Looking sharper

THE 19-YEAROLD HUBBLE Space Telescope.-

THE space shuttle Atlantis, which was launched on May 11 with a crew of seven, returned safely to the earth after one of its most exacting missions, lasting 13 days. In this period the astronauts performed five spacewalks extravehicular activities (EVAs) to fix some of the on-board instruments of the 19-year-old HST and enhance its capabilities by replacing some old instruments with new and more capable ones. A National Aeronautics and Space Administration (NASA) release said: Its a mission to once more push the boundaries of how deep in space and far back in time humanity can see. Hubbles discovery power is now stated to have improved by 10 to 70 times.

Atlantis touchdown was delayed by two days which added about $1.8 million to the cost of the servicing mission of about $1 billion because of inclement weather in Floridas Vanderbilt Air Force Base, and the landing was finally diverted to Edwards Air Force Base in California. Following NASAs decision to retire its shuttle fleet in 2010, this was the fifth and final servicing mission to the HST and, hence, a crucial one from the point of view of science.

Now and only now can we declare this mission a total success the astronauts are safely on the ground, NASA sciences chief Ed Weiler told a press conference in Florida. Safe re-entry of shuttles is itself a major issue for shuttle launches after the Columbia disaster of 2003 in which all the astronauts perished during re-entry. In fact, this service mission got pushed from its scheduled date in 2004 because of the shuttle tragedy. The integrity of the tiles of the shuttles heat shield had to be ensured before the shuttle could disengage itself from the telescope in its 550-kilometre-high orbit and begin its earthward journey. Any problem with the heat shield would have forced the standby shuttle, Endeavour, which has been readied for a June launch to the International Space Station, to take off on a rescue mission and ferry back the astronauts.

The HST is a collaboration between NASA and the ESA, which put in 15 per cent of the total mission cost. The construction alone cost $2.5 billion, and the cumulative mission cost to this day is estimated to be around $5 billion, with the European contribution estimated at around $750 million. Along with the Compton Gamma Ray Observatory, the Chandra X-ray Observatory and the Spitzer Space Telescope, the HST is one of NASAs Great Observatories. For 19 years, the HST, with its powerful suite of instruments, has consistently produced outstanding scientific results and has caught the public imagination with its spectacularly sharp images of the far reaches of the universe. Terrestrial telescopes can never produce such images because of the absorption and distortion caused by the intervening atmosphere above the earth. The HSTs success can be attributed to two things: one, the robust initial design of the spacecraft and, two, the series of servicing missions (SMs) that has kept the complex space platform functioning and up to date.

Before this last set of improvements on what is arguably the most significant satellite ever launched, astronauts visited the HST to carry out repairs and upgrades in 1993 (SM-1), 1997 (SM-2), 1999 (SM-3A) and 2002 (SM-3B). After the current servicing mission designated SM-4 though it is the fifth one because SM-3 was split into two every major component on the spacecraft, apart from the mirrors, has undergone at least one upgrade since its launch. The HST is the only telescope ever designed to be serviced in space by astronauts. After this servicing mission, the HST is expected to function at least until 2014 when its successor, the James Webb Space Telescope (JWST), is expected to be launched. The JWST is intended to be far superior to the HST but will observe only in the IR, much like Herschel. So it will complement Hubbles observations in the near-IR, visible and ultraviolet (UV) parts of the spectrum.

The primary objective of SM-4 was to deliver two new instruments the Cosmic Origins Spectrograph (COS) and the more capable Wide Field Camera 3 (WFC3). COS has replaced the Corrective Optics Space Telescope Axial Replacement (COSTAR), a 16-year-old package that was installed during SM-1 to correct the flawed 2.4-metre-diameter primary mirror of the telescope. This contact lens for the telescope has now been rendered redundant because, since SM-1, all of the HSTs replacement instruments have had technology built into them to correct the telescopes blurred vision. Stowed in the cargo bay of the shuttle, COSTAR has been brought back. The WFC3, with a higher resolution and a larger field of view, has replaced Wide Field Planetary Camera 2 (WFPC2), which was the workhorse of the HST all these years. These two high-technology instruments will dramatically improve the HSTs potential for discovery and enable the telescope to pick up the faint light from the youngest stars and galaxies.

