Managing Institutions:

Mission: Goddard Space Fight Center (http://glast.gsfc.nasa.gov/)

 Project Manager: Elizabeth Citrin, GSFC/NASA

Large Area Telescope: Stanford Linear Accelerator Center (http://glast.stanford.edu/)

 Principal Investigator: Peter Michelson, Stanford University and SLAC

GLAST Burst Monitor: Marshall Space Flight Center (http://gammaray.msfc.nasa.gov/gbm/)

 Principal Investigator: Charles Meegan, MSFC/NASA

Education & Public Outreach: Sonoma State University (http://glast.sonoma.edu/)

 Lead: Lynn Cominsky, SSU

Contributors:

Jerry Bonnell, Neil Gehrels, J.D. Myers, and Christopher Wanjek (NASA/GSFC)

Lynn Cominsky and Phil Plait (Sonoma State University)

Special thanks to Don Kniffen (NASA/HQ), Kathy Turner (DOE), Jonathan Ormes and Steve Ritz (NASA/GSFC), Charles Meegan (NASA/MSFC)

What Are Gamma Rays?

What we call light is really just a tiny fraction of the broad range of the electromagnetic radiation spectrum. The entire span stretches 

from very low-energy radio waves through microwaves, infrared radiation, visible light, ultraviolet light, X-rays, and finally to very 

high-energy gamma rays. The processes producing photons (single particles of electromagnetic radiation) of each type of radiation 

differ, as do their energy, but all of the different forms of radiation emitted are still just part of the electromagnetic spectrum’s family. 

The only real difference between a gamma-ray photon and a visible light photon is the energy. Gamma rays can have over a billion times 

the energy of the type of light visible to our eyes.

In fact, gamma rays are so energetic that they are harmful to life on Earth. Luckily, the Earth’s atmosphere absorbs gamma rays, 

preventing them from affecting life on the ground. However, if you want to observe the Universe in gamma-ray light, this poses a 

problem. The very atmosphere that protects us from gamma rays prevents us from observing them from the ground. Astronomical 

observations of gamma-ray sources in the GLAST energy range are therefore done with high-altitude balloons or satellites launched into 

space, above the protective blanket of the Earth’s atmosphere.

The high energy of gamma rays poses another problem: they can pass right through any lens or mirror, making it very difficult to 

focus them in a telescope. Astronomical observations, therefore, must rely on a different technology to view the gamma-ray Universe. 

Scientists must make use of methods developed in the world of particle physics, where techniques for measuring high-energy particles 

have long been understood. GLAST’s specialized astronomical instruments will therefore employ detectors used and perfected by 

physicists interested in the interactions of subatomic particles.

The Universe is home to exotic 

and beautiful phenomena, some 

of which produce fantastic 

amounts of energy. Supermassive 

black holes, merging neutron 

stars, streams of hot gas moving 

close to the speed of light ... these 

are but a few of the marvels that 

generate gamma rays, the most 

energetic form of electromagnetic 

radiation.

The Gamma-ray Large Area 

Space Telescope (GLAST) will 

explore this high-energy cosmos. 

With GLAST, astronomers will 

have a sensitive tool to study how 

black holes – well known for their 

ability to swallow up matter – can 

actually accelerate jets of matter 

outward at fantastic speeds; 

physicists will be able to search 

for new fundamental interactions 

between subatomic particles, and 

cosmologists will gain valuable 

information about the birth and 

early evolution of the Universe. In 

a real sense, GLAST will explore 

the links between the realm of the 

very small with the Universe of 

the very large.

For this unique endeavor – one 

that combines many scientific 

disciplines – NASA is teaming 

up with the U.S. Department of 

Energy and institutions in France, 

Germany, Italy, Japan, and 

Sweden. The launch is scheduled 

for 2006.

Gamma-ray Large Area

Space Telescope

http://glast.gsfc.nasa.gov/

GLAST Science

A considerable improvement over its successful predecessor – the Compton Gamma Ray Observatory – GLAST will have the ability 

to detect gamma rays in a range of energies from thousands to hundreds of billions of times more energetic than the light visible to the 

human eye. Radiation of such a magnitude can only be generated under the most extreme conditions; therefore GLAST will focus on 

studying the most energetic objects and phenomena in the Universe.

Because of their tremendous energy, gamma rays travel through the Universe largely 

unobstructed. This means GLAST will be able to observe gamma-ray sources near the 

edge of the visible Universe. Gamma rays detected by GLAST will originate near the 

otherwise obscured central regions of exotic objects like supermassive black holes, 

pulsars, and gamma-ray bursts. GLAST will study mechanisms of particle acceleration 

in extreme astrophysical environments. Among topics of cosmological interest will be 

the information obtained about the periods of star and galaxy formation in the early 

Universe and on dark matter.

Studying these high-energy objects and events with the advanced technologies of 

GLAST could give us an entirely new understanding of our Universe, revealing 

unanticipated phenomena. Historically, such new knowledge has eventually given rise 

to altogether new technologies.

The Instruments

GLAST will employ two instruments to observe the gamma-ray Universe. The 

primary instrument is the Large Area Telescope, or LAT. It has a wide field-ofview, allowing it to see large areas of the sky at the same time. It will detect 

gamma rays with 10 million to 150 billion times the energy of the light detected 

by the human eye (in technical terms, it detects gamma-ray energies of 20 million 

electron volts to greater than 300 billion electron volts, where the energy of 

visible light is roughly 2 electron volts).

The LAT will detect gamma rays by using a technique known as pair-conversion. 

When a gamma ray slams into a layer of tungsten in the detector, it creates a pair 

of subatomic particles (an electron and its anti-matter counterpart, a positron). 

