Imagine the Universe!

Observatories in space

Radio observatories

artist concept of VLBI
At present, there are no radio observatories in space. There are plans, however, for two in the coming years. They are the Very Long Baseline Interferometry (VLBI) Space Observatory Program (VSOP), a Japanese mission with a scheduled launch date of January 1997, and RADIOASTRON, a Russian mission schedule for 1998. NASA will be supporting both missions with its Deep Space Network radio telescope facilities around the world.

Radio waves CAN make it through the Earth's atmosphere without significant obstacles (In fact radio telescopes can observe even on cloudy days!). However, the availability of a space radio observatory complements radio telescopes on the Earth in some important ways.

One is a special technique used in radio astronomy called "interferometry". Radio astronomers can combine data from two telescopes that are very far apart and create images which have the same resolution as if they had a single telescope as big as the distance between the two telescopes! That means radio telescope arrays can see incredibly small details. One such array is called the Very Large Baseline Array (VLBA): it consists of ten radio telescopes which reach all the way from Hawaii to Puerto Rica: nearly a third of the way around the world! But by putting a radio telescope in orbit around the Earth, radio astronomers could make images as if they had a radio telescope the size of the entire planet!

artist concept of COBE

Microwave observatories

There are no microwave observatories in space right now nor are there any major microwave observatories planned that I know of... Mail me if you know of any!

The most recent microwave observatory was the Cosmic Background Explorer (COBE) ( which observed the entire sky making very precise measurements of the temperature of the "microwave background".

The sky is a source of microwaves in every direction, most often called the microwave background. This background is believed to be the remnant from the "Big Bang" scientists believe our universe began with. It is believed that a very long time ago all of space was scrunched together in a very small, hot ball. The ball exploded outward and became our universe as it expanded and cooled. Over the course of the past several billion years (the universe's actual age is still a matter of debate, but is believed to be somewhere between ten and twenty billion years), it has cooled all the way to just three degrees above zero. It is this "three degrees" that we measure as the microwave background.

COBE mapped out the entire microwave background, carefully measuring very small differences in temperatures from one direction to another. Astronomers have many theories about the beginning of the universe and their theories predict how the microwave background would look. The very precise measurements made by COBE eliminated a great many of the theories about the Big Bang.

ISO logo

Infrared observatories

The biggest infrared observatory currently in orbit is the brand new InfraredSpace Observatory (ISO) (, launched in November 1995 by the European Space Agency. ISO will operate for at least two years barring unforeseen circumstances. It has been placed in an elliptical orbit with a 24 hour period which keeps it in view of the ground stations at all times, a necessary arrangement since ISO transmits observations as it makes them rather than storing information for later playback. ISO will able to observe from 2.5 to 240 microns.

Late next year, NASA plans to launch the Space Infrared Telescope Facility (SIRTF) ( SIRTF will use an passive cooling system (i.e. it radiates away its own heat rather than requiring an active refrigerator system like most other space infrared observatories) and it will be launched well away from the Earth where it will not have to contend with Earth occultation of sources nor with the comparatively warm environment in near-Earth space.

Another major infrared facility coming soon will be the Stratospheric Observatory for Infrared Astronomy (SOFIA). Although SOFIA will not be an orbiting facility, it will carry a large telescope within a 747 aircraft flying at an altitude sufficient to get it well above most of the Earth's infrared absorbing atmosphere. SOFIA will be replacing the Kuiper Airborne Observatory.

artist concept of HST

Visible spectrum observatories

The only visual observatory in orbit at the moment is the Hubble Space Telescope (HST). Like radio observatories in space, there are visible observatories already on the ground. However, Hubble has several special advantages over them.

HST's biggest advantage is, because it is above the Earth's atmosphere, it does not suffer distorted vision from the air. If the air was all the same temperature above a telescope and there was no wind (or the wind was perfectly constant), telescopes would have a perfect view through the air. Alas, this is not how our atmosphere works. There are small temperature differences, wind speed changes, pressure differences, and so on. This causes light passing through air to suffer tiny wobbles. It gets bent a little, much like light gets bent by a pair of glasses. But unlike glasses, two light beams coming from the same direction do not get bent in quite the same way. You've probably seen this before -- looking along the top of the road on a hot day, everything seems to shimmer over the black road surface. This blurs the image telescopes see, limiting their ability to resolve objects. On a good night in an observatory on a high mountain, the amount of distortion caused by the atmosphere can be very small. But the Space Telescope has NO distortion from the atmosphere and its perfect view gives it many many times better resolution than even the best ground-based telescopes on the best nights.

