Next month marks the 20th anniversary of the Hubble Space Telescope’s launch into space. On Apr. 24, 1990, at 8:33 a.m., the Space Shuttle Discovery (STS-31) took off, carrying Hubble as its primary payload. Hubble is the result of a collaboration between NASA and the European Space Agency (ESA), the first of four space telescopes in NASA’s Great Observatories program. The other three are the Compton Gamma Ray Observatory (de-orbited in 2000), the Chandra X-ray Observatory, and the Spitzer Space Telescope. Hubble is the only Great Observatory that takes images in the visible light that we all see. Hubble, therefore, has captured the public’s imagination like no other telescope.
In 1946, Lyman Spitzer wrote the paper “Astronomical advantages of an extraterrestrial observatory.” In this paper, he discusses the two main reasons to put a telescope above the atmosphere. First of all, our atmosphere distorts images. Have you ever looked up while standing underwater? Did you notice how the water distorts images of thing above the surface? Our air has precisely this effect on the stars. Of course, the air’s effect is less pronounced than the water’s, but we see it when we observe point sources such as stars. A star’s twinkling is in fact our attempt to rectify the position of a star, given that its precise position in the sky continues to change slightly due to the atmosphere. Astronomers quantify this distortion as the atmospheric seeing. The seeing limits the angular resolution of a telescope (the minimum distance between distinguishable objects in an image). A telescope in space can therefore see better than even a much larger telescope on the ground. Secondly, our atmosphere absorbs much of the infrared and ultraviolet light from space, including virtually all UV light less than 310 nm in wavelength. Above the air, Hubble can detect infrared, visible and ultraviolet light. We thus learn more about stars and galaxies by studying more of the light they emit.
Hubble orbits 347 miles above the Earth, a little over twice the distance from Houston to San Antonio. That orbital height places Hubble in the exosphere, the thinnest, outermost layer of the Earth atmosphere which is in fact a transition between Earth’s atmosphere and interplanetary space. It also leaves Hubble close enough to Earth that Earth’s disk blocks much of the potential field of view. Low Earth orbit was required however, so that Space Shuttle crews could reach Hubble and service it. This turned out to be critical as the primary mirror installed and launched in 1990 had an error. Instead of being perfectly hyperbolic, the mirror was too flat at the edges by 2.2 microns (.0022 mm). This was enough to introduce severe spherical aberration into all images. The crew of STS-61, aboard the Space Shuttle Endeavor, installed corrective optics in 1993.
Here are some interesting facts about the Hubble Telescope:
- Hubble travels at 5 miles per second, completing one orbit every 97 minutes. The diameter of the telescope (constrained by the size of the Space Shuttle in which it was launched), is 94.5 inches.
- The Space Telescope Science Institute (STScI) in Baltimore, Maryland, is the science operations center for the Hubble Space Telescope. Astronomers at this institute allocate telescope time and schedule Hubble observations. They also receive, archive, and distribute data taken with Hubble.
- Optically, Hubble is a reflecting telescope with a Cassegrain design. In this design, light entering the telescope first encounters a primary mirror and is then focused onto a secondary mirror which in turn focuses the light through a small hole in the primary mirror to an array of instruments on board.
There are several instruments and sensors on Hubble that allow it to take different images and readings. These include:
- The Wide Field Camera, which takes images in visible light and thus produces most of the beautiful photos associated with Hubble. Earlier versions of this instrument were called ‘Wide Field and Planetary Camera” (WFPC). WFPC 2 snapped a photo of the famous Hubble Deep Field (1994), imaging some of the most distant galaxies known.
- The Space Telescope Imaging Spectrograph, a spectrometer sensitive to ultraviolet, visible, and near-infrared light.
- The Near Infrared Camera and Multi-Object Spectrometer (NICMOS), a spectrometer sensitive to infrared light.
- The Advanced Camera for Surveys (ACS), which became the primary imaging instrument on board HST upon its installation in 2002, replaced the Faint Object Camera (FOC). ACS imaged the Hubble Ultra Deep Field in 2003 and 2004.
