Hubble Space Telescope
| The Hubble Space Telescope as seen from the Space Shuttle Discovery during the second servicing mission of the telescope, STS-82
|Orbit Height||600 km
|Orbit period||100 min
|Launch date||24 April 1990
|Deorbit date||circa 2010
| Physical Characteristics
|Collecting Area||approx. 4.3 m2
|Effective Focal Length||57.6m (189 ft)
|NICMOS||camera and spectrometer
|WFPC2||wide field camera
|STIS||spectrometer and camera (failed)
The Hubble Space Telescope (HST) is a telescope orbiting the Earth at the outer edges of the atmosphere. It is a space observatory in the Great Observatories program. Named after Edwin Hubble, it was launched into orbit in 1990 as a joint project of NASA and the European Space Agency. Initial optical errors were corrected in 1993, and high-quality imaging began in 1994. HST is projected to continue operating until 2009, when funding is expected to be moved to the James Webb Space Telescope (JWST).
Working outside the atmosphere has advantages because the atmosphere obscures images and filters out electromagnetic radiation at certain wavelengths, mainly in the infrared. By carrying diverse instruments and dividing time between many astronomical projects from all over the world, Hubble has contributed to an extraordinary variety of astronomical discoveries. Among the most notable are the confirmation of dark matter, observations supporting the current accelerating universe theory, and studies of extrasolar planets. A common misconception is that the principal benefit of observations from orbit is high-resolution -- in fact the sensitivity to faint objects is the biggest advantage, and ground-based interferometric observations of bright sources have much higher resolution than HST.
HST is located about 600 kilometers above the ground, orbiting the Earth every 97 minutes.
Hubble masses about 11,000 kilograms, is 13.2 meters long, and has a maximum diameter of 4.2 m.
Two solar panels provide electricity, which is mainly used to power the instruments. Four large flywheels are used to orient and stabilize the telescope. The telescope's infrared camera and multi object spectrometer also need to be cooled down to minus 180 degrees Celsius for operation.
The spacecraft is rumored to be based in design on the National Reconnaissance Office's KH-11 reconnaissance satellite, with differences in instrumentation focal point, and light sensitivity. Evidence in favor of this theory include the fact that Hubble and KH-11s were shipped in the same container.
Hubble is a reflecting telescope with two mirrors. The main mirror has a diameter of about 2.4 m. The main mirror was erroneously ground into a slightly incorrect shape; this is corrected by an optics package known as the Corrective Optics Space Telescope Axial Replacement (COSTAR). With COSTAR, the telescope can achieve optical resolutions better than 0.1 arcseconds.
The light collected and focused by the telescope ends up in one of several instruments. There are five instrument bays, designed to allow instruments to be exchanged during servicing missions. Several such replacements have been performed, taking advantage of new technology to improve Hubble's capabilities. One of the instrument bays is occupied by COSTAR.
The current (as of 2004) complement of instruments is:
Each of these instruments has some capability as a spectrometer. Additionally, the telescope's Fine Guidance Sensors (FGS) can be (and have been) used for science.
The instruments formerly hosted on Hubble are:
Hubble was designed to be serviced by spacewalking astronauts from the space shuttle. It has a modular design, easily-removable parts, handrails, and other astronaut-friendly features. Servicing missions are used to swap out failed hardware, perform upgrades, and to counter atmospheric drag by boosting the telescope back into a higher orbit.
Much, but not all, of Hubble's hardware is designed to be replaced in orbit. In particular, the four main instrument bays are designed to allow instruments to be exchanged. It was anticipated that advancing technology would make it possible to build better instruments for Hubble, and using Hubble as a platform would be faster and much cheaper than building a new space telescope for a new instrument.
The Space Shuttle and Hubble has always been closely connected dating back to the 1970s when a decision, later rescinded, was made to launch all new payloads with the Space Shuttle. While the Space Shuttle has allowed Hubble to be serviced in orbit, it has also caused Hubble's fate to be determined by delays in the shuttle schedule. A sizeable fraction of the astronomical community feels that the connection between the Shuttle and Hubble has been much too close and that it would have been better and cheaper to launch Hubble on an unmanned launcher, though for servicing missions, if any, the Space Shuttle would have been needed anyway.
Montage of images taken by the Hubble Space telescope. Clockwise from the upper left: the Tadpole galaxy, the Cone Nebula, two colliding spiral galaxies dubbed "The Mice", and stellar birth in the Omega Nebula.
See also the external link: Additional Hubble Discoveries.
