About The Hubble Space Telescope(HST)

Conception

The idea of space-based observation had been floating around in the heads of many astronomers in the early 1900s. One of those astronomers was Lyman Spitzer, who wrote the paper Astronomical advantages of an extraterrestrial observatory. In this paper he envisioned telescope 500 miles above the earth, above atmospheric layers, equipped with various measuring devices for different phases of astronomic research. He anticipated that “Such a telescope could measure the spectra of stars, planets, etc. without the absorption of the earth’s atmosphere would have astronomical uses…obtaining information on the behavior of matter under conditions not in the laboratory, knowledge of fundamentals physics would thereby be enhanced”(1).

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           Spitzer devoted his career to seeing a telescope to be put in space. While he served as the president of the American Astronomical Society and while working with the US space program in the 1960s, he led a program to design an earth orbiting observatory to study ultraviolet light. This project was a success, and was to be known as the Copernicus satellite. Soon Spitzers dream of launching a telescope into space would become a reality. Spitzer was able to convince Congress and the scientific community, the value of placing a telescope into space, giving birth to the very successful Orbiting Astronomical Observatory.

           The success of the Orbiting Astronomical Observatory was essential for the concept of the space telescope, and gave the confidence to the scientific community to pursue greater and more complex space telescope systems. Spitzer continued to lobby NASA and Congress to develop a space telescope. “In 1975, NASA, along with the European Space Agency, began development of what would become the Hubble Space Telescope” (2).

Journey into space

           The design and creation of the Hubble Space Telescope(HST) would be a collaboration between many institutions. Two of NASAs laboratories, the Marshal Space Flight Center(MSFC) and the Goddard Space Flight Center would be dividing the work. Marshal would be responsible for the design, development, and construction of the HST, while Goddard was responsible for the control of the scientific instruments and the ground control for the missions. Two other organizations were commissioned to work on the HST, Perkin-Elmer for the Optical Telescope Assembly and Guidance sensors, and Lockheed to construct the spacecraft in which all the components would be housed (3).

           In early 1986, close to the planned launch date of the Hubble, the program came to a halt. The Challenger accident had put to a stop to the US space program, causing the Hubble’s launch to be delayed several years(4). The telescope had to be stored until a new launch date could be scheduled. During this time, it did allow for Hubble’s engineers to double check all their work, swap out parts, run tests, and make other improvements to the telescope. In 1988, with the resuming of shuttle flights, the Hubble’s launch was schedule for April 24, 1990. The shuttle mission STS-21, flown by Discovery, successfully launched the Hubble Space Telescope into orbit (4).

Hubble Operation Setbacks

           Almost immediately after the launch of the Hubble, it was clear there was something very wrong. Hubble’s incredibly polished and accurate mirrors, could not focus properly. Hubble's two and a half meter on eight point two foot mirror was rendered incapable by an error in its shapes less than one fiftieth of the width of a human hair. There was an aura of panic in the meetings and although there were actuators on the Hubble, designed to push and pull on the telescope mirror to fix small errors, this aberration was “seven times the dynamic range of the actuators”, meaning the problems is seven times larger than they can correct for (5). Discussions began on how to move forward with talk of using it in its current form, fixing it, or scrapping the project all together. Everyone involved with the Hubble was under fire, and the Hubble became known as a national disaster. Many solutions to the problem were possible, but almost all could not be completed while in space. After months of the Hubble floating in space, being completely unusable, a solution was formulated. Corrective lenses were to be placed in front of the sensor, essentially giving the Hubble glasses. The solution and repair, which I will discuss in more detail in a later section, was NASA’s greatest comeback story to date.

Getting data back to earth

           The raw data collected by the telescope have a long way to go before they become actual Hubble images. As Hubble completes a particular observation, it converts the starlight into digital signals. The digital signals are then relayed down to a ground station at White Sands, New Mexico through two orbiting Tracking and Data Relay Satellites. The ground station then relays the data to Goddard Space Flight Center's ground control system, where staff ensure its completeness and accuracy. “Once the ground station transfers the data to Goddard, Goddard sends it to the Space Telescope Science Institute (STScI), where staff translate the data into scientifically meaningful units — such as wavelength or brightness — and archive the information on 5.25-inch magneto-optical disks”(6). The Hubble transmits a whopping 120 Gigabytes of data every week down to earth.

