Hubble in free orbit

Putting a telescope in space is one way of avoiding this problem. As well as collecting visible light from its orbit high above the atmosphere, the HST also observes the infrared and ultraviolet wavelengths that are completely filtered out by the atmosphere.

The HST has made some of the most dramatic discoveries in the history of astronomy. From its vantage point 600 kilometres above Earth, HST can detect light with 'eyes' five times sharper than the best ground-based telescopes.
 
 
Spacecraft
 
At the heart of HST are a 2.4-metre primary mirror and a collection of four science instruments that work across the entire optical spectrum - from infrared, through the visible, to ultraviolet light. There is one camera, two combined camera/spectrographs and a set of fine guidance sensors. HST was designed to be serviced in space, thus also allowing outdated instruments to be replaced. The telescope was placed in a low-Earth orbit by the Space Shuttle (600 kilometre altitude) and uses modular components so that it could be recovered on subsequent Shuttle missions and faulty or outdated parts more easily replaced before being re-released into orbit.

Power for the computers and scientific instruments onboard is provided by two 2.6 x 7.1 metre solar panels. The power generated by the arrays is also used to charge six nickel-hydrogen batteries that provide power to the spacecraft for about 25 minutes per orbit while HST flies through Earth's shadow.

The telescope uses an elaborate system of attitude controls to improve its stability during observations. A system of reaction wheels manoeuvre the telescope into place and its position in space is monitored by gyroscopes. Fine Guidance Sensors are used to lock onto guide stars to ensure the extremely high pointing accuracy needed to make very accurate observations.
 
 
Journey
 
HST was deployed by the Space Shuttle Discovery (STS-31) into a circular orbit 600 kilometres above the ground, inclined at 28.5 degrees to the equator. The time for one orbit is between 96 and 97 minutes.
 
 
History
 
The idea of sending a telescope into space was first proposed long before the first satellites were launched. German rocket scientist Herman Oberth suggested a space-bound telescope as early as 1923 in his book ‘Die Rakete zu den Planeträumen’.

It took many years before technology caught up with Oberth’s idea. The American Lyman Spitzer proposed a more realistic plan for a space telescope in 1946 and lobbied for his idea for almost 30 years.

In the 1970s, NASA and ESA took up the idea and proposed a 3-metre space telescope. Funding began to flow in 1977 and it was decided to name the telescope after Edwin Powell Hubble who had discovered the expansion of the Universe in the 1920s. Although the Hubble Space Telescope (HST) was down-sized to 2.4 metres, the project started to attract significant attention from astronomers.

The precision-ground mirror was finished in 1981 and the assembly of the entire spacecraft was completed in 1985. The plan called for a launch on NASA’s Space Shuttle in 1986 but, just months before the scheduled launch, the Challenger disaster caused a year-long delay of the entire Shuttle programme.

Hubble was finally launched in 1990 and the tension built up as astronomers examined the first images through Hubble’s eyes. It was soon realised that Hubble’s mirror had a serious flaw. A focusing defect prevented Hubble from taking sharp images – the mirror edge was too flat by a mere fiftieth of the width of a human hair. Over the next months scientists and engineers from NASA and ESA worked together and came up with a superb corrective optics package that would restore Hubble’s eyesight completely.

A crew of astronauts carried out the repairs necessary to restore the telescope to its intended level of performance during the first Hubble Servicing Mission (SM1) in December 1993.

Although the two subsequent servicing missions were at least as demanding in terms of complexity and workload, SM1 captured the attention of both astronomers and the public at large to a degree that no other Shuttle mission since has achieved. Meticulously planned and brilliantly executed, the mission succeeded on all counts. It will go down in history as one of the highlights of human spaceflight. Hubble was back in business.

Since SM1, three other Servicing Missions have been carried out: during SM2 in 1997 two new instruments were installed, in SM3A, 1999, many of Hubble’s crucial technical systems were exchanged, and in 2002 came SM3B when Hubble again got new science instruments.

Two further milestones are currently planned, the next in 2005. Servicing Mission 4 is scheduled to upgrade Hubble’s scientific capabilities again.

In October 2009, the mission was extended until end 2012.
 
 
Partnerships
 
NASA is ESA’s partner for the HST. ESA has a nominal 15% stake in the mission and has, among other things, provided the Faint Object Camera, the first two solar panels that powered the spacecraft and a team of space scientists and engineers at the Space Telescope Science Institute in Baltimore, United States.

Astronomers employed by ESA and the European Southern Observatory at the Space Telescope-European Coordinating Facility work with various aspects of HST in Munich, Germany, including the calibration of HST’s instruments and public outreach. Europe's contribution to HST entitles European astronomers to 15% of the telescope's observing time.
 
 
Last update: 7 October 2009