Besides the complicated operation of replacing the old with the new, two instruments needed repair. These were the Advanced Camera for Surveys (ACS), the instrument that has produced Hubbles most popular and dramatic images, and the Space Telescope Imaging Spectrograph (STIS), the most versatile spectrograph to be flown on Hubble. In January 2007, the ACS suffered a serious power failure, which caused the three observing channels the Wide Field Channel, the Solar Blind Channel and the High Resolution Channel to cease functioning. The SBC was returned to service in February 2007, but the WFC and the HRC remained inoperative. The STIS, too, failed in 2004 following a power supply problem.

This attempt at in-orbit repairs was unprecedented. It was particularly challenging because the instruments were not designed for in-orbit repairs. In fact, they had been specifically designed not to come apart. The repairs could only be successfully accomplished by the use of new tools and special procedures that NASA engineers had developed specifically for the mission.

However, space hardware always springs surprises. In one such instance during SM-4, attempts to remove a bolt on a handrail failed despite all the new tools that were deployed. Supported by real-time, on-ground validation of the required forces, the astronaut eventually had to apply brute force to remove the handrail that was obstructing the STIS repair tool.

In addition, the spacecraft itself needed some repairs and maintenance to make it last longer. The nickel-hydrogen (Ni-H) batteries, for example, that the HST was flying with were the originals ones, sent up in 1990. They lasted much longer than their intended life and had begun to degrade. A degrading Fine Guidance Sensor (FGS), one of the three on board, had to be replaced. The FGS acquires a distant star, locks onto it and fixes the satellites orientation with respect to it and the three gyroscope assemblies (containing two gyroscopes each which have limited lifetimes) that keep the pointing of the telescope rock steady.

Hubbles FGSs and gyroscopes together provide extraordinary pointing stability 0.007 arcsecond of jitter akin to holding a laser beam on a coin 300 km away. The FGSs also provide capability for astrometry the detailed study of stellar dynamics and motions enabling the determination of close binary stars and star-planet systems. These replacement operations required one of the longest spacewalks in history, lasting nearly eight hours.

Servicing Mission 4 was actually scheduled for October 14, 2008. But in late September 2008, just 17 days before the launch, it was discovered that after working for 18 and a half years the primary channel of the Science Instrument Command & Data Handling (SIC&DH) Unit had failed. The unit is actually a computer that sends commands to the HSTs science instruments and formats the data for transmission to the ground. The launch was postponed so that a replacement unit could be made ready and the crew could be trained for the task. Though the telescope was operational with a back-up system despite the failed unit, its replacement restored the redundancy.

To prevent drastic temperature variations to the HSTs subsystems as it moves from daylight into darkness behind the earth and away from the sun, the telescope is thermally insulated by Multi Layer Insulation (MLI). Over the 19 years of exposure to the harsh space environment and radiation, the MLI had degraded considerably. Three large surface areas of the MLI were replaced with panel-type thermal insulation called New Outer Blanket Layer (NOBL) made from thin stainless steel foils. Originally, only two replacements were scheduled. But, as the astronauts were able to complete the tasks on the last EVA day earlier than anticipated, the third area was also replaced. The removal of the damaged MLI did prove to be tricky with some pieces free-floating into space.

An important addition to the telescope hardware is a new soft-capture and docking interface for easy de-orbiting of the spacecraft at the end of its life. Originally, the spacecraft was designed to be returned on board a shuttle. But with the retirement of the shuttle fleet, this is no longer an option. The new device is actually a suitable interface for autonomous docking, say, with a robotic spacecraft of the future. This addition has been made to the berthing side of the telescope.