The use of the Hubble’s data and images by Astronomers around the world is very competitive and there is not enough time for every astronomer to get a turn. Astronomers must submit proposals in order to get access and control of the Hubble, and a review committee chooses what they deem to be the best proposals. “The winning proposals are the ones that make the best use of the telescope’s capabilities while addressing pressing astronomical questions. Each year around 1,000 proposals are reviewed and approximately 200 are selected, for a total of 20,000 individual observations”(6).

Servicing Missions and their Impact

Orbital servicing is the key to keeping Hubble in operating condition. “NASA’s orbital servicing plans address three primary maintenance scenarios: Incorporating technological advances into the science instruments, normal degradation of components , and random equipment failure or malfunction.”(7) Originally, planners considered using the Shuttle to return the telescope to Earth approximately every five years for maintenance. However, the idea was rejected for both technical and economic reasons. Returning Hubble to Earth would entail a significantly higher risk of contaminating or damaging delicate components. Ground servicing would require an expensive clean room and support facilities, including a large engineering staff, and the telescope would be out of action for a year or more—a long time to suspend scientific observations. Shuttle astronauts can accomplish most maintenance and refurbishment within an 11-day on-orbit mission with only a brief interruption to scientific operations and without the additional facilities and staff needed for ground servicing

Service Mission 1

           Because of the problem with the Hubble’s primary mirror, there was a lot of fear that Congress would write off the loss and move on. “The first servicing mission, already planned for 1993, then became much more than a simple scheduled service call: It became the only chance to save the program and the spacecraft from either euthanasia or perhaps resignation to living with its diminished performance” (4). The Optical Systems Failure Review board were grateful to find that the mirror was uniform in error. The team worked to find a ‘prescription’ for the aberrated mirror. When it was determined that the mirror was too flat by roughly 2 micrometers, or about 1/40th of a human hair, they were able to create a reverse prescription to correct the problem and a solution was now in the works. In the 1980s, a device was being constructed called the Space Telescope Axial Replacement(STAR), a potential replacement for one of the scientific instruments on the Hubble.

This device would be repurposed with corrective optics for the Hubble’s mirror, coining the name COSTAR. “COSTAR was a telephone booth-sized instrument which placed 5 pairs of corrective mirrors, some as small as a nickel coin, in front of the Faint Object Camera, the Faint Object Spectrograph and the Goddard High Resolution Spectrograph” (8).  The COSTAR was a complex system of mechanical components, mirrors, and electronics all controllable from earth. Since many of the scientific instruments depended on the Hubble’s primary mirror, the defection caused them to become obsolete, and special corrective changes needed to be made for each instrument to function as intended. In addition, the service mission included installation and replacement of various other parts of the Hubble, including an improved version of the Wide Field Planetary Camera, new solar arrays, sensors, and gyroscopes.

On January 13th, 1994 the mission was declared a success. At 11 days, 5 EVAs or Extra-Vehicular Activity periods, the first servicing mission was one of the most complicated and intensive missions performed of its time. '’It's fixed beyond our wildest expectations,’ Program Scientist Ed Weiler beamed at a mid-January press conference. ‘The performance is as perfect as engineering can achieve and the laws of physics will allow(4).This successful mission not only improved Hubble's vision — which led to a string of remarkable discoveries in a very short time — but it also validated the effectiveness of on-orbit servicing.

Service mission 2

After a successful first mission to correct Hubble’s vision in 1993, a second Servicing Mission (STS-82) was launched to the space telescope in February 1997. The goal of this 10-day operation was to enhance Hubble’s scientific capabilities for discovery by conducting a number of maintenance tasks and refurbishing the existing systems. There is no question that Hubble's 'first generation' cameras gave us remarkable views of very distant galaxies. However, the light from the most distant galaxies is shifted to infrared wavelengths by the expanding universe. To see these galaxies, Hubble needed to be fitted with an instrument that could observe infrared light. STS-82 included the installation of two technologically advanced instruments by a crew of astronauts who reached Hubble aboard the Discovery Space Shuttle. Both devices featured technology that was not available when the first designs of the Hubble Space Telescope were produced. Provides Hubble with unique and powerful spectroscopic capabilities. A spectrograph separates the light gathered by the telescope into its spectral components so that the composition, temperature, motion, and other chemical and physical properties can be analyzed.