- The Cosmic Origins Spectrograph (COS), installed this past May, replaced Hubble’s original corrective optics (the Corrective Optics Space Telescope Axial Replacement, or COSTAR). COS takes spectra in the ultraviolet range.
|Jupiter in Ultraviolet (about 2.5
hours after R’s impact). The black
dot near the top is a Galilean moon
In 1993, as Hubble’s optics were restored to their full power, it was discovered that Comet Shoemaker-Levy 9 was on a collision course with Jupiter. That collision occurred in July 1994. With Hubble, astronomers could get much clearer and more detailed images of a space collision. Hubble has provided us with unprecedented telescopic views of all the planets except Mercury, which is too close to the sun in our sky.
Hubble has contributed to the discovery of exoplanets (planets around stars other than our Sun). In 2008, NASA released a composite of two photographs taken by the ACS in 2004 and 2006. These photos showed that the bright star Fomalhaut has a companion planet, designated Fomalhaut b.
Astronomers have used Hubble to measure the distances to Cepheid variables (stars whose variation in brightness depends on their luminosity) more accurately. By comparing this luminosity to the apparent brightness of the star, astronomers could determine the distance to the star and thus to distant galaxies containing them. This helps astronomers constrain the value of the Hubble constant, the rate at which the universe is expanding.
Perhaps the most striking results from Hubble are the Hubble Deep Field and Hubble Ultra Deep Field. In these images, Hubble’s sensitive optics produced images of galaxies billions of light years away. HUDF includes galaxies up to 13 billion light years away (the accepted age of the universe is 13.7 billion years.
The foregoing is just a sample of the science done with Hubble. Over 8,000 scientific papers based on Hubble data have been published in peer-reviewed journals.
Unfortunately, Hubble cannot last forever. Even in the exosphere, there is a slight drag on Hubble than causes it to lose energy and slowly fall towards Earth. Further, Hubble’s instruments, like any machines, degrade and become inoperable if not serviced.
After the Space Shuttle Columbia exploded on re-entry on February 1, 2003, the NASA Administrator at the time, Sean O’Keefe, decided that all future Space Shuttle flights must have the option of docking at the International Space Station in the event of an emergency. Since no shuttle flight can reach both the Hubble Space Telescope and the ISS on the same orbit, this rule canceled a servicing mission to Hubble planned for 2005. An outcry from astronomers, the public, and elected officials prompted O’Keefe’s successor, Michael Griffin, to reconsider and reverse that decision. Space Shuttle Atlantis launched on May 11, 2009, marking the fifth and final mission to service Hubble. Atlantis astronauts installed a new Cosmic Origins Spectrograph and a third Wide Field Camera to replace the second. They also replaced two batteries, a Fine Guidance Sensor and six gyroscopes which help orient the telescope. With the refurbishments, Hubble should function at least until 2014.
One of Hubble successors, slated for launch in June 2014, is the James Webb Space Telescope. This telescope will orbit the Sun (not the Earth) at the second Lagrangian point of the Earth-sun system. An object at this point remains in line with the Earth and Sun, on the far side of the Earth. This telescope will look for light from the earliest stars and galaxies in the universe, at infrared wavelengths. Because it images light only in the infrared, James Webb will not be a full successor to Hubble, however.
A fuller successor, should it be approved, built, and launched, would be the Advanced Technology Large Aperture Space Telescope (ATLAST). This telescope, like Hubble, would form images in infrared, visible, and ultraviolet light. However, its mirror would be much larger, between 320 and 660 inches in size. Such a telescope is far in the future, however. If Hubble is gone after 2014, there will be some years without anything quite like it.
Hubble may be in its final years, but we can still experience its fantastic discoveries. An IMAX film crew and camera accompanied Space Shuttle Atlantis astronauts of STS-125 on their May 2009 mission to service Hubble. We are thus proud and excited to present to you Hubble 3D, a new IMAX film opening today in IMAX. Blast off with Hubble 3D and travel across space and time on this amazing adventure.
Check out the preview below.
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