Friday, 26 January 2007

Teleskop Luar Angkasa Hubble VI

Scientific results

Key Projects

In the early 1980s, NASA and StScI convened four panels to discuss Key Projects. These were projects that were both scientifically important and would require significant telescope time, which would be explicitly dedicated to each project. This guaranteed that these particular projects would be completed early, in case the telescope failed sooner than expected. The panels identified three such projects: (1) a study of the nearby intergalactic medium using quasar absorption lines to determine the properties of the intergalactic medium and the gaseous content of galaxies and groups of galaxies;[83] (2) a medium deep survey using the Wide Field Camera to take data whenever one of the other instruments was being used[84] and (3) a project to determine the Hubble Constant within ten percent by reducing the errors, both external and internal, in the calibration of the distance scale.[85]

Important discoveries

One of Hubble's most famous images,Pillars of Creation shows stars forming in the Eagle Nebula
The Hubble has helped to resolve some long-standing problems in astronomy, as well as turning up results that have required new theories to explain them. Among its primary mission targets was to measure distances to Cepheid variable stars more accurately than ever before, and thus constrain the value of the Hubble constant, the measure of the rate at which the universe is expanding, which is also related to its age. Before the launch of HST, estimates of the Hubble constant typically had errors of up to 50%, but Hubble measurements of Cepheid variables in the Virgo Cluster and other distant galaxy clusters provided a measured value with an accuracy of ±10%, which is consistent with other more accurate measurements made since Hubble's launch using other techniques.[86]
While Hubble helped to refine estimates of the age of the universe, it also cast doubt on theories about its future. Astronomers from the High-z Supernova Search Team and theSupernova Cosmology Project[87] used the telescope to observe distant supernovae and uncovered evidence that, far from decelerating under the influence of gravity, the expansion of the universe may in fact be accelerating. This acceleration was later measured more accurately by other ground-based and space-based telescopes, confirming Hubble's finding. The cause of this acceleration remains poorly understood;[88] the most common cause attributed is dark energy.[89]
The high-resolution spectra and images provided by the HST have been especially well-suited to establishing the prevalence of black holes in the nuclei of nearby galaxies. While it had been hypothesized in the early 1960s that black holes would be found at the centers of some galaxies, and work in the 1980s identified a number of good black hole candidates, it fell to work conducted with Hubble to show that black holes are probably common to the centers of all galaxies.[90][91][92] The Hubble programs further established that the masses of the nuclear black holes and properties of the galaxies are closely related. The legacy of the Hubble programs on black holes in galaxies is thus to demonstrate a deep connection between galaxies and their central black holes.
The collision of Comet Shoemaker-Levy 9 with Jupiter in 1994 was fortuitously timed for astronomers, coming just a few months after Servicing Mission 1 had restored Hubble's optical performance. Hubble images of the planet were sharper than any taken since the passage of Voyager 2 in 1979, and were crucial in studying the dynamics of the collision of a comet with Jupiter, an event believed to occur once every few centuries.
Other major discoveries made using Hubble data include proto-planetary disks (proplyds) in the Orion Nebula;[93] evidence for the presence of extrasolar planets around sun-like stars;[94] and the optical counterparts of the still-mysterious gamma ray bursts.[95] HST has also been used to study objects in the outer reaches of the Solar System, including the dwarf planets Pluto[96] and Eris.[97]
A unique legacy of Hubble are the Hubble Deep Field and Hubble Ultra Deep Field images, which utilized Hubble's unmatched sensitivity at visible wavelengths to create images of small patches of sky that are the deepest ever obtained at optical wavelengths. The images reveal galaxies billions of light years away, and have generated a wealth of scientific papers, providing a new window on the early Universe.
The non-standard object SCP 06F6 was discovered by the Hubble Space Telescope (HST) in February 2006.[98][99]

