More than 26 years since launch, the Hubble Space Telescope continues its role at the forefront of astronomy, ranging from our own Solar System to the high-redshift universe.
About the Mission
The Hubble Space Telescope (HST) is a cooperative program of the European Space Agency (ESA) and the National Aeronautics and Space Administration (NASA) to operate a space-based observatory for the benefit of the international astronomical community. HST is an observatory first envisioned in the 1940s, designed and built in the 1970s and 80s, and operational since the 1990. Since its preliminary inception, HST was designed to be a different type of mission for NASA — a long-term, space-based observatory. To accomplish this goal and protect the spacecraft against instrument and equipment failures, NASA planned on regular servicing missions. Hubble has special grapple fixtures, 76 handholds, and is stabilized in all three axes. HST is a 2.4-meter reflecting telescope, which was deployed in low-Earth orbit (600 kilometers) by the crew of the space shuttle Discovery (STS-31) on 25 April 1990.
Responsibility for conducting and coordinating the science operations of the Hubble Space Telescope rests with the Space Telescope Science Institute (STScI) on the Johns Hopkins University Homewood Campus in Baltimore, Maryland. STScI is operated for NASA by the Association of Universities for Research in Astronomy, Inc. (AURA).
HST's current complement of science instruments includes three cameras, two spectrographs, and fine guidance sensors (primarily used for accurate pointing, but also for astrometric observations). Because of HST's location above the Earth's atmosphere, these science instruments can produce high-resolution images of astronomical objects. Ground-based telescopes are limited in their resolution by the Earth’s atmosphere, which causes a variable distortion in the images. Hubble can observe ultraviolet radiation, which is blocked by the atmosphere and therefore unavailable to ground-based telescopes. In the infrared portion of the spectrum, the Earth’s atmosphere adds a great deal of background, which is absent in Hubble observations.
Hubble Through the Years
When originally planned in the early 1970s, the Large Space Telescope program called for return to Earth, refurbishment, and re-launch every 5 years, with on-orbit servicing every 2.5 years. Hardware lifetime and reliability requirements were based on that 2.5-year interval between servicing missions. In the late 70s, contamination and structural loading concerns associated with return to Earth aboard the shuttle eliminated the concept of ground return from the program. NASA decided that on-orbit servicing might be adequate to maintain HST for its 15-year design life. A three-year cycle of on-orbit servicing was adopted. HST servicing missions in December 1993, February 1997, December 1999, March 2002 and May 2009 were enormous successes and validated the concept of on-orbit servicing of Hubble.
The years since the launch of HST in 1990 have been momentous, with the discovery of spherical aberration in its main mirror and the search for a practical solution. The STS-61 (Endeavour) mission of December 1993 corrected the effects of spherical aberration and fully restored the functionality of HST. Since then, servicing missions have regularly provided opportunities to repair aging and failed equipment as well as incorporate new technologies in the telescope, especially in the Science Instruments that are the heart of its operations.
Mission Operations and Observations
Although HST operates around the clock, not all of its time is spent observing. Each orbit lasts about 95 minutes, with time allocated for housekeeping functions and for observations. "Housekeeping" functions includes turning the telescope to acquire a new target, switching communications antennas and data transmission modes, receiving command loads and downlinking data, calibrating the instruments and similar activities. On average, the telescope spends about 50% of the time observing astronomical targets. About 50% of the time the view to celestial targets is blocked by the Earth, and that time is used to carry out these support functions.
Each year the STScI solicits ideas for scientific programs from the worldwide astronomical community. All astronomers are free to submit proposals for observations. Typically, 700-1200 proposals are submitted each year. A series of panels, involving roughly 100 astronomers from around the world, are convened to recommend which of the proposals to carry out over the next year. There is only sufficient time in a year to schedule about 1/5 of the proposals that are submitted, so the competition for Hubble observing time is tight.
After proposals are chosen, the observers submit detailed observation plans. The STScI uses these to develop a yearlong observing plan, spreading the observations evenly throughout the period and taking into account scientific reasons that may require some observations to be at a specific time. This long-range plan incorporates calibrations and engineering activities, as well as the scientific observations. This plan is then used as the basis for detailed scheduling of the telescope, which is done one week at a time. Each event is translated into a series of commands to be sent to the onboard computers. Computer loads are uplinked several times a day to keep the telescope operating efficiently.
When possible, two scientific instruments are used simultaneously to observe adjacent target regions of the sky. For example, while a spectrograph is focused on a chosen star or nebula, a camera can image a sky region offset slightly from the main viewing target. During observations the Fine Guidance Sensors (FGS) track their respective guide stars to keep the telescope pointed steadily at the right target.
Engineering and scientific data from HST, as well as uplinked operational commands, are transmitted through the Tracking Data Relay Satellite (TDRS) system and its companion ground station at White Sands, New Mexico. Up to 24 hours of commands can be stored in the onboard computers. Data can be broadcast from HST to the ground stations immediately or stored on a solid-state recorder and downlinked later.
The observer on the ground can examine the "raw" images and other data within a few minutes for a quick-look analysis. Within 24 hours, GSFC formats the data for delivery to the STScI. STScI is responsible for calibrating the data and providing them to the astronomer who requested the observations. The astronomer has a year to analyze the data from the proposed program, draw conclusions, and publish the results. After one year the data become accessible to all astronomers. The STScI maintains an archive of all data taken by HST. This archive has become an important research tool in itself. Astronomers regularly check the archive to determine whether data in it can be used for a new problem they are working on. Frequently they find that there are HST data relevant for their research, and they can then download these data free of charge.
Hubble has proven to be an enormously successful program, providing new insight into the mysteries of the Universe.