The Hubble Space Telescope (HST) was the first and flagship mission of NASA's Great Observatories program. Designed to complement the wavelength capabilities of the other spacecraft in the program (CGRO, AXAF, and SIRTF), HST was a 2.4 m, f/24 Ritchey-Chretien telescope capable of performing observations in the visible, near-ultraviolet, and near-infrared (1150 A to 1 mm).
Placed into a low-earth orbit by the space shuttle, HST was designed to be modular so that on subsequent shuttle missions it could be recovered, have faulty or obsolete parts replaced with new and/or improved instruments, and be re-released. HST was roughly cylindrical in shape, 13.1 m end-to-end and 4.3 m in diameter at its widest point.
HST used an elaborate scheme for attitude control to improve the stability of the spacecraft during observations. Maneuvering was performed by four of six gyros, or reaction wheels. Pointing could be maintained in this mode (coarse track) or the Fine Guidance Sensors (FGSs) could be used to lock onto guide stars (fine lock) to reduce the spacecraft drift and increase the pointing accuracy.
Power to the two on-board computers and the scientific instruments was provided by two 2.4 x 12.1 m solar panels. The power generated by the arrays was also used to charge six nickel-hydrogen batteries which provided power to the spacecraft during the roughly 25 minutes per orbit in which HST was within the Earth's shadow.
Communications with the satellite were maintained with the TDRS satellites. Observations taken during the time when neither TDRS was visible from the spacecraft were recorded on tape recorder and dumped during periods of visibility. The spacecraft also supported real-time interactions with the ground system during times of TDRS visibility, enabling observers to make small offsets in the spacecraft pointing to perform their observations. HST was the first scientific spacecraft designed to utilize the full capabilities of TDRSS, communicating over either multiple-access or single-access channels at any of the supported transmission rates.
HST was operated in three distinct phases. During the first phase of the mission (Orbital Verification or OV), responsibility for the spacecraft was given to Marshall Space Center. OV consisted of an extended, eight-month checkout of the spacecraft, including test of the on-board computers, pointing control system, solar arrays, etc. This phase was followed by the Science Verification (SV) phase, lasting nearly another year, during which each of the six science instruments was tested to verify their capabilities and set limits on their safe operations during the remainder of the mission. Responsibility for the spacecraft during SV was given to Goddard Space Flight Center. The last phase of the mission, known as the General Observer (GO) phase, was planned to last from the end of SV through the end of the mission and was the responsibility of the Space Telescope Science Institute. General observations were phased in gradually, however, during the SV phase because the OV and SV portions of the mission were considerably longer than expected prior to deployment. HST is now in the 11th year of GO operations.
The mission was troubled soon after launch by the discovery that the primary mirror was spherically aberrated. In addition, problems with the solar panels flexing as the spacecraft passed from the Earth's shadow into sunlight caused problems with the pointing stability. Steps were taken to correct these problems, including replacement of the solar panels, replacement of the Wide Field and Planetary Camera with a second-generation version with built-in corrective optics, and replacement of the High-Speed Photometer with COSTAR (Corrective Optics Space Telescope Axial Replacement) to correct the aberration for the remaining instruments.
The following are some of the key science results from HST:
Frosty white water ice clouds and swirling orange dust storms above a vivid rusty landscape reveal Mars as a dynamic planet in this sharpest view ever obtained by an Earth-based telescope. Hubble Space Telescope took the picture on June 26, when Mars was approximately 43 million miles (68 million km) from Earth -- the closest Mars has ever been to Earth since 1988. Hubble can see details as small as 10 miles (16 km) across. The colors have been carefully balanced to give a realistic view of Mars' hues as they might appear through a telescope. Especially striking is the large amount of seasonal dust storm activity seen in this image. One large storm system is churning high above the northern polar cap [top of image], and a smaller dust storm cloud can be seen nearby. Another large dust storm is spilling out of the giant Hellas impact basin in the Southern Hemisphere [lower right].
The Horsehead, also known as Barnard 33, is a cold, dark cloud of gas and dust, silhouetted against the bright nebula, IC 434. The bright area at the top left edge is a young star still embedded in its nursery of gas and dust. But radiation from this hot star is eroding the stellar nursery. The top of the nebula also is being sculpted by radiation from a massive star located out of Hubble's field of view. Only by chance does the nebula roughly resemble the head of a horse. Its unusual shape was first discovered on a photographic plate in the late 1800s. Located in the constellation Orion, the Horsehead is a cousin of the famous pillars of dust and gas known as the Eagle nebula. Both tower-like nebulas are cocoons of young stars. The Horsehead nebula lies just south of the bright star Zeta Orionis, which is easily visible to the unaided eye as the left-hand star in the line of three that form Orion's Belt. Amateur astronomers often use the Horsehead as a test of their observing skills; it is known as one of the more difficult objects to see visually in an amateur-sized telescope.
