Named in honor of astronomer Edwin Hubble, the Hubble Space Telescope is the first major optical telescope placed by NASA in space to observe the most distant galaxies and stars as well as planets in our solar system. Hubble space observatory was launched in April 24, 1990 through Space Shuttle Discovery (STS-31) from Kennedy Space Center in Florida, and deployed into a circular orbit about 600 km above the ground. The launch of Hubble marked the most significant progress in the field of astronomy since Galileo’s telescope.
Hubble is placed far above rain clouds and light pollution, and therefore is able to get an unobstructed view of the universe. It has been designed to access the otherwise invisible ultraviolet part of the spectrum, and can also access areas of the infrared not visible from the ground. Hubble whirls around Earth in a circular low Earth orbit at a speed of about 17,000 miles per hours, taking pictures of stars, galaxies and planets. At the time of launch, the weight of this scientific instrument was about 24,000 pounds, which has now increased to about 27,000 pounds following the final servicing mission (SM) in 2009. Since the beginning of its mission in 1990, Hubble has traveled more than 3 billion miles and made over 1.2 million observations. More than 12,800 scientific papers have been published by astronomers using the data provided by Hubble.
Hubble has been designed as a permanent space-based observatory that could be visited by the astronauts for replacement or up-gradation of outdated science instruments. So far, NASA has flown five servicing missions (SM 1, 2, 3A, 3B, and 4) to conduct repairs, replace degraded/failed components, upgrade equipment, and install new instruments on Hubble. The first servicing mission was flown in December 1993 and the last in May 2009.
The current scientific instruments installed on Hubble are:
Advanced Camera for Surveys (ACS)
ACS is a third-generation imaging camera which has been optimized to perform surveys or broad imaging campaigns. ACS was installed during Servicing Mission 3B and replaced Hubble’s Faint Object Camera. The wavelength range of this instrument extends from ultraviolet, through the visible and to the near-infrared. The wide field of view of ACS is about twice that of Hubble’s former WFPC2. ACS provides excellent image quality and high sensitivity, and can map large areas of the sky in great detail. A special optical tool ‘grism’ in ACS provides it capabilities to perform spectroscopy. ACS is made up of three sub-instruments: (1) Wide Field Camera (WFC), High Resolution Camera (HRC), and Solar Blind Camera (SBC). WFC is an optical and near-infrared camera (with wide field) which has been optimized to search galaxies and galaxy clusters in ancient and remote Universe. HRC has been designed to capture high resolution, detailed images of the light coming from the centers of galaxies with huge black holes, as well as of images of star clusters, ordinary galaxies, and gaseous nebulae. The SBC detects faint ultraviolet radiation by blocking visible light.
Wide Field Camera 3 (WFC3)
The WFC3 was installed during Servicing Mission 4 to enhance the observational capabilities of Hubble. WFC3 provides improved resolution over a wider field of view and, together with COS, is leading the way to new discoveries. The combination of enhanced field of view and broader waveband makes WFC3 a powerful instrument. WFC3 features two channels – one for near infrared (NIR) and other from ultraviolet and visible light (UVIS). The NIR channel uses a 1 megapixel array made from mercuric cadmium telluride (HgCdTe), while the UVIS channel uses a silicon-based CCD. WFC3 aims to make observations in the ultraviolet and visible range to study stellar archeology and the distribution of galaxies at high redshift. The observations in the infrared range are aimed to study the highest redshift galaxies and to detect presence of water and ice on Mars.
Cosmic Origins Spectrograph (COS)
A spectroscope can be used to determine the density, temperature, chemical composition and many other properties of any object which absorbs or emits light. COS was constructed at the Center for Astrophysics and Space Astronomy at the University of Colorado and was installed during Servicing Mission 4. This instrument performs spectroscopic functions by breaking up light into its individual components. Scientists use COS to study: (1) the large-scale structure of the Universe, (2) formation and ages of stars, galaxies, and planets, and (3) origins of stellar and planetary systems. The Far Ultraviolet (FUV) channel of COS covers wavelengths from 115 to 177 nanometers, while the Near Ultraviolet (NUV) channel can cover wavelengths from 175 to 300 nanometers.
Space Telescope Imaging Spectrograph (STIS)
Installed during Servicing Mission 2, STIS is a versatile “combi-instrument”, which allows scientists obtain high resolution spectra of resolved objects. A unique feature of STIS is that it can simultaneously provide spectra from many different points along a target. STIS combines a camera with a spectrograph, and is able to function with a wide range of wavelengths from near-infrared to ultraviolet. The light gathered by the telescope is spread by the spectrograph to determine different properties of celestial objects, such as temperature, chemical composition, magnetic fields, radial velocity, and rotational velocity. Switching the spectrograph to “long slit spectroscopy” mode enables scientists to simultaneously obtain spectra of many different points across an object simultaneously. With “echelle spectroscopy” mode, the spectrum of one object is spread over the detector providing better wavelength resolution in a single exposure. The coronograph of STIS allows study of nearby fainter objects by blocking light from bright objects.
Near Infrared Camera and Multi-Object Spectrograph (NICMOS)
Weighing about 370 kg and installed during Servicing Mission 2, NICMOS is able to carry out infrared imaging and spectroscopic observations of astronomical targets. It can detect infrared light (wavelengths between 8,000 to 25,000 Ångström) which is invisible to human eyes. Due to its infrared capabilities, NICMOS can look through the dense clouds of gas and dust in the central region of our Milky Way. An electrical cooler was installed in NICMOS during Servicing Mission 3B, which has enabled it produce the best results.
