The Hubble Space Telescope

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The first large orbiting telescope to study the Universe in visible light, the Hubble Space Telescope (HST) has revolutionized our view of the cosmos, answering some key questions in modern astronomy and raising new ones.

Astronomers have long been aware of the benefits of a telescope in space. Visible light is one of the few types of radiation that pass through our atmosphere relatively unscathed, but its passage through the air to even the highest mountaintop observatories is still affected by blurring and distortion. A telescope above the atmosphere would be immune to these problems. While Earthbound telescopes might have larger mirrors and therefore greater light grasp (the theoretical ability to see fainter objects), an orbiting observatory would see everything in pin-sharp detail.

The orbiting Hubble Space Telescope was championed by Lyman Spitzer (see panel, right). It finally received funding in 1977 and was scheduled for an October 1986 launch when tragedy struck the Space Shuttle Challenger (see pp.202-203). Although the remaining Shuttles returned to flight in late 1988, a slot could not be found to deploy the HST until Discovery's STS-31 mission of April 1990.

Teething troubles

The optical design of the HST was similar to many Earthbound telescopes, using a series of mirrors to bend light to a focus behind the large primary mirror, in any of four instrument modules. The initial instrument suite incorporated five instruments (two cameras, two spectrographs for analyzing light, and aperture door ^^

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BIOGRAPHY

solar panels assembly instrument module secondary assembly

radio antenna

BIOGRAPHY

ten a photometer for measuring the precise brightness of objects). The HST was always designed for a long life, with occasional visits from the Shuttle to perform repairs and install new instruments. This was just as well, because when scientists at Baltimore's Space Telescope Science Institute began to put the telescope through its paces, they discovered that the primary mirror had a minute flaw in its shape - and as a result, all Hubble's images were blurred.

While the HST was not entirely crippled by the problem, it was a huge embarrassment. Fortunately, a way was found to compensate for Hubble's short sight (see panel, opposite). A rescue mission in December 1993 was an outstanding success, paving the way for three more servicing missions.

Since the repair, the HST has been one of the world's most productive scientific instruments, making countless observations and helping to solve many conundrums about the Universe. It has also been a source of good publicity for NASA, its beautiful, spectacular, and sometimes profound images frequently making the headlines. Such is the telescope's popularity that in 2006, NASA gave in to public pressure and lifted its own ban on non-ISS Shuttle flights, scheduling one final servicing mission for 2008. This may allow the HST to run until its replacement is ready in the mid-2010s.

ANATOMY OF A SPACE TELESCOPE

Light entering the HST is reflected off a convex primary mirror, which bounces rays back on converging paths to a smaller secondary mirror. From here, it is folded back again, through a hole in the centre of the primary and brought to a focus at the instruments.

RELEASED IN ORBIT

Discovery's robot arm delicately places the HST in orbit 612km (380 miles) above the Earth during initial deployment in April 1990. Placing the telescope well above the atmosphere meant taking the Shuttle to an unusually high altitude.

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TECHNOLOGY

FIXING HUBBLE

Since there was no way to replace or repair Hubble's original primary mirror in orbit, engineers and scientists devised an optical box of tricks called the Corrective Optics Space Telescope Axial Replacement (COSTAR). This used ten precisely ground mirrors to adjust the path of light through the telescope and bring it into focus. It was built to slot into one of Hubble's existing instrument bays, diverting corrected light back into any of the other three. With COSTAR in place, the improvement in Hubble's images was spectacular.

After correction

Before correction

LOOKING BACK IN TIME

The Hubble "Deep Fields" are among the HST's most iconic images - panoramas produced by pointing the telescope at the same patch of sky for several days. They show galoxies so distant that their light started its journey to us billions of years ago.

HUBBLE HIGHLIGHTS

In well over a decade of operation, the Hubble Space Telescope has captured some of the most spectacular images of the cosmos ever seen. These distant vistas show objects and events that our spacecraft and probes may never reach, yet with Hubble's help, we can still explore them.