Space record broken

Since the shuttle Discovery launched the HST on April 24, 1990, Hubble has orbited the earth more than 97,000 times and provided more than 4,000 astronomers access to stars and galaxies not observable using ground-based telescopes. It has provided some of the most stunning images of the sky. On December 28, 2008, the orbiting telescope broke one of the longest-standing space records. It registered 6,823 days in orbit and overtook the continuous orbital observation held by the ESA/NASA/United Kingdom International Ultraviolet Explorer (IUE), a mission that ended in 1996. It has helped resolve some long-standing problems in astronomy but has also thrown up observations that have needed to be explained with new theories. These have helped answer some of physicists key questions about the universe. The HST has actually seen an object that emitted light about 13 billion years ago. Since the universe is 13.7 billion years old which estimate is also thanks to the HST it is light from the universe at its infancy. The HST has brought to us startling images from the nearest parts of our solar system to the farthest reaches of the universe or, equivalently, further back in time than ever before.

One of its major contributions is to constrain the value of the Hubble Constant, the measure of the rate at which the universe is expanding, which is related to its age. This it did by measuring more accurately than ever before the distances to the so-called Cepheid variable stars, which vary periodically in brightness. Before the HST, the Hubble Constant typically had errors of up to 50 per cent, but the HSTs measurements of Cepheid variables in the Virgo Cluster and other distant galaxy clusters provided a value with 10 per cent error. This has helped refine the age of the universe to 13.7 billion years.

Dark energy

The use of the HST to observe distant supernovae threw up a puzzling aspect of the universes fate in the future. These observations showed that, instead of a decelerating universe under the action of mutual gravitational tugs between galaxies and stars, it may, in fact, be accelerating. This accelerating universe has now been seen by other astronomical measurements as well but the cause of this acceleration is poorly understood but hypothesised to be because of a mysterious force of dark energy, which is believed to constitute about 70 per cent of the universes content.

Before the HST, in the 1980s, astronomers suspected, but had no proof, that supermassive black holes lurk at the centre of galaxies. The high-resolution images from the WFPC2, together with the high-resolution spectroscopic data from Hubble, showed that most galaxies in the universe do indeed harbour monstrous black holes up to a billion times the mass of our sun. The legacy of the Hubble programme on black holes in galaxies has thus demonstrated a deep and profound connection between galaxies and their central black holes.

The WFPC2 gave the world a stunning view of Comet Shoemaker-Levy 9 plunging into the gas giant Jupiter in 1994. The images showed the event in great detail, including the ripples expanding outwards from the impact. The Hubble images were much sharper than any taken since the passage of Voyager 2 in 1979 and were crucial for the study of the dynamics of the collision of a comet with Jupiter, an event believed to occur once every few centuries.

The WFPC2 has captured exquisite pictures of the birth and death of stars. The HSTs famed picture of the Pillars of Creation and other images of colourful dying stars offered the first detailed view of life of stars. The camera also took the first pictures of the dusty discs around stars where planets are born, particularly the proto-planetary discs in the Orion Nebula, and provided evidence for the presence of extrasolar planets around sun-like stars, thus demonstrating that planet-forming environments are a common feature in the universe.

The HSTs Hubble Deep Field and Hubble Ultra Deep Field images, which made use of the telescopes high sensitivity at optical wavelengths to create images of small patches of sky, are the deepest ever obtained in the visible part of the spectrum. These images reveal galaxies billions of light years away and have provided a new window on the early universe. This unique legacy of Hubbles has generated a wealth of scientific research, and the telescope continues to have a significant impact on astronomical research by discovering several uncommon and non-standard stellar objects. But all these discoveries would have been impossible if the bold decision to correct the flaw in the grinding and polishing of the primary mirror by Perkin-Elmer in the first of its servicing missions in 1993. This was followed by three other crucial servicing missions, all of which are worth recounting to make the Hubbles story complete.

Within weeks of the launch of the telescope, the images returned showed that there was a serious problem with the telescopes optical system. The telescope failed to achieve a final sharp focus and the best image quality obtained was drastically lower than expected. Images of point sources had a spread of more than one arcsecond, instead of the design criterion of less than 0.1 arcsecond. The first servicing mission, therefore, assumed great significance because of the extensive work that needed to be carried out on the telescope to install corrective optics. The complex SM-1 involved the installation of several instruments and other equipment over 10 days and five EVAs.