Fine Guidance Sensors (FGS)
There are three FGS’es onboard Hubble, and these sensors are located at 90-degree intervals around the circumference of the telescope. Two of these FGS’es are used to point and lock the telescope on the target, while the third FGS is used for position measurements. These FGS’es can also be used to investigate binary star systems or to measure stellar distances.
- Wide Field and Planetary Camera 2 (WFPC2): Installed during Servicing Mission 1; removed during Servicing Mission 4
- Corrective Optics Space Telescope Axial Replacement (COSTAR): Installed during Servicing Mission 1; removed during Servicing Mission 3B
- Faint Object Camera (FOC): Part of original complement of instruments; replaced by ACS during Servicing Mission 3B
- Faint Object Spectrograph (FOS): Part of original complement of instruments; replaced by NICMOS during Servicing Mission 2
- Goddard High Resolution Spectrograph (GHRS): Part of original complement of instruments; replaced by STIS during Servicing Mission 2
- High Speed Photometer (HSP): Part of original complement of instruments; replaced by COSTAR during Servicing Mission 1)
- Wide Field and Planetary Camera 1 (WFPC1): Part of original complement of instruments; replaced by WFPC2 during Servicing Mission 1
Hubble Servicing Missions
Hubble is the only space telescope that was designed to be serviced in space by astronauts and to accommodate regular equipment upgrades. Since its launch in 1990, NASA has flown five servicing missions (SM 1, 2, 3A, 3B, and 4) to replace, repair and upgrade instruments and system on Hubble. The first servicing mission was launched in December 1993 and the last in 2009. Each servicing mission included performing delicate operations, such as intercepting the telescope in orbit, using shuttle’s mechanical arm to retrieving the telescope, carrying out tethered spacewalks, conducting repairs, replacing degraded components, and installing new instruments.
Servicing Mission 1 (SM1)
Soon after the launch of the Hubble space telescope in 1990, NASA scientists discovered problems with Hubble’s mirror. A focusing defect in the mirror prevented the telescope to capture sharp images. This forced NASA to send the first servicing mission (SM1) to Hubble to install corrective optics. Scientists from NASA and ESA built a corrective optics package to restore Hubble’s eyesight completely. NASA selected seven astronauts for the mission, who were trained to use a variety of specialized tools. The 10-day mission was launched using Endeavor in December 1993, and involved replacing the High Speed Photometer with COSTAR corrective optics package. Other instrument replaced in the mission included WFPC, solar arrays and their drive electronics, two electrical control units, four gyroscopes in the telescope pointing system, two magnetometers, and other electrical components. WFPC was replaced with the Wide Field and Planetary Camera 2 (WFPC2). The computers onboard Hubble were also upgraded. This mission – which was meticulously planned and perfect executed – captured the attention of the public as well as astronomers. It was declared a complete success after NASA started getting sharper images from Hubble. After SM1, Hubble was back in business.
Servicing Mission 2 (SM2)
SM2 was launched using Discovery in February 1997. The main objective of SM2 was to replace the Faint Object Spectrograph (FOS) and Goddard High Resolution Spectrograph (GHRS) with the Near Infrared Camera and Multi-Object Spectrometer (NICMOS) and the Space Telescope Imaging Spectrograph (STIS). The mission also involved replacing an Engineering and Science Tape Recorder with a Solid State Recorder as well as repairing thermal insulation.
Servicing Mission 3A (SM3A)
SM3A was flown by Discovery in December 1999. Before the start of SM3A, failure of four of the six gyroscopes on Hubble made the telescope incapable of performing scientific observations. The SM3A replaced all the six gyroscopes as well as a Fine Guidance Sensor and the thermal insulation blankets. A new Voltage/temperature Improvement Kit (VIK) was installed to prevent battery overcharging. The old computer (DF-224) was replaced with a new computer that was 20 times faster and had six times more memory. The throughput was increased as some computer tasks were now shifted from ground to the spacecraft.
Servicing Mission 3B (SM3B)
SM3B was launched using Columbia in March 2002. Astronauts installed a new instrument in this mission and replaced FOC with Advanced Camera for Surveys (ACS). COSTAR now became redundant as all new instruments had built-in correction for the main mirror aberration. Installation of a closed-cycle cooler revived the NICMOS. Solar arrays were replaced again to provide 30% more power.
Servicing Mission 4 (SM4)
NASA had planned the next servicing mission for February 2005, but the Columbia disaster in 2003 led to canceling of all future manned service missions. NASA Administrator Sean O’Keefe announced that all future shuttle missions should have capabilities to reach the International Space Station in case there are any in-flight problems. Many astronomers criticized the decision of O’Keefe, arguing that Hubble was highly valuable for humanity and should be saved at any cost. In late 2004, many Congressional members, getting much support from public, urged NASA and Bush Administration to reconsider the decision to drop Hubble rescue mission. In 2005, appointment of Michael D. Griffin as NASA’s new administrator changed the situation completely. In October 2006, Griffin gave his approval for the 11-day Hubble maintenance mission using Atlantis. The mission was originally scheduled for October 2008, but was postponed due to failure of Hubble’s main data-handling unit in September 2008.
SM4 was finally flown using Atlantis in May 2009. This last scheduled shuttle mission for Hubble installed the replacement data-handling unit as well as two new observation instruments – the Cosmic Origins Spectrograph (COS) and Wide Field Camera 3 (WFC3). The Soft Capture and Rendezvous System and New Outer Blanket Layers (NBLs) were also installed. SM4 also involved repairing of STIS and ACS systems and installing of superior nickel hydrogen batteries.