Impact on astronomy

Distant galaxies in deep space in aHubble Ultra Deep Field photograph
Many objective measures show the positive impact of Hubble data on astronomy. Over 9,000 papers based on Hubble data have been published in peer-reviewed journals,[100] and countless more have appeared in conference proceedings. Looking at papers several years after their publication, about one-third of all astronomy papers have no citations, while only 2% of papers based on Hubble data have no citations. On average, a paper based on Hubble data receives about twice as many citations as papers based on non-Hubble data. Of the 200 papers published each year that receive the most citations, about 10% are based on Hubble data.[101]
Although the HST has clearly had a significant impact on astronomical research, the financial cost of this impact has been large. A study on the relative impacts on astronomy of different sizes of telescopes found that while papers based on HST data generate 15 times as many citations as a 4 m ground-based telescope such as the William Herschel Telescope, the HST costs about 100 times as much to build and maintain.[102]
Making the decision between investing in ground-based versus space-based telescopes in the future is complex. Even before Hubble was launched, specialized ground-based techniques such as aperture masking interferometry had obtained higher-resolution optical and infrared images than Hubble would achieve, though restricted to targets about 108 times brighter than the faintest targets observed by Hubble.[103][104] Since then, advances in adaptive optics have extended the high-resolution imaging capabilities of ground-based telescopes to the infrared imaging of faint objects. The usefulness of adaptive optics versus HST observations depends strongly on the particular details of the research questions being asked. In the visible bands, adaptive optics can only correct a relatively small field of view, whereas HST can conduct high-resolution optical imaging over a wide field. Only a small fraction of astronomical objects are accessible to high-resolution ground-based imaging; in contrast Hubble can perform high-resolution observations of any part of the night sky, and on objects that are extremely faint.


The Hubble Space Telescope as seen from Space Shuttle Discovery during its second servicing mission (STS-82).
To commemorate Hubble Telescope's 20th Birthday, NASA, along with ESA and Space Telescope Institute, released findings from Hubble.
Anyone can apply for time on the telescope; there are no restrictions on nationality or academic affiliation.[105] Competition for time on the telescope is intense, and the ratio of time requested to time available (the oversubscription ratio) typically ranges between 6 and 9.[106]
Calls for proposals are issued roughly annually, with time allocated for a cycle lasting approximately one year. Proposals are divided into several categories; 'general observer' proposals are the most common, covering routine observations. 'Snapshot observations' are those in which targets require only 45 minutes or less of telescope time, including overheads such as acquiring the target; snapshot observations are used to fill in gaps in the telescope schedule that cannot be filled by regular GO programs.[107]
Astronomers may make 'Target of Opportunity' proposals, in which observations are scheduled if a transient event covered by the proposal occurs during the scheduling cycle. In addition, up to 10% of the telescope time is designated Director's Discretionary (DD) Time. Astronomers can apply to use DD time at any time of year, and it is typically awarded for study of unexpected transient phenomena such as supernovae.[108] Other uses of DD time have included the observations that led to the production of the Hubble Deep Field and Hubble Ultra Deep Field, and in the first four cycles of telescope time, observations carried out by amateur astronomers.

Amateur observations

The first director of STScI, Riccardo Giacconi, announced in 1986 that he intended to devote some of his Director Discretionary time to allowing amateur astronomers to use the telescope. The total time to be allocated was only a few hours per cycle, but excited great interest among amateur astronomers.[109]
Proposals for amateur time were stringently peer reviewed by a committee of leading amateur astronomers, and time was awarded only to proposals that were deemed to have genuine scientific merit, did not duplicate proposals made by professionals, and required the unique capabilities of the space telescope. In total, 13 amateur astronomers were awarded time on the telescope, with observations being carried out between 1990 and 1997. One such study was Transition Comets — UV Search for OH Emissions in Asteroids. The very first proposal, A Hubble Space Telescope Study of Post Eclipse Brightening and Albedo Changes on Io, was published in Icarus,[110] a journal devoted to solar system studies. After that time, however, budget reductions at STScI made the support of work by amateur astronomers untenable, and no further amateur programs have been carried out.[111]

