Tuesday, 6 February 2007

Teleskop Luar Angkasa Hubble VIII


Equipment failure

A WFPC2 image of a small region of theTarantula Nebula in the Large Magellanic Cloud
Past servicing missions have exchanged old instruments for new ones, both avoiding failure and making possible new types of science. Without servicing missions, all of the instruments will eventually fail. In August 2004, the power system of the Space Telescope Imaging Spectrograph (STIS) failed, rendering the instrument inoperable. The electronics had originally been fully redundant, but the first set of electronics failed in May 2001.[131] This power supply was fixed during servicing mission 4 in May 2009. Similarly, the main camera (the ACS) primary electronics failed in June 2006, and the power supply for the backup electronics failed on January 27, 2007.[132] Only the instrument's Solar Blind Channel (SBC) was operable using the side-1 electronics. A new power supply for the wide angle channel was added during SM 4, but quick tests revealed this did not help the high resolution channel.[133]As of late May 2009, tests of both repaired instruments are still ongoing.
HST uses gyroscopes to stabilize itself in orbit and point accurately and steadily at astronomical targets. Normally, three gyroscopes are required for operation; observations are still possible with two, but the area of sky that can be viewed would be somewhat restricted, and observations requiring very accurate pointing are more difficult.[134] There are further contingency plans for science with just one gyro,[135] but if all gyros fail, continued scientific observations will not be possible. In 2005, it was decided to switch to two-gyroscope mode for regular telescope operations as a means of extending the lifetime of the mission. The switch to this mode was made in August 2005, leaving Hubble with two gyroscopes in use, two on backup, and two inoperable.[136] One more gyro failed in 2007.[137] By the time of the final repair mission, during which all six gyros were replaced (with two new pairs and one refurbished pair), only three gyros were still working. Engineers are confident that they have identified the root causes of the gyro failures, and the new models should be much more reliable.[138]
In addition to predicted gyroscope failure, Hubble eventually required a change of nickel hydrogen batteries. A robotic servicing mission including this would be tricky, as it requires many operations, and a failure in any might result in irreparable damage to Hubble. Alternatively, the observatory was designed so that during shuttle servicing missions it would receive power from a connection to the space shuttle, and this capability could have been utilized by adding an external power source (an additional battery) rather than changing the internal ones.[139] In the end, however, the batteries were simply replaced during service mission 4.

Orbital decay

Hubble orbits the Earth in the extremely tenuous upper atmosphere, and over time its orbit decays due to drag. If it is not re-boosted by a shuttle or other means, it will re-enter the Earth's atmosphere sometime between 2019 and 2032, with the exact date depending on how active the Sun is and its impact on the upper atmosphere. The state of Hubble's gyros also affects the re-entry date, as a controllable telescope can be oriented to minimize atmospheric drag. Not all of the telescope would burn up on re-entry. Parts of the main mirror and its support structure would probably survive, leaving the potential for damage or even human fatalities (estimated at up to a 1 in 700 chance of human fatality for a completely uncontrolled re-entry).[140] With the success of STS-125, the natural re-entry date range has been extended further as the mission replaced its gyroscopes, even though Hubble was not re-boosted to a higher orbit.
NASA's original plan for safely de-orbiting Hubble was to retrieve it using a space shuttle. The Hubble telescope would then have most likely been displayed in the Smithsonian Institution. This is no longer considered practical because of the costs of a shuttle flight, the mandate to retire the space shuttles years prior, and the risk to a shuttle's crew. Instead NASA looked at adding an external propulsion module to allow controlled re-entry.[141] The final decision was not to attach a de-orbit module on STS-125, but to add a grapple fixture so a robotic mission could more easily attach such a module later.[142]

