NASA's Hubble Space Telescope is providing a daily bonanza of images and data for astronomers
Tim Furniss/LONDON
NASA has scheduled another Space Shuttle servicing mission to the Hubble Space Telescope (HST) for 1997. The STS82/Discovery mission, scheduled for launch in February 1997, is the second of five planned to extend the life of the HST to 2005 or beyond.
Such were the heroics of the first servicing mission, the STS61/Endeavour, in December 1993, that this second mission is likely to be regarded as a routine "oil change" job, despite it being a major undertaking.
The STS61 was dubbed, perhaps unfairly, as a rescue, rather than a servicing mission, after the discovery that the HST's primary mirror was suffering from a spherical aberration because it had been manufactured to the wrong shape. Its first images had fallen short of the resolution predicted (Flight International, 24-30 November, 1993) after the HST had been deployed in orbit in April 1990 by the STS31/Discovery.
Compared with the desired profile-versus -radius, the mirror surface is too low by an amount that grows from zero to 0.002mm at the outer edge. This minute aberration meant than several of the HST's instruments were not able to perform to their predicted image quality.
Despite the fact that the HST was already sending back spectacular images for astronomers, the planned STS61 servicing mission was transformed to a major repair effort, particularly as the HST solar panels needed replacing earlier than expected. This was because they suffered from the "jitters", induced by the effect of temperature variations in orbit on the panels' booms.
During six unprecedented STS61 spacewalks, astronauts fixed a pair of "spectacles" on the HST and replaced or restored an array of other equipment. The "spectacles", called the Corrective Optics Space Telescope Axial Replacement (COSTAR), were actually an intricate set of ten small mirrors which re-focused light from the primary mirror.
The Ball Aerospace-built COSTAR restored the scientific potential of three instruments - the Goddard High Resolution Spectrograph, a Faint Object Spectrograph and the European Space Agency's Faint Object Camera. The COSTAR may have to be replaced during the STS82 mission.
The STS61 astronauts also changed the solar panels and installed as planned a new wide field and planetary camera, called the WFPC 2. This is an autonomous system which does not benefit from the COSTAR, as it already had corrective optics installed.
Although all the HST instruments are returning excellent data and images, it is the WFPC 2 which is the star of the show, as illustrated by the four photographs shown.
Two cameras in one
The WFPC 2 is actually two cameras in one. It can be operated in either "wide-field" (rather a misnomer) or planetary mode. It is sensitive to light wavelengths from 1,150A (ultraviolet) to 11,000A (infra red) and it can photograph an area no larger than 2.6 arc/min across, or less than one-tenth of the diameter of the Moon, thus trading field-of-view for greater resolving power.
The camera can be used to photograph an area as small as 1.1 arc/min across, but can resolve detail on objects, or separations between objects, only 0.043 arc/sec across. The WFPC directs light gathered by the HST on to an optical pyramid of four mirrors, which can direct the light on to four charged-coupled devices, which are sensitive silicon detectors.
During the third HST servicing mission, planned for November 1999, the WFPC 2, or another instrument, will be replaced by the Hubble Advanced Camera for Exploration (HACE). Also built by Ball Aerospace, the $30 million HACE will be a major enhancement of the WFPC2, promising spectacular sights.
The violent birth
Compared with a view taken by an Earth-bound telescope of the NGC starburst spiral galaxy, 8 million light years away in the constellation of the Sculptor the WFPC 2 takes astronomers into its very core, revealing a violent star formation within a region 1,000 light years across. A starburst galaxy has an exceptionally high rate of star birth, first revealed by its excess of infra-red radiation from warm dust.
WFPC 2's view shows luminous star clusters, dust lanes which trace regions of dense gas, and filaments of glowing gas. The HST reveals several regions of intense star formation, which include a bright, super-compact star cluster. (This image was processed by NASA, the University of Wisconsin and Lowell Observatory, and released via the Space Telescope Science Institute, by Carnegie Institutions of Washington DC).
The Saturn storm
The WFPC has captured images of most of the planets of the Solar System, including this one of Saturn, featuring a rare storm appearing as a white arrowhead-shaped feature near the equator. The storm is generated by an upwelling of warmer air, similar to a thunderhead on an Earth thunderstorm, and consists of ammonia ice crystals which form when an upward flow of warmer gases pushes its way through the planet's frigid cloud tops. The east-west extent of the storm is equal to the diameter of the Earth. The picture was taken when Saturn was 1,446 million km from the Earth and is a composite of images taken through different colour filters within a 6min period, to create a true-colour rendition. (This image was processed by New Mexico State University, the Space Telescope Science Institute and NASA and released by the Space Telescope Science Institute).
The shock wave
The constellation of Cygnus, the Swan, is sometimes called the Northern Cross and contains the Veil nebula, NGC 6992, which is part of the so-called Cynus Loop, the remnants of a supernova (exploding star), which astronomers calculate blew up 15,000 years ago. The Cynus Loop covers an area in the sky which is six times the diameter of the Moon as seen in the night sky. It is an expanding blastwave from the stellar cataclysm. Here part of the blastwave has recently hit a cloud of denser-than-average interstallar gas and the collision has driven shock waves into the cloud which heats interstellar gas, causing it to glow.
The colour is produced by a composite of three different images. Blue shows emission from doubly-ionized oxygen atoms, produced by the heat behind the shock wave. Red shows light given off by singly-ionized sulphur atoms. This emission arises well behind the shock wave. Green shows light emitted by hydrogen atoms. Much of the hydrogen emission comes from an extremely thin zone (only several times the distance between the Earth and the Sun) immediately behind the shock wave itself. These thin regions appear as sharp, green filaments. (This image was processed by the Arizona State University and NASA and was released by the Space Telescope Science Institute.)
The tale of two clusters
The WFPC 2 views rich detail of star birth regions, only previously seen in our own galaxy, in a closely knit pair of star clusters, 166,000 light years away in the Large Magellanic Cloud (LMC) in the southern constellation of Doradus. The field-of-view is 130 light years across. The image includes almost 10,000 stars down to 25th magnitude, which is more than twice as many as can be seen over the entire sky seen by the naked eye on a clear night on the Earth. Earth-based observations of this region resolves less than 1,000 stars in the same region. About 60% of the stars belong to a dominant yellow cluster called NGC 1850, and are estimated to be 50 million years old. About 20% are massive white stars about four million years old. The remainder are field stars in the LMC. Although the clusters lie along the same line of sight, the younger, more open cluster lies 200 light years beyond the older. To observe two well-defined star populations separated by such a "small" gap of space is unusual. This juxtaposition suggests that supernova explosions in the older cluster might have triggered the birth of the younger cluster. (The image was processed by the Space Telescope Science Institute, the European Space Agency, Jet Propulsion Laboratory and NASA and released by the Space Telescope Science Institute).
Source: Flight International