20th birthday

The Hubble Telescope celebrated its 20th birthday on April 22, 2010. To commemorate the occasion, NASA, ESA, and Space Telescope Institute (STScI) released an image from the Carina Nebula.[112]



Wednesday, 24 January 2007

Teleskop Luar Angkasa Hubble V

Servicing missions and new instruments

Servicing Mission 1

Astronauts Musgrave and Hoffman installing corrective optics during SM1
Improvement in Hubble images after SM1
Astronauts replacing gyroscopesduring SM3A
Hubble on the payload bay just prior to release during SM3B
The telescope had always been designed so that it could be regularly serviced, but after the problems with the mirror came to light, the first servicing mission assumed a much greater importance, as the astronauts would have to carry out extensive work on the telescope to install the corrective optics. The seven astronauts selected for the mission were trained intensively in the use of the hundred or more specialized tools that would be needed.[66] The Space Shuttle Endeavour mission STS-61 took place in December 1993, and involved installation of several instruments and other equipment over a total of 10 days.
Most importantly, the High Speed Photometer was replaced with the COSTAR corrective optics package, and WFPC was replaced with the Wide Field and Planetary Camera 2 (WFPC2) with its internal optical correction system. In addition, the solar arrays and their drive electronics were replaced, as well as four of the gyroscopes used in the telescope pointing system, two electrical control units and other electrical components, and two magnetometers. The onboard computers were upgraded, and finally, the telescope's orbit was boosted, to compensate for the orbital decay from 3 years of drag in the tenuous upper atmosphere.[50]
On January 13, 1994, NASA declared the mission a complete success and showed the first of many much sharper images.[67] At the time, the mission had been one of the most complex ever undertaken, involving five lengthy periods of extra-vehicular activity, and its resounding success was an enormous boon for NASA, as well as for the astronomers who now had a fully capable space telescope.

Servicing Mission 2

Servicing Mission 2, flown by Discovery (STS-82) in February 1997, replaced the GHRS and the FOS with the Space Telescope Imaging Spectrograph (STIS) and the Near Infrared Camera and Multi-Object Spectrometer (NICMOS), replaced an Engineering and Science Tape Recorder with a new Solid State Recorder, repaired thermal insulation and again boosted Hubble's orbit.[68] NICMOS contained a heat sink of solid nitrogen to reduce the thermal noise from the instrument, but shortly after it was installed, an unexpected thermal expansion resulted in part of the heat sink coming into contact with an optical baffle. This led to an increased warming rate for the instrument and reduced its original expected lifetime of 4.5 years to about 2 years.[69]

Servicing Mission 3A

Servicing Mission 3A flown by Discovery (STS-103), took place in December 1999, and was a split-off from Servicing Mission 3 after three of the six onboard gyroscopes had failed. (A fourth failed a few weeks before the mission, rendering the telescope incapable of performing science observations.) The mission replaced all six gyroscopes, replaced a Fine Guidance Sensor and the computer, installed a Voltage/temperature Improvement Kit (VIK) to prevent battery overcharging, and replaced thermal insulation blankets.[70] Although the new computer is hardly a powerhouse (a 25 MHz radiation hardened Intel 486 with two megabytes of RAM), it is still 20 times faster, with six times more memory, than the DF-224 it replaced. The new computer increases throughput by moving some computing tasks from the ground to the spacecraft, and saves money by allowing the use of modern programming languages.[71]

