The Hubble Space Telescope (abbreviated to HST) was carried into space in the year 1990 by a space shuttle mission. It is still operational in the low orbit of the earth and has been one of NASA‘s most successful missions.
Hubble is at a distance of 353 miles above the surface of the earth. The goal of sending the Hubble to space was to see further and deeper into space. Named after the astronomer Edwin Hubble, the telescope orbits the earth in a position just above its atmosphere. This position enables the telescope to get unique views of the universe without any distortion unlike any telescope on the ground. A telescope in space is capable of observing infrared and ultraviolet light which otherwise gets strongly absorbed by the atmosphere.
Though Hubble does not have the privilege of being the first telescope in space, it is by far the most versatile and one of the largest. Hubble has beamed thousands of pictures back to the earth and has helped to unravel many mysteries of the universe, including existence of dark energy, quasars’ identities, and the age of the universe.
The HST was built by NASA with help from the European Space Agency. It is being operated by the Space Telescope Science Institute. The premier observatories of NASA other than the HST include Spitzer Space Telescope, Chandra X-ray Observatory, etc.
Though work of constructing Hubble started in the early 1970s when the funding was allocated, the proposed launch in 1983 was severely delayed owing to budget constraints and the Challenger disaster. Immediately after launch in 1990, it was found out that the mirror was not ground correctly which compromised the capabilities of the telescope. However, it was set right in 1993 through a service mission.
The Hubble is a telescope that is capable of being serviced in space. There were four service missions between the years 1993 and 2002 that helped to repair and service systems on the space telescope. However, the fifth service mission was canceled on grounds of safety after the disaster of Columbia. Notwithstanding this, one final mission by space shuttle Atlantis was approved by Mike Griffin, the NASA administrator, which completed the required service and repair in 2009. The telescope is expected to be functional up to 2013. Hubble’s successor, James Webb Space Telescope is expected to be launched in 2018.
The actual construction and engineering work was split among many institutions. Whereas Marshall Space Flight Center took over the design, development and the construction, Goddard Space Flight Center took care of the instrumentation and the ground controls. Perkin-Elmer was assigned to develop the Fine Guidance Sensors and Optical Telescope Assembly of the Hubble. The spacecraft construction and assembly of the housing was assigned to Lockheed.
Moving at a rate of about five miles per second, it completes one spin around the earth in 97 minutes. As it travels, the primary mirror attached to Hubble captures light and directs it to another secondary mirror which in turn focuses this light through a hole in the primary mirror to the many instruments that are placed inside. Once the mirror captures the light, some of the instruments work individually and others together to produce different results and gather different information.
Though most people think that the capability of a telescope is in its capacity to magnify objects, it works to create a picture by collecting more light than the normal human eye can. The larger the mirror of a telescope, the greater is the amount of light that it can capture.
The primary mirror of Hubble is about 2.5 m in diameter. Though it is much smaller than many ground-based telescopes’ mirrors, its location beyond the earth’s atmosphere gives it a very remarkable clarity.
Optically, the HST is a Cassegrain Reflector that uses two hyperbolic mirrors for imaging over a wide field of view. The mirror used in HST was polished to an accuracy nearing 10 nanometers with the help of computer controlled polishing machines.
The mirror was made of ultra-low expansion glass (Corning) to reduce its weight to a minimum. The mirror is coated with a reflective coating of aluminum that is 65 nanometers thick and a proactive coating of magnesium fluoride that is about 25 nanometers thick.
Knowing that the spacecraft would have frequent transitions between sunlight and shade, and this would cause as many variations in temperature, multi-layered insulation is used to keep the housing temperature stable. This stability is required for accurate pointing of the telescope. The telescopes and other instruments are placed in a light aluminum shell. A graphite epoxy frame placed within the shell keeps the parts of the telescope always aligned. A nitrogen gas purge prior to the launch ensured that the instruments would later not be covered with ice.
Some of the scientific instruments, among many others, placed in the housing are described below.
The Wide Field Planetary Camera is designed to see near-ultraviolet, near-infrared, and visible light one at a time. Its field of view and resolution is higher than most of Hubble’s other instruments; it is among the newest of the telescope’s instruments. It is used to study dark energy and formation of stars.
The Goddard High Resolution spectrograph was designed to make ultraviolet observations. It uses a photon-counting digital detector. It is used for observing extremely faint objects. The spectrograph is used to create a wavelength fingerprint of the object being observed and creates unique information about it.
The Advanced Camera for Surveys is designed to detect visible light and studies the earliest activity in the universe. It hunts for the biggest planets and the most distant objects. Though it stopped working in 2007 owing to a minor fault, it was repaired during the service mission in 2009.
The Fine Guidance Sensors, in addition to keeping Hubble pointed in the right direction always, they are also used to measure the precise distances between stars.
Hubble’s activities are completely powered by sunlight. The attached solar arrays convert the sun’s rays to electricity. The extra energy is stored in batteries that supply the power to Hubble when it is away from the sun.
Hubble’s collection of data has led on to many astronomical discoveries and more than 10000 scientific articles have been published on the basis of data gathered by Hubble. The credit for the multitude of findings that have come through Hubble goes to the policy governing the use of the telescope. Any astronomer has the liberty to submit a proposal and request telescope’s time. The observations to be made are then selected by a team of experts and after they are completed, the astronomers are given a time of one year to complete the relevant study and then publish the findings to the scientific community.
Hubble discovered that the age of the universe is close to 14 billion years and the presence of dark energy that has caused the acceleration of expansion of the universe. Hubble has pictures of many galaxies at different points in their evolution, dusk and debris that cause formation of planets and protoplanetary disks. Hubble also discovered that gamma-ray explosions occurred in distant galaxies during the collapse of massive stars.