STIS's two-dimensional detectors have allowed the instrument to gather 30 times more spectral data and 500 times more spatial data than the previous spectrographs on Hubble. These were capable of only looking at one place at a time. One of the greatest advantages to using STIS is in the study of supermassive black holes. STIS searches for massive black holes by studying the star and gas dynamics around galactic centers. It measures the distribution of matter in the universe by studying quasar absorption lines. It also uses its high sensitivity and spatial resolution to study star formation in distant galaxies and perform spectroscopic mapping of solar system objects.

The Near Infrared Camera and Multi-Object Spectrometer (NICMOS) has let us gain valuable new information on the dusty centers of galaxies and the formation of stars and planets. NICMOS consists of three cameras. It is capable of both infrared imaging and spectroscopic observations of astronomical targets. NICMOS gave astronomers their first clear view of the universe at near-infrared wavelengths between 0.8 and 2.5 micrometers - longer wavelengths than the human eye can see. (The expansion of the universe shifts the light from very distant objects toward longer red and infrared wavelengths.)

NICMOS's near infrared capabilities have provided views of objects too distant for research by previous Hubble optical and ultraviolet instruments. NICMOS's detectors also perform more efficiently than previous infrared detectors. With its cryogenics depleted, NICMOS is now dormant and awaiting the installation of a new cooling system in SM3B.

Service Mission 3a and 3b

           Service mission 3, which took place in December of 1999, ended up being split into two missions. This was because three of the 6 gyroscopes had failed, parts that allow the Hubble to accurately point at specific locations. “Servicing Mission 3A successfully replaced equipment and performed maintenance upgrades to the Hubble Space Telescope. Although no new scientific instruments were installed, many activities took place over 3 EVA days” (10). Various components, including the Fine Guidance Sensor, which allows for stabilization during observation, thermal insulation blankets, and a more advanced computer system were installed during this mission.

           Service mission 3b, flown by the space shuttle Columbia in 2002, set out to upgrade the Hubble’s optics. The new instrument that was installed, the Advanced Camera for Surveys(ACS), would bring the decade old telescope into the 21st century. “ASC sees in wavelengths ranging from visible to far ultraviolet. It is actually a team of three different cameras with specialized capabilities” (11). This new camera doubled the Hubble’s field of view and allowed for 10 times the amount of data to be captured than its predecessor, the Wide Field and Planetary camera. Other components installed included new solar arrays, increasing power efficiency by 30 percent, and an update to the NICMOS, an infrared camera and spectrometer. These new additions to the Hubble allowed astronomers to conduct new, more efficient surveys of the universe.

Service mission 4

The Hubble Space Telescope was reborn with Servicing Mission 4 (SM4), the fifth and final servicing of the orbiting observatory. During SM4, two new scientific instruments were installed – the Cosmic Origins Spectrograph (COS) and Wide Field Camera 3 (WFC3). Two failed instruments, the Space Telescope Imaging Spectrograph (STIS) and the Advanced Camera for Surveys (ACS), were brought back to life by the first ever on-orbit repairs With these efforts, Hubble has been brought to the apex of its scientific capabilities. To prolong Hubble's life, new batteries, new gyroscopes, a new science computer, a refurbished fine guidance sensor and new insulation on three electronics bays were also installed over the 12-day mission with five spacewalks. Additionally, a device was attached to the base of the telescope to facilitate de-orbiting when the telescope is eventually decommissioned.

Scientific discoveries

The Birth of the Universe

           One the biggest justification for building the Hubble Telescope was to determine the age of the universe. Before Hubble, the age of universe was estimated to be 10-20 billion years old, not a particular accurate guess. Hubble would be able to narrow down this estimate through the observation of Cepheid stars.” Cepheids are a special type of variable star with very stable and predictable brightness variations. The period of these variations depends on physical properties of the stars such as their mass and true brightness”(13). By observing Cepheids, astronomers can measure the changes of the stars intensity and dimness, which determines their ‘true brightness’. When comparing their ‘true brightness’ to their observed brightness, astronomers can then determine the distance to those stars and the galaxies they lay in.

