Telescopes, Space. Selected Space Telescope Missions

The ability to place astronomical telescopes and other detectors in space has given rise to the term ‘‘space astronomy,’’ which describes astronomy performed from space rather than earthbound observatories. Its main advantage over terrestrial astronomy is its elimination of the deleterious effects of the earth’s atmosphere, which not only absorbs some wavelengths of radiation but also distorts the images of stars and other objects of interest.

Indeed, astronomy at some wavelengths (e.g., far infrared, submillimeter and x-ray astronomy) has advanced only through space astronomy, simply because the radiation is absorbed by the earth’s atmosphere.

Although the term ‘‘space telescope’’ may refer to a module or payload on a space station, it is more commonly applied to a dedicated astronomical satellite or space observatory. There have been literally hundreds of space astronomy missions, observing in a wide variety of wavelengths from the radio end of the spectrum to x-rays and gamma rays.

The first space-based observatory was the National Aeronautics and Space Administration’s (NASA’s) Orbiting Solar Observatory, OSO-1, which was launched in March 1962 to expand on the work of the Explorer spacecraft series. OSO-1 operated for almost two years, during which time it transmitted data on more than 140 solar flares. The data collected from its 13 experiments was tape- recorded on board the satellite and transmitted to earth in a 5-minute period on each orbit (a communications method known as store-and- forward).

Previous science spacecraft had been designed as individual instrument carriers with no attempt at standardization, but the nine OSO spacecraft were based on a standardized observatory platform on which a variety of different payloads could be mounted—a design method that has become the norm for most types of spacecraft. Thus, although OSO-1 weighed only 206 kilograms at launch, by the time OSO-8 was launched in 1975, the standardized platform was supporting a spacecraft weighing 1052 kilograms.

The OSO platform was spin-stabilized by rotating the base section, using nitrogen thrusters known as ‘‘gas jets,’’ while the upper section, which contained the pointing-dependent part of the astronomical payload, remained pointing towards the sun. The spacecraft’s spin axis was kept perpendicular to the solar vector by magneto-torquer coils in the base section, which aligned themselves, and thus the spacecraft, with the earth’s magnetic field. Meanwhile, a gyroscope in the upper section acted as a memory to ensure that the spacecraft acquired the sun’s light quickly on each orbit after emerging from the earth’s shadow.

The early OSO launches were followed in 1968 by NASA’s Orbiting Astronomical Observatory (OAO), which carried no less than eleven telescopes, enabling it to observe stars in the infrared, ultraviolet, x-ray and gamma-ray parts of the spectrum. Telescope mirrors up to 96 centimeter in diameter could be mounted inside the satellite’s cylindrical core.

Having realized that a spin- stabilized platform could not provide the stability required for accurate astronomical observations, a three-axis stabilized platform was designed for OAO. Its attitude was controlled by a number of rotating wheels mounted inside the spacecraft and aligned with each of its three axes, a type of stabilization system now used for the majority of manned and unmanned spacecraft.

The best-known space observatory is NASA’s Hubble space telescope (HST), an optical telescope of the Cassegrain type named after the American astronomer Edwin Powell Hubble. It incorporates a number of camera and spectrometer payloads tuned to various frequency bands, and was designed to be launched to, serviced in, and eventually retrieved from, low-earth orbit by the Space Shuttle. Some indication of the progress made between the OSO series and the HST is provided by the increase in mass: the HST weighed some 11,250 kilograms at the time of its launch in April 1990.

The pointing accuracy of space telescopes has also increased markedly since OSO-1, which was accurate only to about 1 arc-minute. OSO-8, for example, equaled the arc-second accuracy of terrestrial telescopes and the three-axis stabilized OAO series reached 0.03 arcsecond. For comparison, the HST has a pointing stability of 0.007 arcseconds and the European Space Agency’s (ESA’s) Hipparcos astrometry satellite has an incredible 0.001 arcsecond stability.

Prior to launch, the capabilities of the HST’s revolutionary payload were well publicized. Its 800-kilogram, 2.4-meter-diameter primary mirror, for example, would enable the telescope to resolve something the size of a small coin from a distance of 20 kilometers and detect the light of a firefly from about 16,000 kilometers. Unfortunately, it was discovered after launch that the mirror had been ground incorrectly as a result of a measurement error, to the extent that it was 2 micrometers (0.002 millimeters) too flat at the edges.

A servicing mission to correct the mirror’s spherical aberration was conducted in December 1993, replacing one of HST’s five main payloads with an optical instrument known as COSTAR (corrective optics space telescope axial replacement), which deployed five pairs of small corrective mirrors between the primary mirror and three of the remaining payloads.

Subsequent servicing missions have changed or upgraded the original payloads and HST has made many successful observations and discoveries, including the derivation of a new value for the Hubble constant (the constant of proportionality between the recession speeds of galaxies and their distances from each other) and thus an improved estimate of the age of the universe. In fact, the HST is so popular among astronomers that observing time, which has to be booked in advance, is many times oversubscribed.

Table 1. Selected Space Telescope Missions

A selection of some of the more recent space- based telescopes is provided in Table 1. At the time of writing, several advanced missions are being planned by space agencies, including NASA’s Next Generation Space Telescope, the European Space Agency’s Integral gamma ray source mapper, and a number of infrared telescopes.