April 24th 2011 marks the 21st anniversary of the launch of the Hubble Space Telescope (HST) and NASA has released a new image to mark the occasion. This shows the interaction of spiral galaxy UGC 1810 with its companion UGC 1813.
Galaxies UGC 1810 (top) and UGC 1813 (bottom). Image credit: NASA, ESA, A. Riess, L. Macri, and Hubble Heritage Team/Public domain.
The success of the HST took decades of persistence and hard work. German physicist Hermann Oberth was the first to consider a space-based telescope in 1923[1], and American astronomer Lyman Spitzer discussed the two main advantages to Oberth's idea in 1946[2][3a].
Firstly, Spitzer argued that a space-based telescope would have a much greater resolution than an equivalently sized ground-based telescope. The stars appear to twinkle because their light changes velocity as it moves through the air. This problem is known as 'seeing'. The best ground-based telescopes are placed at high altitudes in order to minimise the amount of air that the light has to travel through. A space-based telescope would not face this problem and would therefore be able to observe fainter, older, objects.
Secondly, Spitzer argued that a space-based telescope would be able to observe objects at a greater range of wavelengths. Ground-based telescopes cannot easily observe infrared or ultraviolet light because they are absorbed by the atmosphere.
A final advantage of a space-based telescope is that it does not suffer from as much light pollution as a ground-based one.
Spitzer continued to campaign for a space-based telescope and, in 1965, he was appointed to head a committee that would define the scientific objectives of such a mission[4]. In the 1970s, NASA began to lobby Congress for funding and, after a nationwide campaign by astronomers, the Senate finally agreed to provide half the requested amount[5a]. The European Space Agency (ESA) provided extra funding, as well as staff, instruments, and solar cells. In return, they were promised 15% of the observing time[6]. In 1978, they began designing the telescope, which was due to launch in 1983[3b].
The telescope was named the Hubble Space Telescope (HST) after American astronomer Edwin Hubble in 1983[7]. Hubble was the first to show that the universe is expanding, and the expansion rate of the universe can be determined using the Hubble parameter[8]. One of the main objectives for the HST was to accurately determine the Hubble parameter, which can be used to calculate the age of the universe[9].
A number of technical problems meant that the launch date was delayed until October 1986. The Challenger disaster occurred in January of that year. Seven people were killed, including Christa McAuliffe, a teacher from New Hampshire, and shuttle missions were suspended until 1988. The launch date was finally set for April 24th 1990, when the telescope was transported into orbit by the Space Shuttle Discovery[5b].
Over all this time, the cost of the HST had inflated from its original estimate of $400 million to over $2.5 billion[10]. The telescope was too sensitive to be stored in normal atmospheric conditions, and it cost about $6 million a month to maintain on Earth[11].
Within weeks of the launch, it became apparent that images of faint objects were suffering from spherical aberration. This occurs when the lens does not focus properly, and the primary mirror was calculated to be about 2200 nm too flat (a nm is a billionth of a metre)[12].
The HST was designed to be serviced by astronauts, but it would be impossible to replace the whole mirror while in orbit, and so optical components had to be added to counter the effect. These were installed by seven astronauts over a period of ten days in 1993[13]. Four service missions have since been conducted, the final one in 2009[14].
The HST has been responsible for thousands of scientific papers and it has been used to make a number of important discoveries[15]. Anyone can apply for telescope time, regardless of their nationality, and time has been rewarded to thirteen amateur astronomers since its launch[16].
In 1997, the HST showed that supermassive black holes are contained at thei centre of most galaxies[17] and two years later, it was used to determine the Hubble parameter with 10% accuracy[18]. This allowed the age of the universe to be calculated, and it was found to be between 9 and 14 billion years old. This value was later refined by the Wilkinson Microwave Anisotropy Probe (WMAP), which was launched in 2001[19]. This was the same year that the HST helped confirm the existence of dark energy, which is accelerating the expansion of the universe[20].
In 2005, the HST was used to help determine how planets form, by observing them in phases that had never been seen before[21]. Three years later, it was used to make the first detection of an organic molecule in the atmosphere of an exoplanet[22].
The HST is still being used to make new discoveries. Only last week, NASA announced that it had observed galaxies that formed 200 million years after the big bang, the oldest that have ever been seen[23].
The HST has also provided some of the most iconic images of the universe.
Star-Forming Region LH 95 in the Large Magellanic Cloud galaxy. Image credit: NASA, ESA, and the Hubble Heritage Team/CC-A.
The HST is expected to continue working until at least 2014[24], when its successor, the James Webb Space Telescope (JWST), is due to be launched[25]. The JWST has been developed by NASA with help from the ESA and the Canadian Space Agency. It's named after James E. Webb, the second administrator of NASA, and will be able to detect stars that are hundreds of millions of years older than those observed by the HST.
The HST was due to return to Earth in a Space Shuttle. This is no longer possible as NASA is due to retire its shuttles in July 2011. Instead, it will remain in orbit until a robotic mission can safely bring it back to Earth[26].
UPDATE: As of 2017, the HST is still operating and the JWST is currently scheduled for launch in 2018.