NGC 3132 is a striking example of a planetary nebula. This expanding cloud of gas, surrounding a dying star, is known to amateur astronomers in the southern hemisphere as the "Eight-Burst" or the "Southern Ring" Nebula. The name "planetary nebula" refers only to the round shape that many of these objects show when examined through a small visual telescope. In reality, these nebulae have little or nothing to do with planets, but are instead huge shells of gas ejected by stars as they near the ends of their lifetimes. NGC 3132 is nearly half a light year in diameter, and at a distance of about 2000 light years is one of the nearer known planetary nebulae. The gases are expanding away from the central star at a speed of 9 miles per second. This image clearly shows two stars near the center of the nebula, a bright white one, and an adjacent, fainter companion to its upper right. (A third, unrelated star lies near the edge of the nebula.) The faint partner is actually the star that has ejected the nebula. This star is now smaller than our own Sun, but extremely hot. The flood of ultraviolet radiation from its surface makes the surrounding gases glow through fluorescence. The brighter star is in an earlier stage of stellar evolution, but in the future it will.
This stellar swarm is M80 (NGC 6093), one of the densest of the 147 known globular star clusters in the Milky Way galaxy. Located about 28,000 light-years from Earth, M80 contains hundreds of thousands of stars, all held together by their mutual gravitational attraction. Globular clusters are particularly useful for studying stellar evolution, since all of the stars in the cluster have the same age (about 15 billion years), but cover a range of stellar masses. Every star visible in this image is either more highly evolved than, or in a few rare cases more massive than, our own Sun. Especially obvious are the bright red giants, which are stars similar to the Sun in mass that are nearing the ends of their lives.
M51, also known as NGC 5194, is having a close encounter with a nearby companion galaxy, NGC 5195, just off the upper edge of this image. The companion's gravitational pull is triggering star formation in the main galaxy, as seen in brilliant detail by numerous, luminous clusters of young and energetic stars. The bright clusters are highlighted in red by their associated emission from glowing hydrogen gas. This Wide Field Planetary Camera 2 image enables a research group to clearly define the structure of both the cold dust clouds and the hot hydrogen and link individual clusters to their parent dust clouds. Intricate structure is also seen for the first time in the dust clouds. Along the spiral arms, dust "spurs" are seen branching out almost perpendicular to the main spiral arms. The regularity and large number of these features suggests to astronomers that previous models of "two-arm" spiral galaxies may need to be revisited. The new images also reveal a dust disk in the nucleus, which may provide fuel for a nuclear black hole.
HST's greatest talent is its ability to see at finer resolution than any ground-based telescope.
This allows for imaging of individual stars in nearby galaxies, a huge boom to understanding the size and nature of the Universe.
The above image is a distant cluster of galaxies surrounding a quasar. The top panel is the region as observed by ground-based telescopes, the bottom panel is the same region from HST. Although not a large telescope, HST has the ability to concentrate more light into smaller pixels, allowing astronomers to see extremely distant objects.
Webb Space Telescope:
The James Webb Space Telescope (JWST) is a planned space telescope optimized for observations in the infrared, and a scientific successor to the Hubble Space Telescope and the Spitzer Space Telescope. The main technical features are a large and very cold 6.5-meter (21 ft) diameter mirror, an observing position far from Earth, orbiting the Earth-Sun L2 point, and four specialized instruments. The combination of these features will give JWST unprecedented resolution and sensitivity from long-wavelength visible to the mid-infrared, enabling its two main scientific goals studying the birth and evolution of galaxies, and the formation of stars and planets.
In planning since 1996, the project represents an international collaboration of about 17 countries led by NASA, and with significant contributions from the European Space Agency and the Canadian Space Agency. It is named after James E. Webb, the second administrator of NASA, who played an integral role in the Apollo program.
JWST's capabilities will enable a broad range of investigations across many subfields of astronomy. One particular goal involves observing some of the most distant objects in the Universe, beyond the reach of current ground and space based instruments. This includes the very first stars, the epoch of reionization, and the formation of the first galaxies. Another goal is understanding the formation of stars and planets. This will include imaging molecular clouds and star-forming clusters, studying the debris disks around stars, direct imaging of planets, and spectroscopic examination of planetary transits.