Spectroscopy in Space

Why is it important to perform spectroscopy in different portions of the EM spectrum? Well, there are many answers to this question. Basically the reason is that the optical window is so tiny that we are just missing a lot of the action! Surveys of the sky at different wavelengths have shown us that there are many objects in the Universe that emit very little optical light but are extremely bright at, say, infrared or X-ray wavelengths. In particular, it takes a lot of energy to create X-rays and gamma rays, and observing objects in these spectral regions tells us about some of the most energetic phenomena in the Universe. Optical spectra just don't give us the whole picture.

Also, the spectral lines of different chemical elements are concentrated in different portions of the spectrum. Elements such as carbon, silicon, and magnesium, for instance, have few if any spectral lines in the optical, but have numerous spectral lines in the ultraviolet. Hence, to determine the abundances of these elements ultraviolet spectra are needed. It's the same with other spectral regions, too.

It is only over the last 40 years that we have had the ability to lift telescopes (and spectrographs!) above the atmosphere, which has been akin to opening entirely new windows on the Universe. Here we highlight some of the many space missions that have had (or will have) spectroscopy as a primary activity. These descriptions are necessarily brief, but we provide links to Web sites where more detailed information can be found.

The International Ultraviolet Explorer (IUE) satellite

IUE has been called "the little satellite that could." This astronomical workhorse was launched in 1978 for a nominal three year mission, but has put in more than 18 years of service to date, obtaining over 100,000 high and low resolution ultraviolet spectra of thousands of celestial objects. IUE operates over the 1200 - 3200 Angstrom region of the near and far ultraviolet.

The Space Telescope Imaging Spectrograph, the Faint Object Spectrograph (FOS) and the Goddard High Resolution Spectrograph (GHRS), all instruments that have been or are currently on the Hubble Space Telescope

Although Hubble's public persona is one of taking spectracular pictures (which it certainly does!), in reality several of its major scientific instruments are spectrographs. The STIS is the current "workhorse" spectrograph on Hubble, replacing the FOS and GHRS instruments which were originally launched with Hubble in 1990. The above link takes you to the hubble "Instruments" web page where you can get more information. (However, this is mostly technical information!)

The Copernicus Satellite (OAO-3)

The third Orbiting Astronomical Observatory, nicknamed Copernicus, was a high-resolution spectrograph launched in 1972 that observed in the far-UV (900-1200 Angstroms). Copernicus observations demonstrated how rich this tiny spectral region was in "astronomical information," but was only sensitive enough to look at bright, hot stars relatively near the sun. A satellite is now in development by NASA that will observe in this same spectral range, but with much higher sensitivity. (See FUSE below.)

The Hopkins Ultraviolet Telescope (HUT)

The Hopkins Ultraviolet Telescope has flown two missions on the Space Shuttle, and was used directly from the payload bay as part of a package of telescopes called the Astro Observatory. HUT is a 36-inch (0.9 meter) telescope with a far-ultraviolet spectrograph attached that operated in the 850 - 1850 Angstrom region. Hence, although HUT overlapped with other telescopes such as IUE and Hubble, its spectral coverage extended to shorter wavelengths. HUT observed several hundred objects of all kinds, effectively opening the spectral window from 900 to 1200 Angstroms to general observations for the first time, but at low spectral resolution compared with Copernicus (see above). The HUT link connects you to both general and technical information, as well as some marvelous earth and shuttle pictures taken during the two Astro missions!

The Extreme Ultraviolet Explorer (EUVE) Satellite

EUVE was launched in 1991 and is still in operation. It was the first and only satellite launched to study the extreme ultraviolet spectral region, which is mostly cloaked by absorption from interstellar hydrogen in our galaxy. EUVE found, however, that a larger number of objects were observable in this region than expected. Spectra by EUVE have led to new insights about white dwarf stars, cool stars like the sun, and certain types of binary stars.

The Far Ultraviolet Spectroscopic Explorer (FUSE) Satellite

FUSE is a telescope and far-ultraviolet spectrograph that has been in the planning stages for over a decade and is now being developed for launch in May 1999. This instrument will observe a small region in the far ultraviolet (from about 900 - 1180 Angstroms), but at very high spectral resolution. This spectral region contains many spectral lines of interest to astronomers, and the high resolution will permit FUSE to study this spectral region in great detail for many kinds of objects.

The Einstein X-ray Observatory (HEAO-2)

The Einstein Observatory was the second in a series of High Energy Astrophysical Observatory satellites. Launched in 1978, it operated for 3 years and revolutionized the field of X-ray astronomy. Einstein included several imaging and spectroscopic X-ray instruments.

The Advanced Satellite for Composition and Astronomy (ASCA)

This X-ray satellite is a joint venture between NASA and the Japanese agency NASDA. Launched in 1994, it has a broader spectral coverage, higher spectral resolution, and greater sensitivity than the Einstein Observatory had, and is breaking much new ground in our understanding of the X-ray universe.

The Chandra X-ray Observatory (previously called AXAF)

The Chandra Observatory could be called the "Hubble Telescope of the X-ray region." This satellite, to be launched in late 1999, will revolutionize X-ray astronomy the way Hubble did for the UV/optical regions. Also like Hubble, it will combine X-ray imaging and spectroscopy in one satellite.

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Bill Blair ([email protected])