The Crab Nebula (left) is all that remains of a once-brilliant star - it detonated in a supernova explosion almost a thousand years ago, and since then its shredded remnants have been expanding outwards to form a gas bubble in space. Another type of bubble is the Cat's Eye Nebula (right) - the outer layers of a less violent star that is nearing the end of its life and blowing most of its material away into space. Much further away lie colliding galaxies such os the Antennae (above). The HST's snapshots capture only an instant in a process that lasts millions of years and which will ultimately create new generations of stars.

June 2000

NASA scientists announce Chandra's discovery of rings in the Crab Nebula supernova remnant.

25 August 2003

The Spitzer Space Telescope is launched on a Delta rocket.

22 March 2005

NASA announces Spitzer's detection of light from planets around other stars.

September 2001

Scientists announce Chandra's detection of X-ray flares from a giant black hole at the centre of the Milky Way.

CHANDRA: X-RAYS

The Chondra X-ray Observatory detects high-energy radiation from objects such as supernova remnants and material being pulled into black holes.

SPITZER: INFRARED

The Spitzer Space Telescope studies infrared radiation, allowing it to peer through opaque gas and dust clouds and to detect relatively cold, dark material in our galaxy and beyond.

COMPTON: GAMMA RAYS

The CGRO measured radiation from some of the most violent events in the Universe, including supernova explosions and matter-antimotter collisions.

5 April 1991

The Compton Gamma Ray Observatory (CGRO) is deployed into low Earth orbit from the Space Shuttle Atlantis.

19 September 1994

CGRO makes the surprising discovery of gamma rays generated by Earth's thunderstorms.

23 July 1999

The Chandra X-ray Observatory is deployed from Columbia.

4 June 2000

For safety reasons, CGRO is deliberately taken out of orbit while NASA can still control its re-entry.

June 2000

NASA scientists announce Chandra's discovery of rings in the Crab Nebula supernova remnant.

Great observatories

CHANDRA: X-RAYS

The Chondra X-ray Observatory detects high-energy radiation from objects such as supernova remnants and material being pulled into black holes.

SPITZER: INFRARED

The Spitzer Space Telescope studies infrared radiation, allowing it to peer through opaque gas and dust clouds and to detect relatively cold, dark material in our galaxy and beyond.

COMPTON: GAMMA RAYS

The CGRO measured radiation from some of the most violent events in the Universe, including supernova explosions and matter-antimotter collisions.

September 2001

Scientists announce Chandra's detection of X-ray flares from a giant black hole at the centre of the Milky Way.

25 August 2003

The Spitzer Space Telescope is launched on a Delta rocket.

Since 1991, NASA has put a series of astrophysical instruments into orbit, complementing the Hubble Space Telescope (HST) with observations of the Universe made by collecting radiation from beyond the visible spectrum.

22 March 2005

NASA announces Spitzer's detection of light from planets around other stars.

In nine years of operation, CGRO detected more than 400 new gamma-ray sources (ten times the number previously known), associated with some of the most bizarre objects in the Universe. Despite this remarkable success, NASA deliberately brought the mission to an end in 2000. A failure in one of CGRO's three gyroscopic stabilisers meant that if another had failed ground controllers would have been unable to guide the massive spacecraft to re-entry over an unpopulated area.

X-ray observations

The Chandra X-ray Observatory, third of the Great Observatories, was launched in 1999. Named after Indian-American astrophysicist Subramanyan Chandrasekhar, Chandra studies the Universe at slightly less energetic radiations than CGRO. However, it too faced a problem bringing the radiation to a focus, since X-rays approaching a mirror head-on would pass straight through it.

Chandra uses an ingenious series of curved mirror sections nested one inside the other. X-rays striking the mirrors at a shallow angle ricochet off each in turn, eventually coming to a focus in the X-ray instrumentation. Chandra's cameras produce a much higher resolution than any previously put into orbit, allowing it to produce far better-defined images of X-ray sources.

HEAT SEEKER

The Spitzer Space Telescope combines a large telescope with three cryogenically 1 cooled instruments to study infrared radiation. The liquid-helium coolont slowly leaks into space, giving Spitzer a limited life, but it is currently expected to operote fo more than five years.