The HSTs High Speed Photometer (HSP) was replaced by the COSTAR corrective package and the WFPC was replaced with the WFPC2, which had an internal corrective optics system. Further, the solar arrays and their drive electronics were replaced, as were four of the six gyroscopes, two electrical control units and other electrical components and two magnetometers. The on-board computer was upgraded and finally the telescopes orbit was boosted to compensate for the orbits decay from three years of drag in the tenuous upper atmosphere. On January 13, 1994, the mission was declared a success when the first sharp images were received, thus heralding a fully capable space telescope.

The second servicing mission, in February 1997, replaced the Goddard High Resolution Spectrograph (GHRS) and the Faint Object Spectrograph (FOS) with STIS and the Near Infrared Camera and Multi-Object Spectrometer (NICMOS). The mission also replaced an Engineering and Science Tape Recorder with a new Solid State Recorder, repaired the thermal insulation and again boosted the telescopes orbit. NICMOS had a heat sink of solid nitrogen to reduce thermal noise from the instrument but shortly after it was installed an unexpected thermal expansion caused the coolant to come into contact with an optical baffle, which led to warming at an increased rate. The lifetime of NICMOS degraded from 4.5 years to two years, which naturally called for an upgrade servicing mission in 1999.

However, SM-3 had to be split into two when it was discovered that three of the six on-board gyroscopes had failed. Three serve as back-ups but just before the mission the fourth gyro too failed, rendering the telescope incapable of observing because it lacked pointing accuracy. During SM-3A, the replacement of thermal insulation blankets, all six gyroscopes, an FGS and the computer chip DF-224 with the 20-times-faster Intel 486 was carried out and a Voltage/temperature Improvement Kit (VIK) was installed.

A new instrument, the ACS, was installed in SM-3B, which was launched in March 2002. This replaced the Faint Object Camera (FOC). The mission also revived NICMOS by replacing the cooling system. The ACS enhanced the HSTs capabilities. Together with the repaired NICMOS, the ACS enabled Hubbles Ultra Deep Field imaging. The mission also replaced the solar array for the second time. The new smaller-sized arrays, built for the Iridium satellite constellation, helped reduce the spacecrafts drag. The HSTs power distribution unit was also replaced. The job required a complete power shutdown, for the first time since Hubbles launch.

With SM-4, the HST has, in fact, acquired a new life. With its new suite of five instruments, the WFC3, COS, the ACS, STIS and NICMOS, Hubble is once again ready for a performance peak higher than before and promising many more discoveries. COS will be the most sensitive UV spectrograph ever flown on the HST. It will probe the cosmic web the large-scale structure of the universe whose form is determined by the gravity of the unseen dark matter and is traced by galaxies and intergalactic gas. COS will sample the chemical content and physical state of the gases in distant galaxy halos, providing important insights into the building process of early galaxies. COS and STIS are highly complementary and together are expected to provide a full set of new tools for astrophysical research. The WFC3 will cover a broader range of the spectrum than its predecessor, ranging from near-UV, through the visible to the near-IR. The WFC3 will be the only instrument with this panchromatic ability. It will be able to probe the nature of dark energy more seriously. Together with the ACS, the WFC3 is expected to open a new era in Hubble imaging in the years to come.

According to a press release, functional testing has confirmed the successful repair of the ACS, except its high-resolution channel, and of STIS. In addition, all the subsystems and units that provide the HST with its new operational capabilities and potentially extending its in-orbit life even up to 2020 have been successfully repaired or replaced and their functionalities too have been validated. According to the two space agencies, NASA and the ESA, SM-4 has been completely successful and expectations have been exceeded. As an ESA release put it, equipped with new eyes representing most advanced detector capabilities, a new brain, stabilisation units and shiny new clothes, the telescope now appears as a new star in orbit. A gift to astronomers in the International Year of Astronomy!