These particles in turn hit another, deeper layer of tungsten, each creating further 

particles and so on. The direction of the incoming gamma ray is determined by 

tracking the direction of these cascading particles back to their source using highprecision silicon detectors. Furthermore, a separate detector counts up the total 

energy of all the particles created. Since the total energy of the particles created 

depends on the energy of the original gamma ray, counting up the total energy 

determines the energy of that gamma ray. In this way, GLAST will be able to 

make gamma-ray images of astronomical objects, while also determining the 

energy for each detected gamma ray.

The secondary instrument onboard is the GLAST Burst Monitor, or 

GBM. The GBM is designed to observe gamma ray bursts, which 

are sudden, brief flashes of gamma rays that occur about once a 

day at random positions in the sky. These bursts are still a mystery 

to astronomers; no one knows what causes them, or what physical 

forces are at work. All that is known is that they are among the most 

powerful explosions in the Universe. The GBM has such a large 

field-of-view that it will be able to see bursts from over 2/3 of the 

sky at one time, providing locations for follow-up observations of 

these enigmatic explosions. The GBM is composed of two sets of 

detectors – 12 sodium iodide (NaI) scintillators and two cylindrical 

bismuth germanate (BGO) detectors. When gamma rays interact 

with these crystalline detectors, they produce flashes of visible 

light, which the detector can use to locate the gamma-ray burst on 

the sky. The GBM works at a lower energy range than the LAT, so 

together they provide the widest range of energy detection in the 

gamma-ray regime for any satellite ever built.

High Energy BGO

Detectors (1 of 2)

Low Energy NaI

Detectors (3 of 12)

Image Credits:

Black Hole on Cover – A. Kamajian/NASA

EM Spectrum – NASA

GLAST in Orbit – Hytec

LAT – LAT Team/Hytec

GBM – GBM Collaboration

Gamma-ray Sky – S. Digel, LAT Team

Active Galactic Nuclei – NASA/STScI

Unidentified EGRET Sources – EGRET Collaboration

Solar Flares – ISAS, Yohkoh Project, SXT Group

Gamma Ray Bursts – NASA/Swift/Spectrum Astro

Cosmology and Particle Astrophysics – NASA/STScI

Pulsars – NASA/CXC/SAO
Simulation of the gamma-ray sky (E > 100 MeV) as 

observed by the GLAST Large Area Telescope (LAT).

The Gamma-ray Sky

Of the 271 gamma-ray sources detected by the EGRET instrument 

onboard the Compton Gamma-Ray Observatory, 172 are unidentified 

sources. Possible candidates for these sources include active galactic 

nuclei, pulsars, supernova remnants, dense molecular clouds, and 

stellar-mass black holes within our Galaxy. It is even quite possible 

that entirely new phenomena could account for some portion of 

these unidentified sources. GLAST will localize the positions of 

the unidentified sources accurately enough to make searches for 

counterparts at other wavelengths much more feasible. Above: 

Compton Observatory all sky gamma-ray image of the unidentified 

sources

Unidentified EGRET Sources

There is strong evidence that matter that radiates across the entire 

electromagnetic spectrum is only 10% of the total mass of the 

universe. In other words, 90% of the mass of the universe does not 

emit light at any wavelength. Currently the only way to detect this socalled dark matter is by its gravitational effects on luminous matter. 

One theoretical candidate for what dark matter might be is a type of 

hypothetical particle called a weakly interacting massive particle, or 

WIMP. The theory that predicts the existence of WIMPS also predicts 

that they may annihilate one another, producing gamma rays. If the 

theory is correct, then GLAST may be capable of observing radiation 

from these particles in the galactic halo, helping to unravel the mystery 

of dark matter. Above: Hubble Space Telescope optical image of a 

gravitationally lensed cluster of galaxies

Cosmology and Particle Astrophysics

Gamma Ray Bursts

Intense flashes of gamma rays lasting from fractions of a second to hours, 

some with fading afterglows visible for months. Apparently associated 

with star forming regions in galaxies, these are among the most powerful 

explosions in the universe. Left: Artist’s conception of a gamma-ray 

burst

Example: GRB 990123

Distance – 10 billion light-years

Size – emitting region is light-seconds across

Power – at maximum up to 1,000,000,000,000,000,000

(quintillion) times the Sun’s power

Distant galaxies, some of which are intense and highly variable flaring 

sources of high-energy gamma rays. AGN are powered by supermassive 

black holes with up to billions of times the Sun’s mass. The gamma rays 

seen from some AGN arise from relativistic jets of material that are aimed 

in our direction. Left: Hubble Space Telescope optical image of M87’s jet.

Example: 3C279

Distance – 7 billion light-years

Size – central region 3 light-days across

Power – at flare maximum near 1,000,000,000,000

(trillion) times the power output of the Sun

Active Galactic Nuclei (AGN)

Pulsars

Rotating neutron stars with strong magnetic fields. Neutron stars are 

collapsed stellar cores with the density of an atomic nucleus (about 1 

billion tons per cubic centimeter!) and are slightly more massive than the 

Sun. Left: Chandra Observatory X-ray image of the Crab Nebula pulsar

Example: The Crab Nebula Pulsar

Distance – 7,000 light-years

Size – 10 kilometer diameter neutron star rotating 30 times

per second

Power – 75,000 times the power output of the Sun

Solar Flares

Intense energetic solar explosions which produce gamma rays and 

accelerate atomic particles. Solar flares are generated within the Sun’s 

highly magnetic active regions and can bombard Earth with high-energy 

radiation. Left: X-ray image of the flaring Sun from the Yohkoh satellite

Example: Solar Flare 11 June 1991

Distance – 8 light-minutes

Size – solar active regions are several times the size of

planet Earth

Power – a typical flare has one-tenth the power output of

the entire quiet Sun