burst mystery, namely the failure of
the on-board data tape recorders. CGRO was meant to perform observations
semi-autonomously, taking data, storing it onboard, and then sending it by
radio to a receiving station. When the data recorders failed, data had to
be sent live, directly to ground station. This was done via NASA's
Tracking and Data Relay satellite system (TDRSS), and it worked ok. Because
TDRSS couldn't be used exclusively for CGRO, only about 80 percent of the
data taken by the spacecraft was actually recorded. A little bit more
data was lost as a result of the spacecraft passing through the
South Atlantic Anomaly -- the charged particles there would overwhelm the
signals from gamma rays, so the spacecraft was usually turned off for a short
time on each orbital passage.
One last note, on the death of CGRO. As I said at the top, and as you almost
certainly heard on the news, CGRO was deliberately brought down from orbit
by NASA. This was done because the spacecraft only had two functioning
gyroscopes left to stabilize the craft, and one of these was not
in good health. Because the spacecraft was so massive, parts of it would
have survived a re-entry, much like parts of Skylab survived to crash-land
in remote Australia in 1979. To avoid inadvertently causing injury or
death if the spacecraft were to make an uncontrolled re-entry over a populated
area, it was brought down in a "controlled descent". The best guess for the
re-entry location was in the eastern pacific, several hundred kilometers off the northwest coast of
(*) Please see my writeup in Air Cherenkov Telescope for a gamma ray
telescope that doesn't fly in space.
(**) see http://www.journals.uchicago.edu/ApJ/journal/issues/ApJS/v116n2/36180/36180.pdf (I am author #4).
I used many resources for this writeup, but the best semi-technical site was
http://imagine.gsfc.nasa.gov is an excellent site for young people and
those interested in non-technical and historical information.