Debate over final servicing mission

Columbia was originally scheduled to visit Hubble again in February 2005. The tasks of this servicing mission would have included replacing a fine guidance sensor and two broken gyroscopes, placing protective "blankets" on top of torn insulation, replacing the Wide Field and Planetary Camera 2 with a new Wide Field Camera 3 and installing the Cosmic Origins Spectrograph (COS). However, then-NASA Administrator Sean O'Keefe decided that, in order to prevent a repeat of the Columbia accident, all future shuttles must be able to reach the 'safe-haven' of the International Space Station (ISS) should an in-flight problem develop that would preclude the shuttle from landing safely. The shuttle is incapable of reaching both the Hubble Space Telescope and the International Space Station during the same mission, and so future manned service missions were canceled.[143]
This decision was assailed by numerous astronomers, who felt that Hubble was valuable enough to merit the human risk. HST's successor, the James Webb Space Telescope (JWST), will not be ready until well after the 2010 scheduled retirement of the space shuttle. While Hubble can image in the ultraviolet and visible wavelengths, JWST is limited to the infrared. The break in space-observing capabilities between the decommissioning of Hubble and the commissioning of a successor is of major concern to many astronomers, given the great scientific impact of HST taken as a whole.[144] The consideration that the JWST will not be located in low Earth orbit, and therefore cannot be easily repaired in the event of an early failure, only makes these concerns more acute. Nor can JWST's instruments be easily upgraded. On the other hand, many astronomers felt strongly that the servicing of Hubble should not take place if the costs of the servicing come from the JWST budget.
In January 2004, O'Keefe said he would review his decision to cancel the final shuttle servicing mission to HST due to public outcry and requests from Congress for NASA to look for a way to save it. On 13 July 2004 an official panel from the National Academy of Sciencesmade the recommendation that the HST should 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". In August 2004, O'Keefe requested the Goddard Space Flight Center to prepare a detailed proposal for a robotic service mission. These plans were later canceled, the robotic mission being described as "not feasible".[145] In late 2004, several Congressional members, led by Sen. Barbara Mikulski (D-MD), held public hearings and carried on a fight with much public support (including thousands of letters from school children across the country) to get the Bush Administration and NASA to reconsider the decision to drop plans for a Hubble rescue mission.[146]
The arrival in April 2005 of the new NASA Administrator, Michael D. Griffin, changed the status of the proposed shuttle rescue mission. At the time, Griffin stated he would reconsider the possibility of a manned servicing mission. Soon after his appointment, he authorized Goddard Space Flight Center to proceed with preparations for a manned Hubble maintenance flight, saying he would make the final decision on this flight after the next two shuttle missions. In October 2006 Griffin gave the final go-ahead for the mission. The 11-daySTS-125 mission by Atlantis was scheduled for launch in October 2008.[147][148] However, the main data-handling unit failed in late September 2008, halting all reporting of scientific data. This unit has a backup, and on October 25, 2008 Hubble was successfully rebooted and was reported to be functioning normally.[149] However, since a failure in the backup unit would now leave the HST helpless, the service mission was postponed to allow astronauts to repair this problem. This mission got underway on May 11, 2009[150] and completed all the long planned replacements as well as additional repairs, including replacing the main data-handling unit.

Planned successors

Several space telescopes are claimed to be successors to Hubble, and some ground based astronomy lays claim to higher optical achievements. None of the near-term space-based telescopes will duplicate Hubble's wavelength coverage (near ultra-violet to near infrared wavelength), instead concentrating on the farther infrared bands. These bands are preferred for studying high Z and low temperature objects, but cannot be studied from the ground, and cannot be retrofitted to the Hubble since it lacks the requisite cooled optics. None of the space-based successors are designed to be serviced on orbit. In contrast, ground based astronomy includes roughly the same wavelengths as Hubble, is catching up in terms of resolution (via adaptive optics), has much larger light gathering power, and is easily upgraded. However, it cannot yet match the Hubble's excellent resolution over a wide field of view, and the very dark background of space.

JWST plans to detect stars in the early Universe approximately 280 million years older than stars HST now detects.
The James Webb Space Telescope (JWST) is a planned infrared space observatory, and lays claim to being a planned successor of Hubble.[151] The main scientific goal is to observe the most distant objects in the universe, beyond the reach of existing instruments. JWST will be able to detect stars in the early Universe approximately 280 million years older than stars HST now detects.[152][153]
JWST is a NASA-led international collaboration between NASA, the European Space Agencyand the Canadian Space Agency. Formerly called the Next Generation Space Telescope (NGST), it was renamed after NASA's second administrator, James E. Webb, in 2002. The telescope's launch is planned for no earlier than June 2014. It will be launched on an Ariane 5rocket.[154]
Another similar effort is the European Space Agency's Herschel Space Observatory, launched on May 14, 2009. Like JWST, Herschel has a mirror substantially larger than Hubble's, but observes only in the far-infrared.
Much further out is the Advanced Technology Large-Aperture Space Telescope (AT-LAST)[155] is a proposed 8 to 16-meter (320 to 640-inch) optical space telescope that if approved, built, and launched (using the planned Space Launch System), would be a true replacement and successor for the Hubble Space Telescope (HST); with the ability to observe and photograph astronomical objects in the opticalultraviolet, and Infrared wavelengths, but with substantially better resolution than Hubble.
Selected space telescopes & instruments[156]
GALEX20030.135-0.280 μm0.5 m
Spitzer20033-180 μm0.85 m
Hubble STIS19970.115-1.03 μm2.4 m
Hubble WFC320090.2-1.7 μm2.4 m
Herschel200960-672 μm3.5 m
JWSTPlanned0.6-10 μm6.5 m
Existing ground based telescopes, and various proposed Extremely Large Telescopes, certainly exceed the HST in terms of sheer light gathering power, due to their much larger mirrors. In some cases, they may also be able to match or beat Hubble in resolution by using adaptive optics (AO). However, AO on large ground-based reflectors will not make Hubble and other space telescopes obsolete. Most AO systems sharpen the view over a very narrow field – Lucky Cam, for example, produces crisp images just 10" to 20" wide, whereas Hubble's cameras are super sharp across a 2½' (150") field. Furthermore, space telescopes can study the heavens across the entire electromagnetic spectrum, most of which is blocked by Earth's atmosphere. Finally, the background sky is darker in space than on the ground, because air absorbs solar energy during the day and then releases it at night, producing a faint—but nevertheless discernible—airglow that washes out faint, low-contrast astronomical objects.[157]

See also