Servicing Mission 3B

Servicing Mission 3B flown by Columbia (STS-109) in March 2002 saw the installation of a new instrument, with the FOC (the last original instrument) being replaced by the Advanced Camera for Surveys (ACS). This meant that COSTAR was no longer required, since all new instruments had correction for the main mirror aberration built in.[65]
The mission also saw the revival of NICMOS, which had run out of coolant in 1999. A new cooling system was installed that reduced the instrument's temperature enough for it to be usable again. Although not as cold as its original design called for, the temperature is more stable, in many ways a better tradeoff.[69] ACS in particular enhanced Hubble's capabilities; it and the revived NICMOS together imaged the Hubble Ultra Deep Field.
The mission replaced the solar arrays for the second time. The new arrays were derived from those built for the Iridium comsat system and were only two-thirds the size of the old arrays, resulting in less drag against the tenuous reaches of the upper atmosphere while providing 30 percent more power. The additional power allowed all instruments on board the HST to be run simultaneously, and reduced a vibration problem that occurred when the old, less rigid arrays entered and left direct sunlight. Hubble's Power Distribution Unit was also replaced in order to correct a problem with sticky relays, a procedure that required the complete electrical power down of the spacecraft for the first time since it was launched.[72]

Servicing Mission 4

Astronauts work on Hubble during SM4

Hubble floats free from Atlantis after SM4.
Servicing Mission 4 (SM4), which took place in May 2009, was the last scheduled shuttle mission (STS-125) for the Hubble Space Telescope.[73] The mission was first planned for October 14, 2008.[74] However on September 27, 2008, the Science Instrument Command and Data Handling (SI C&DH) unit on HST failed. All science data pass through this unit before they can be transmitted to Earth. Although it had a backup unit, if the backup were to fail, the HST's useful life would be over.[75] Therefore on September 29, 2008, NASA announced that the launch of SM4 would be postponed until 2009 so the SI C&DH unit could be replaced as well.[76] SM4, with a replacement SI C&DH unit,[77] was launched aboardSpace Shuttle Atlantis on May 11, 2009.[78]
On SM4, astronauts, over the course of five spacewalks, installed two new instruments, Wide Field Camera 3 (WFC3), and the Cosmic Origins Spectrograph (COS). WFC3 will increase Hubble's observational capabilities in the ultraviolet and visible spectral ranges by up to 35 times due to its higher sensitivity and wider field of view. The telephone-booth sized COS assembly replaced the Corrective Optics Space Telescope Axial Replacement (COSTAR) that was installed in 1993 to correct Hubble's spherical aberration problems. (COSTAR was no longer needed after the replacement of the FOC in 2002, the last original instrument without the necessary correction built in.[65]) The COS will do observations in the ultraviolet parts of the spectrum, complementing the measurements done by the repaired STIS system.
The mission repaired two instruments that had failed, the Advanced Camera for Surveys(ACS) and the Space Telescope Imaging Spectrograph (STIS). The astronauts also performed other component replacements, including all three Rate Sensor Units (each containing two gas-bearing gyroscopes); one of three Fine Guidance Sensor (FGS) units used to help keep pointing accuracy and increase platform stability; the SI C&DH unit; all six of the 125-pound (57 kg) nickel-hydrogen batteries used to provide all Hubble's electrical power to support operations during the night portion of its orbit; and three New Outer Blanket Layer (NOBL) thermal insulation protective blankets. The batteries had never been replaced and were more than 13 years over their original design life.[79]
Atlantis released the Hubble Space Telescope on May 19, 2009 back into space after all repairs were successfully made. After testing and calibration, Hubble resumed routine operation in September 2009.[80] These efforts are expected to keep the telescope fully functioning at least into 2014, and perhaps longer.[81]
Hubble was originally designed to be returned to earth on board a shuttle. With the retirement of the shuttle fleet, in July 2011, this will no longer be possible. NASA engineers developed the Soft Capture and Rendezvous System (SCRS), a ring-like device that was attached to Hubble’s aft bulkhead during SM4, which will enable the future rendezvous, capture, and safe disposal of Hubble by either acrewed or robotic mission.[82] The next mission will be to deorbit Hubble at the end of its service life.