- Hubble provided dramatic pictures of the collision of comet Shoemaker-Levy 9 and Jupiter in 1994.
- The theory that most galaxies host a black hole in their nucleus has been partially confirmed by many Hubble observations.
- Additional evidence for planets surrounding stars other than the Sun was obtained with Hubble.
- Some of the observations leading to the current model of an accelerating universe were performed using the Hubble space telescope.
- In December 1995, Hubble photographed the Hubble Deep Field, a region covering one 30-millionth of the area of the sky and containing several thousand faint galaxies. A similar patch of southern sky was also imaged and looked remarkably similar, strengthening the position that the Universe is uniform over large scales, and that Earth occupies a typical place in the Universe. Another Deep Field photograph was taken in 1998.
- In March 2004, Hubble photographed the Hubble Ultra Deep Field. The image is made from a million-second-long exposure, taken off and on over a period of about three months.
In general, when astronomers discover something new using another telescope, Hubble is the first-choice instrument for doing follow-up observations. Hubble is rarely used for the sort of "speculative" observations which might discover new objects for the first time.
Scheduling of observations
Requests for allocation of HST observation time are processed by the Space Telescope Science Institute (STScI), located at Johns Hopkins University in Baltimore, Maryland. The requests undergo peer review; about 1 in 10 of the requests are granted. Observation time is free, and the affiliation or nationality of the submitter is irrelevant.
Once approved, data about the observation is sent to Goddard's Space Telescope Operations Control Center, which determines the exact schedule of daily observations and sends commands to the telescope's computer, via the Tracking Data Relay Satellite (TDRS) system with ground station in White Sands, New Mexico.
Data from the telescope is collected and analyzed by astronomers and scientists at STScI. Every day, STScI archives 3 to 5 gigabytes of Hubble Space Telescope data and delivers between 10 and 15 gigabytes to astronomers.
Hubble was designed in the 1970s. It cost about US$ 2 billion ($2,000,000,000) to build and launch.
The telescope was launched by Space Shuttle Discovery mission STS-31 on April 24 1990. This had been postponed from a 1986 launch date by the Space Shuttle Challenger disaster in January that year.
The first images back from the telescope were generally regarded as a big disappointment for astronomers and all concerned in the project. They were blurred, and despite image processing could not match the predicted resolution. It was determined that the main mirror had been ground slightly too flat at the edges, resulting in spherical aberration. This was eventually traced to a miscalibrated measuring instrument, a mistake that most in the field consider a sign of gross negligence and incompetence.
Servicing mission 1
Servicing Mission 1 (STS-61) in December 1993 installed several instruments and other equipment:
- replaced the High Speed Photometer (HSP) with the Corrective Optics Space Telescope Axial Replacement (COSTAR);
- replaced the Wide Field and Planetary Camera (WFPC) with the Wide Field and Planetary Camera 2 (WFPC2);
- replaced the solar arrays and their drive electronics;
- replaced two of the three Rate Sensing Units (RSUs), containing four gyroscopes, to leave Hubble with a full complement of six working gyroscopes;
- replaced two electrical control units and other electrical components;
- replaced two magnetometers;
- upgraded the computer, adding more memory and a processor;
- installed improvised covers on the magnetometers, which threatened to shed debris due to UV decay;
- fitted an electrical connection box on the Goddard High Resolution Spectrograph (GHRS);
- boosted Hubble's orbit.
The most important change was the installation of the corrective optics package. On January 13 1994 NASA declared the mission a complete success, and showed the first of many much sharper images.
Servicing mission 2
Servicing Mission 2 (STS-82) in February 1997 replaced two instruments and various other hardware:
The mission was completely successful.
Servicing mission 3A
Servicing Mission 3A (STS-103) in December 1999:
- replaced all six gyroscopes;
- replaced a Fine Guidance Sensor;
- replaced the computer;
- installed a Voltage/temperature Improvement Kit (VIK) to prevent battery overcharging;
- replaced an S-Band Single Access Transmitter (SSAT);
- replaced an Engineering and Science Tape Recorder with a new Solid State Recorder (SSR);
- replaced thermal insulation blankets.
Servicing mission 3B
Servicing Mission 3B (STS-109) in March 2002:
Replacing the Power Converter Unit was particularly tricky, because it was not designed for in-orbit replacement. It required taking the satellite completely off-line for the first time since it was put into operation.
The new solar arrays were derived from those built for the Iridium comsat system. They are only two-thirds the size of the old tattered arrays, resulting in less drag against the tenuous reaches of the upper atmosphere, while providing 30% more power. The additional power permits all instruments on board the Hubble to be run simultaneously. They also reduce a vibration problem that occurred when the old, more rigid arrays entered and left direct sunlight.