Another advantage of the Space Telescope is that without the atmosphere in the way, it can see more than just the visible spectrum. The Space Telescope can also see ultraviolet light which normally is absorbed by the Earth's atmosphere and cannot be seen by regular telescopes. So the Space Telescope can see a much wider portion of the spectrum.

artist concept of IUE

Ultraviolet observatories

Right now there are no dedicated ultraviolet observatories in orbit. The Hubble Space Telescope can perform a great deal of observing at ultraviolet wavelengths, but it has a very fairly small field of view. Until September 1996, the International Ultraviolet Explorer (IUE) was operating and observing ultraviolet radiation. Its demise, although unfortunate, was hardly premature: IUE was launched in January, 1978 with planned operations of three years. IUE functioned more or less like a regular ground based observatory save that the telescope operator and scientist do not actually visit the telescope, but send it commands from the ground. Other than some care in the selection of materials for filters, a UV telescope like IUE is very much like a regular visible light telescope.

In addition to IUE, there have been fairly important recent UV space missions. A reusable shuttle package called Astro has been flown twice in the cargo bay of the space shuttle: it consisted of a set of three UV telescopes. Unlike HST, the Astro UV telescopes had very large fields of view and so could take images of larger objects in the sky -- like galaxies. For instance, if the Hubble Space Telescope and the Astro telescopes were used to look at the Comet Hale-Bopp, Hubble would be able to take spectacular pictures of the core of the comet. The Astro telescopes would be able to take pictures of the entire comet, core and tail.

artist concept of EUVE

Extreme Ultraviolet observatories

There are two extreme ultraviolet observatories in space at the moment. One of them is the very first extreme ultraviolet observatory ever, the ExtremeUltraviolet Explorer (EUVE) ( Astronomers have been somewhat reluctant to explore from space at the extreme ultraviolet wavelengths since all theory strongly suggests that the interstellar medium (the tiny traces of gases and dust between the stars) would absorb radiation in this portion of the spectrum. However, when the Extreme Ultraviolet Explorer (EUVE) was launched, observations showed that the solar system is located within a bubble in the local interstellar medium. The region around the Sun is relatively sparse of gas and dust which allows the EUVE instruments to see much further than theory predicted.

Another extreme ultraviolet observatory currently operating is the Array of Low Energy X-ray Imaging Sensors (ALEXIS) ( Although its name indicates that it is a X-ray observatory, the range of energy ALEXIS is exploring is at the very lowest end of the X-ray spectrum and often considered to be extreme ultraviolet. ALEXIS was launched in 1995 on a Pegasus XL launch vehicle, but a failure during the launch tore off one of the solar panels and the satellite is tumbling through space. Nonetheless, the ALEXIS operations team has been able to use the tumbling of the satellite to map out the entire sky.

artist concept of RXTE

X-ray observatories

There are several X-ray observatories currently operating in space with many more to be launched in the next few years.

The Rossi X-ray Timing Explorer (RXTE) was launched on December 30, 1995. RXTE is able to make very precise timing measurements of X-ray objects, particularly those which show patterns in their X-ray emissions over very short time periods, such as certain neutron star systems and pulsars.

Other X-ray observatories currently operating in space include the Advanced Satellite for Cosmology and Astrophysics (ASCA), a joint U.S.-Japan venture; the Kvant astrophysics module attached to the Russian space station Mir, and SAX, an Italian X-ray satellite. The most recent addition to these X-ray observatories is NASA's Chandra X-ray Observatory (CXO), launched from the space shuttle in July, 1999.

artist concept of GRO

Gamma-ray observatories

The Compton Gamma-Ray Observatory (CGRO) was launched by the space shuttle in April 1991. The observatory's instruments are dedicated to observing the gamma-ray sky, including locating gamma-ray burst sources, monitoring solar flares, and other highly energetic astrophysical phenomenon. An unexpected discovery which Compton has made was the observation of gamma-ray burst events coming from the Earth itself at the top of thunderstorm systems. The cause of this phenomenon is not known, but it is currently suspected to be related to "Sprites": lightening flashes which are occasionally seen jumping upwards from cloud tops to the upper stratosphere.

The Russian gamma-ray observatory Granat has exhausted its control fuel. Its last maneuver was to initiate a roll which has allowed it to perform a continuous all-sky survey.

The next major gamma-ray mission in the near future include the joint U.S.-Russia missions Spectrum X-Gamma ( which will make pointed observations in both X-ray and gamma-ray wavelength regimes.

Imagine the Universe is a service of the High Energy Astrophysics Science Archive Research Center (HEASARC), Dr. Alan Smale (Director), within the Astrophysics Science Division (ASD) at NASA's Goddard Space Flight Center.

The Imagine Team
Acting Project Leader: Dr. Barbara Mattson
All material on this site has been created and updated between 1997-2012.

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