During the 1980s, many astronomers realized the benefits of a series of parallel observatories that could study the sky at different wavelengths. Optical (and near-ultraviolet) images from Hubble could then be complemented by simultaneous observations from other telescopes, revealing how the visible appearance of an object related to its changing properties in other wavelengths. This was the origin of the Great Observatories Program, a series of four satellites (including the HST) to observe the Universe in visible and near-ultraviolet light, gamma rays, X-rays, and the infrared.

The high-energy Universe

The Compton Gamma-Ray Observatory (CGRO) was launched in 1991 and set a new record for a Shuttle payload, weighing in at 17,000kg (37,400lb). Onboard were four instruments, each measuring different gamma-ray properties and "tuned" to detect gamma rays of different energies.

Gamma rays pass through most materials and so cannot be focused by reflectors in the same way as other radiations. In order to work out the source of rays, CGRO's imaging telescope used a unique solution - stacking layers of detectors on top of one another and measuring the sequence in which they recorded gamma rays passing through them. The telescope was named after Arthur Compton, the physicist who discovered the "Compton scattering" process on which all its detectors relied.

X-ray image Ultraviolet radiation Visible light (filtered) Visible light

Mid-infrared (Spitzer)

Far-infrared

Ground-based radio image

Mid-infrared (IRAS)

I TECHNOLOGY

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ORBITING RADIO TELESCOPES

This visible-light image shows the brightest parts of Kepler's Star, the shredded remains of a star that destroyed itself in a supernova explosion in 1604. Only a few glowing filaments are visible.

THE WHOLE STORY

A combined image using invisible rodiotion from X-rays through to the infrared reveals the true bubble-like structure of Kepler. Only the upper-right portion (shown in the picture on the left) shines in visible light.

X-ray image Ultraviolet radiation Visible light (filtered) Visible light

Mid-infrared (Spitzer)

Far-infrared

Ground-based radio image

Mid-infrared (IRAS)

SAME GALAXY, DIFFERENT WAVELENGTHS

The nearest major galaxy to our own, the Andromeda Galaxy (M31), has been studied at every possible wavelength, revealing an array of hidden features - many associated with ring-like dust clouds or the giant black hole at its core.

The cold end

The Spitzer Space Telescope, launched in 2003 and named in honour of Lyman Spitzer (see p.252), studies the opposite end of the energy spectrum from Chandra and CGRO - it is the most sophisticated telescope yet built for detecting infrared radiation. The 85cm (34in) telescope mirror focuses light onto three different infrared detectors. Spitzer is designed to peer through galactic dust clouds and look at dim objects such as the faintest stars, as well as planets in the process of formation.

One wavelength not studied by the Great Observatories was radio. Many radio waves reach the ground intact - the challenge is to create detailed images from them. The far longer wavelength of radio compared to light waves calls for much larger telescopes (such as the one shown here), but even the largest radio dish cannot compete with optical resolution. For this reason, signals from distant telescopes are combined in a technique called interferometry. In 1997, Japan launched HALCA, an orbiting receiver that created an interferometer larger than the Earth.

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Luna 16, the first robot probe to return samples of Moon rock, launches.

17 November 1970

Luna 17 releases its Lunokhod rover onto the Moon.

12 September 1970

Luna 16, the first robot probe to return samples of Moon rock, launches.

17 November 1970

Luna 17 releases its Lunokhod rover onto the Moon.

28 September 1971

Luna 19, the first in a new wave of Soviet orbiters, is launched.

21 February 1972

The Luna 20 sample-return mission lands in the Apollonius highlands.

19 August 1976

Luna 24's sample-return capsule blasts off from the Sea of Crises.

19 February 1994

Clementine enters orbit around the Moon.

6 January 1998

NASA launches its Lunar Prospector probe.

31 July 1999

Lunar Prospector is crashed into the Moon's south polar region.

27 September 2003

ESA launches its SMARM Moon probe.

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