The completion of this servicing mission considerably enhanced Hubble's capabilities, some enthusiasts claiming that it was effectively a new instrument. The two instruments primarily affected by this mission, the ACS and NICMOS, together imaged the Hubble Ultra Deep Field in 2003-2004.
Hubble's future and beyond
Servicing Mission 4, planned for February 2005, was due to be the last servicing mission, as Hubble reached the end of its life expectancy. After the Space Shuttle Columbia disaster, all future shuttles must be inspected externally on orbit before reentry, a task which NASA has decided is too expensive to be done without the facilities of the International Space Station (ISS). The shuttle is incapable of reaching both HST and ISS during the same mission. Therefore, further manned service missions have been cancelled, and an unmanned one is being considered. Among astronomers, there is also popular belief that this decision was motivated by the Bush administration's new manned space agenda.
Lack of a future service mission can result in failure of the telescope in four ways: instruments, gyroscopes, batteries, and reentry.
Each of the four instruments installed on the telescope will, over time, fail individually. Past servicing missions have swapped out old instruments in favour of new ones, both avoiding failure and making possible new types of science. Without any servicing missions, all of the instruments will eventually fail, each failure degrading the scientific possibilities.
On August 3 2004, the power system of the Space Telescope Imaging Spectrograph (STIS) failed, rendering every part of the instrument inoperable. The electronics had originally been fully redundant, but the first set of electronics failed in May 2001. The failure of the other set leaves the instrument effectively dead. It seems unlikely that any science functionality can be salvaged without a servicing mission.
Hubble uses gyroscopes to stabilize itself in orbit. Without them it will be unable to remain steady long enough to take meaningful pictures. The current gyroscopes are expected to have all failed by 2012, resulting in the end of Hubble's science mission. However, as three gyroscopes are currently required for operation, the mission may end earlier; work is underway to allow Hubble to operate on only two. The gyroscopes also allow ground controllers to position the telescope. Without them, the difference in gravity between the top and the bottom of the telescope will cause it always to point perpendicular to the earth in what is known as a gravity-gradient position.
In addition to predicted gyroscope failure, Hubble will require a change of batteries. A robotic servicing mission including this would be tricky, as it requires many operations, and a failure in any would likely result in loss of all research capabilities for Hubble. However, the observatory was designed so that during Shuttle servicing missions it would receive power from a connection to the Space Shuttle. This fact may be utilized by adding an external power source (an additional battery) rather than changing the internal ones. Plans for such a mission are in the works.
Hubble is currently (July 2003) in a 569 km orbit. If it is not reboosted by a shuttle or other means, it will reenter the Earth's atmosphere sometime between 2010 and 2032. The exact date is dependent on how active the Sun is and its impact on the upper atmosphere, though it is likely to be earlier rather than later. The state of Hubble's gyros also impact the reentry date, as a controllable telescope can be made to minimize atmospheric drag.
Not all of the telescope will burn up on reentry. Parts of the main mirror and its support structure are expected to survive, leaving the unfortunate potential for damage or even human fatalies (estimated at up to a 1 in 700 chance of human fatality for a completely uncontrolled reentry). The most recent deorbit plan involved attaching a propulsion module to the satellite in a future mission, which would provide a controlled reentry in 2010. At this time only Russia has the automatic docking capabilities to complete such a plan. Even this option is not simple, as Hubble has none of the active docking hardware required for the Russian automated docking systems to function.
Other Hubble mission options include:
- Retrieval by a space shuttle. This was the original plan for Hubble disposal. It would then most likely be displayed in the Smithsonian. The problems with this method are the cost (~$500,000,000) and risk of a shuttle's crew. Recommendations by the Columbia Accident Investigation Board impose a restriction of only two further missions (after 26 August 2003), making it unlikely that one of these missions will be dedicated to servicing Hubble. Also, this would require a rebuild of the cargo space of the space shuttle sent to retrieve Hubble, since the only space shuttle unmodified since Hubble's launch (and thereby able to hold it in its cargo space) was the destroyed Columbia shuttle.
- Addition of an external propulsion module to allow controlled reentry. This option is currently being investigated by NASA, and would not have to be executed until the expected natural reentry date, potentially after Hubble has completed its operational lifetime. One potential model involves a Pac-Man shaped unit entirely enclosing the satellite.
