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25 Years of the Hubble Space Telescope

April 30, 2015

The Hubble Space Telescope was launched with the Space Shuttle Discovery (Mission STS-31) on April 24, 1990, and it was deployed from Discovery the next day, so it's just had its 25th anniversary. Because of its low Earth orbit, Hubble will fall back to Earth around 2030, or a little later. It's anticipated that Hubble will be replaced by the more advanced James Webb Space Telescope, scheduled to be launched in October, 2018.

Much was expected from Hubble, since it was a Ritchey–Chrétien telescope with a 2.4 meter primary mirror operating beyond Earth's atmosphere. Polishing the mirror to the proper shape (a process called "figuring") was an involved task, since the mirror was fabricated on Earth, where it felt the affect of gravity, but it would be operating in a weightless space environment.

Despite very careful polishing and testing, the geometry of the mirror was discovered to be flawed after it became operational on May 20, 1990. Point sources were spread out over more than an arcsecond, which was much worse that the hundred milliarcsecond specification. Analysis showed that the mirror was polished too flat, by just 2.2 micrometers, but still flat enough to cause what's termed spherical aberration (see figure)

spherical aberrationViewing a square grid with spherical aberration.

Central objects are reasonably imaged, as was the case for the original Hubble Space Telescope mirror, but the image becomes less sharp towards the periphery.

(Illustration by the author, using Inkscape.)

Data analysis showed that the null corrector, an optical instrument used for testing the mirror during polishing, was assembled incorrectly. Hubble was still useful in this state for some observations through use of sophisticated image processing techniques. Still, astronomers craved for the full resolution of the original specification, and there was fortunately an easy solution; easy, that is, if you take a trip to the telescope.

The mirror was not just polished to the wrong shape, it was precisely polished to the wrong shape. In December, 1993, astronauts on Space Shuttle Mission STS-61 modified the optical path of the telescope with two mirrors, one of which had a surface that corrected the spherical aberration. As can be seen in the photograph, Hubble was brought to good working order.

HST images of galaxy M100
Hubble Space Telescope images of the galaxy, M100, taken before and after the installation of corrective optics. The left image, before correction, was taken on November 27, 1993, and the right image, after correction, was taken on December 31, 1993. (NASA images, via Wikimedia Commons.)

So, what has Hubble given us in those twenty-five years? One of its more important findings was an accurate measurement of the rate of universal expansion, and the idea that the rate was not constant throughout the life of the universe.[1,4-6] This was done through careful observation of pulsating stars known as Cepheid variables. This led to the theory that dark matter was affecting the universe through a repulsive force that counteracts gravity.

Before the Hubble Space telescope, the age of universe was only known to be in the range, 10-20 billion years. My preferred value, many decades ago, was 18 billion years, based on a survey by Allan Sandage and his colleagues. Accurate measurements by the Hubble Space Telescope gave 13.8 billion years with an uncertainty of just 40 million years.[4,6]

Hubble confirmed the existence of black holes, and its observations supported the theory that nearly every galaxy has a supermassive black hole at its core.[5-6] The superior resolution of the Hubble Space Telescope has allowed observations of the atmospheres of extrasolar planets.[1] Most impressive are the Hubble deep field images of extremely distant galaxies (see photo).[4]

Abell 2744An example of a Hubble Space Telescope deep field image, a photograph of 3,000 distant galaxies focused through gravitational lensing by the foreground galaxy, Abell 2744. These galaxies appear as they were 12 billion years ago.

(Image: NASA, ESA, J. Lotz, M. Mountain, A. Koekemoer, and the HFF Team, via the Space Telescope Science Institute.)[1]

The universe is not just stars and galaxies. Hubble has imaged many clouds of gas and dust thought to be the material from which new stars are formed.[6] Who can forget Hubble's images of the impressive July, 1994, impact of the comet Shoemaker-Levy 9 with Jupiter. This impact resulted in the disturbance of huge volumes of Jupiter's atmosphere.[6]

Taking a commemorative photo is common on significant anniversaries, and one was released for this 25-year ("silver") anniversary of the Hubble Space Telescope. For this, NASA selected a photograph of a stellar nursery located in the star cluster, Westerlund 2. This cluster, in the constellation Carina, is about 20,000 light-years away, and it contains thousands of nascent stars(see photo).[1-4]

Hubble Space Telescope image of star cluster, Westerlund 2Hubble Space Telescope image of star cluster, Westerlund 2

(Image: NASA, ESA, the Hubble Heritage Team (STScI/AURA), A. Nota (ESA/STScI), and the Westerlund 2 Science Team.)

Many stars in Westerlund are stellar youngsters, aged about 2 million years, as compared with our Sun's 4.6 billion years. It contains some of the most massive stars in our galaxy, and these are also some of the brightest.[2] The anniversary image is a combination of a visible light image from Hubble's Advanced Camera for Surveys, and a near-infrared image from its Wide Field Camera 3.[3] A fly-through video can be found at ref.7.[7]

As any aged satellite observatory, Hubble has had its problems. A telescope needs great pointing stability, and this is accomplished on Hubble using six gyroscopes. These gyroscopes have had their problems, but a replacement set is performing well.[4] Ken Sembach, interim deputy director at the Space Telescope Science Institute, is quoted by BBC News as saying,
"It's also worth noting that two of the instruments repaired on the last servicing mission - the Advanced Camera for Surveys, and the Space Telescope Imaging Spectrograph - are both working five years after the servicing mission, which is longer than they worked originally with their original electronics."[4]
There are no further repair visits planned, so here's hoping for continued longevity of what's already in orbit.


  1. Irene Klotz, "Defying the odds, Hubble telescope still going strong after 25 years," Reuters, April 23, 2015.
  2. Alexandra Witze, "Cosmic confetti celebrates Hubble's 25th birthday, Nature News, April 23, 2015.
  3. Andrew Griffin, "Hubble at 25: anniversary of space telescope celebrated with stellar new images," Independent (UK), April 23, 2015.
  4. Jonathan Amos, "Hubble issues 25th birthday image," BBC News, April 23, 2015.
  5. Daniel Schwartz, "How the Hubble space telescope changed our view of the universe," CBC News April 24, 2015.
  6. Dan Malerbo, "Let's talk about science: Hubble Telescope's 25th anniversary," Pittsburgh Post-Gazette, April 9, 2015.
  7. Hubble Space Telescope, Flight to Star Cluster Westerlund 2, NASA, ESA, STScI, et al., YouTube Video, April 23, 2015.

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