Mission

Hubble in free orbit

The Hubble Space Telescope (HST), a collaboration between ESA and NASA, is a 2.4 m-diameter space telescope optimised to observe from the ultraviolet to the infrared. Launched in 1990 and designed to be refurbished in space by astronauts, Hubble is one of the greatest scientific projects of all time. Since launch, it has opened our eyes to the wonders of our ‘planetary’ backyard and beyond. In so many ways, Hubble has revolutionised modern astronomy, not only by being an efficient tool for making new discoveries, but also by changing the way astronomical research is done.

What's special?

Light can travel through the Universe virtually undisturbed for thousands of millions of years. However, if it happens to be heading for Earth, it must travel through our turbulent atmosphere in the last few microseconds of its journey before arriving at a telescope. To the casual observer this sets the stars twinkling in the night sky but to the astronomer it blurs out the fine cosmic details.

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, Hubble observes the infrared and ultraviolet wavelengths that are filtered out by the atmosphere.

Hubble has made some of the most dramatic discoveries in the history of astronomy. When it was launched, planets outside our Solar System (exoplanets) had not yet been observed. One of Hubble’s major achievements has been the characterisation of exoplanet atmospheres. It has made the first measurements of the composition of a planet atmosphere around another star (HD 209458), finding evidence of sodium, carbon and oxygen. Hubble has found evidence of methane in the atmosphere of another Jupiter-sized planet, HD 189733b. Very recently, it has directly imaged an exoplanet orbiting the nearby star Fomalhaut.

When Hubble was launched, it was also thought that expansion of the Universe would be slowing by now, as gravity acts to reduce the remnant velocity from the Big Bang. Instead, studying ‘Type Ia supernovae’ – stars that explode at the end of their life – as distance indicators, Hubble found that the speed with which the Universe is presently expanding had been increasing for the last several billion years. This points to the existence of ‘dark energy’, a force that comes to exceed gravity and causes the Universe’s expansion to accelerate.

Prior to Hubble, the presence of dust discs around a small number of young stars had been inferred from observations by infrared satellites, but only one disc, around Beta Pictoris, had been directly imaged with a ground-based coronagraph. For centuries it has been believed that such a disc must have been the precursor to our own Solar System. Hubble has revolutionised this area of science, showing at high resolution that in the Orion Nebula about half of the young stars are surrounded by gas and dust structures, many of which are clearly discs.

During its lifetime, Hubble has pushed back the observation boundaries of the Universe. In coordination with other observatories, Hubble has taken long exposures of small regions of sky, to bring out the most distant and most ancient galaxies. With the more sensitive ‘WFC3’ panchromatic camera, installed during the fifth servicing mission, Hubble has been able to see galaxies that formed just 600-800 million years after the Big Bang. These galaxies are now showing signs of a link to their origin from the first stars. They are so blue that they must be extremely deficient in heavy elements, thus representing a population that has nearly primordial characteristics.

Spacecraft

At the heart of HST is a 2.4 m-diameter primary mirror. This supplies light to a collection of five science instruments that work across the entire optical spectrum: from infrared, through the visible, to ultraviolet light.

It has three cameras, two spectrographs and a set of Fine Guidance Sensors that allow Hubble to accurately point to targets on the sky. HST was placed in a low orbit and was designed to be serviced in space by astronauts on the Space Shuttle, thus allowing instruments to be replaced as technology improved, and observatory subsystems to be repaired and modernised.

Power for the computers and scientific instruments is provided by two solar wings. The solar wings also charge six nickel-hydrogen batteries that power the spacecraft for about 25 minutes per orbit while it flies through Earth's shadow.

The telescope uses an elaborate system of attitude controls to improve its stability during observations. Reaction wheels manoeuvre the telescope into place, and gyroscopes monitor its position in space. Fine Guidance Sensor units are used to lock onto guide stars to ensure the extremely high pointing accuracy needed to make precise observations.

Journey

Hubble was deployed by the Space Shuttle Discovery (STS-31) into a circular orbit 575 km above the ground, inclined at 28.5° to the equator. The time taken for one orbit is between 96 and 97 minutes.

History

The idea of sending a telescope into space was 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 it up and proposed a 3 m-diameter space telescope. Funding began 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 it was downsized to 2.4 m, the project started to attract significant attention from astronomers. The 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 led to 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 the main 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 corrective optics package to restore Hubble’s eyesight completely.

A crew of astronauts carried out the repairs 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, four other Servicing Missions have been carried out. During SM2 in 1997 two new instruments were installed; in 1999 during SM3A many of Hubble’s crucial technical systems were exchanged; and in 2002 came SM3B, when Hubble again received new science instruments. Servicing Mission 4 was scheduled for October 2008 but, just a month before the flight, HST’s Science Instrument Command and Data Handling unit failed. The servicing mission was put back in order to build a replacement for this key piece of equipment. So it was not until May 2009 that Space Shuttle Atlantis lifted off and performed the final servicing mission of HST.

A brilliant astronaut crew, commanded by veteran astronaut Scott Altman, and including Navy Capt. Gregory C. Johnson, spacewalkers John Grunsfeld and Mike Massimino, and first-time space fliers Andrew Feustel, Michael Good and Megan McArthur completed a very ambitious 11-day mission.

During the five planned spacewalks, astronauts installed two new instruments: the Wide Field Camera 3 (WFC3) and the Cosmic Origins Spectrograph (COS), and repaired two inactive ones: the Space Telescope Imaging Spectrograph (STIS) and the Advanced Camera for Surveys (ACS).

The astronauts also performed a number of crucial component upgrades, including new batteries, new gyros, a new Science Instrument Command and Data Handling Unit, a new Fine Guidance Sensor and new insulation. After an intense commissioning period, all the instruments are performing normally, and Hubble has resumed its scientific observations, with expanded science capabilities and an extended life.

Partnerships

NASA is ESA’s partner in the project. ESA has a 15% stake in the mission and has, among other things, provided the Faint Object Camera, the first two solar wings that powered the spacecraft and a team of space scientists and engineers at the Space Telescope Science Institute (STScI) in Baltimore, USA. Fifteen European astronomers work at STScI while another group employed by ESA and the European Southern Observatory runs the Space Telescope European Coordinating Facility (ST-ECF), near Munich, Germany, to provide a direct connection to European astronomers. The astronomers at the ST-ECF work on various aspects, including the calibration of its instruments and public outreach. Europe's contribution to HST entitles European astronomers to 15% of the telescope's observing time.