- Addition of a one-time-use external propulsion module to boost the telescope into a ~2300km holding or disposal orbit. Hubble would likely remain in such an orbit for hundreds if not thousands of years.
The Hubble Robotic Vehicle would consist of
- the de-orbit module
- a grapple arm
- a dexterous robot
- an ejection module to carry the grapple arm and robot
Hubble was designed for 15 years of operation, and it will end up serving for at least 22.
Now the space agency and the astronomy community have to sit down and figure out what, if anything, should follow the Hubble. The James Webb Space Telescope (JWST, formerly known as the Next Generation Space Telescope, NGST) may replace the HST in 2012. However, the JWST is an infrared telescope, while the Hubble covered the range from the near infrared through the visible into the near ultraviolet.
What complicates the question are the breathtaking advances in Earth-based astronomy since the Hubble was conceived. During the 1970s when Hubble was designed, the conventional wisdom was that ground based telescopes would never have the resolution of space telescopes because the atmospheric seeing limited the resolution of ground telescopes. In fact, optical imaging observations of bright sources using speckle interferometry or optical interferometry in the 1980s had far higher resolution than Hubble ever achieved, and microcomputer technology starting in the 1990s allowed for adaptive optics imaging of faint objects.
This means that there is not any need to replace the Hubble to obtain better astronomical imagery in the visible range. The new ground-based telescopes can do the job, and even the most ambitious of them, like the Keck in Hawaii and the Very Large Telescope (VLT) in Chile, are much less expensive than the Hubble and much more sensitive to near infrared light (although Hubble still has by far the highest sensitivity in the visible regime, and existing ground-based telescopes will never compete with the Hubble deep fields in the visible due to the effects of atmospheric airglow). This naturally is much easier to service and update. For example, the VLT cost was roughly 1/7 of the HST cost, and gave the astronomical community four 8.2 meter telescopes, each with a resolution almost as high as the Hubble, and probably representing better value for money in terms of the science returned.
The other side of this, of course, is that ground based telescopes are only just now approaching the sensitivity of a 20 year old 2.4 meter space telescope at visible wavelengths. Much of the detector technology developed for these ground telescopes is applicable to new space telescopes. A new space telescope employing these new technologies and other general advancements made in spaceflight over the last 20 years could once again revolutionize astronomy.
Space telescopes are needed for wavelengths outside of visible wavelengths. In particular, Hubble was eventually used largely for observations of the near-ultraviolet, a frequency for which no new telescopes are currently planned.
On 29 January 2004, NASA's Administrator said that that he would review his decision to cancel the final servicing mission of the Hubble Space Telescope due to public outcry and requests from Congress for NASA to look for a way to save the Hubble Space Telescope. Adm. Hal Gehman , chairman of the NASA board that investigated the Space Shuttle Columbia incident would review and send his opinion to NASA according to a letter sent from NASA Administrator Sean O' Keefe to Maryland Senator Barbara Mikulski; Mikulski is the ranking Democrat on the Senate subcommittee that oversees NASA's budget. Websites have been set up dedicated to saving the Hubble Space Telescope.
Federal lawmakers noted that NASA's next generation space telescope, named the James Webb Telescope is not scheduled for launch until 2010, many years after the Hubble Space Telescope was originally expected to cease functions. They also noted that about $200 million has already been spent on two new instruments designed for the Hubble Space Telescope and it might cost $300 million for a mission to return the Hubble Space Telescope safely to the Earth. 
On 13 July 2004, an official panel from the National Academy of Sciences made the recommendation that the Hubble telescope be preserved despite the apparent risks. Their report urged "NASA should take no actions that would preclude a space shuttle servicing mission to the Hubble Space Telescope."
On August 11, 2004, Sean O'Keefe requested the Goddard Space Flight Center to prepare a detailed proposal for a robotic service mission. It is expected that the proposal will take 12 months to produce - any such mission, likely to cost in excess of $1 billion, will not take place before 2007.
In early August 2004 NASA announced that MD Robotics of Canada would be the sole bidder for the robot component of the rescue mission, given its experience with Canadarm and the space station's Mobile Servicing System. The last component of the system is the recently completed Special Purpose Dexterous Manipulator which is nearly an "off-the-shelf" answer to the requirements of the repair mission.
- The Hubble Wars, Eric J. Chaisson, 1998, ISBN 0-674-41255-9, a book by a former member of the science team at the Space Telescope Science Institute, about the launch of the HST and the endless struggles between the science and engineering teams over operational problems and inherent flaws in the telescope, as well as the attempts by publicists and politicians to spin the problems to the public.