           Because astronomers have Cepheid distances which can be accurately measured, they can now take those measurements over time to measure the distance they are moving away from us. “Hubble performed the definitive study of 31 Cepheid variable stars, helping to determine the current expansion rate and thereby narrow the age of the universe down to the most accurate it's ever been…Its observations of Cepheid variable stars, combined with other measurements pinned down the age to 13.7 billion years old, plus or minus a few hundred million years”(14) By calculating the age of the universe, it gives us a timeline of how stars and galaxies are developed, and helped redefine how the universe and everything in it formed.

Hubble deep field

           One of Hubble’s most well-known accomplishments was an image created in 1995. This image, called the Hubble Deep Field, was created in a way not typical of the other Hubble images. The idea was to point the Hubble at nothing in particular, an area of sky where no observation had been made before. The point was to see how well the Hubble could observe distinct galaxies over long exposures with its various instruments. “The first Deep Field, the Hubble Deep Field North (HDF-N), was observed over 10 consecutive days during Christmas 1995. The resulting image[18] consisted of 342 separate exposures, with a total exposure time of more than 100 hours, compared with typical Hubble exposures of a few hours” (19). The results of this image were astonishing. “Almost 3000 galaxies were seen in the image. Scientists analyzed the image statistically and found that the HDF had seen back to the very young Universe where the bulk of the galaxies had not, as yet, had time to form stars” (19). By observing this small dark patch of sky, it revealed a world never seen before. It has inspired other subsequent observations of the sky in various other regions, revealing equally astonishing pictures. These images provided new glimpses of the early universe and generated a mass of scientific discoveries.

One of the most important changes brought on by the Deep Field images is how it changed the way astronomers share data. Instead of keeping the images and data to themselves, typical of scientific groups of the day, the team behind Deep Field immediately released their findings to the scientific community and to the public. This precedent was the starting point for changing the culture of astronomy, allowing for teams across the Astronomy community to be more open with their data and discoveries(19).

Black holes

Astronomers have found convincing evidence for a supermassive black hole in the center of our own Milky Way galaxy, the galaxy NGC 4258, the giant elliptical galaxy M87, and several others. Scientists verified the existence of the black holes by studying the speed of the clouds of gas orbiting those regions. In 1994, Hubble Space Telescope data measured the mass of an unseen object at the center of M87. Based on the motion of the material whirling about the center, the object is estimated to be about 3 billion times the mass of our Sun and appears to be concentrated into a space smaller than our solar system.

For many years, X-ray emissions from the double-star system Cygnus X-1 convinced many astronomers that the system contains a black hole. With more precise measurements available recently, the evidence for a black hole in Cygnus X-1 -- and about a dozen other systems -- is very strong.

Birth of stars        

           Observing the birth of stars has always been tricky, as they seem to take place in dusty environments, the view obstructed by ginormous clouds of gas “Dust clouds scatter visible light, but let infrared light through unimpeded, meaning infrared observations are often the only way to see young stars” (16). The Wide Field Camera 3 (WFC3) upgraded in service mission 4, is fitted with an infrared camera. This allows the Hubble to make both visible and infrared observations of the same location, as shown by the image.

           In the top half of the image, which was taken in visible light, shows the top of a “three-light-year-long pillar, bathed in the glow of light from hot, massive stars off the top of the image. Scorching radiation and fast winds from these stars are sculpting the pillar and causing new stars to form within it” (17).

           By contrast, the bottom image was taken with Hubble’s Infrared camera. The clouds of green and orange gas disappear almost completely, revealing the fledgling stars underneath. ‘’’This is the first time that we have actually seen the process of forming stars being uncovered by hotoevaporation,’ John Hester, lead designer on the WFC3 emphasized. ‘In some ways it seems more like archaeology than astronomy. The ultraviolet light from nearby stars does the digging for us, and we study what is unearthed’’’(17).

Future of the Hubble

The Hubble Space Telescope has been in orbit for over 28 years, 18 years over its planned 15 year lifep. Although there has been talk of a service mission extending the lifep of the Hubble into the 2020s, eventually the time of the Hubble will come to an end. Fortunately, there are telescopes being designed that will fulfil the legacy of the Hubble. One in particular, the James Webb Telescope, is designed to be the successor to the Hubble. The James Webb telescope is an improved version of the Hubble in almost every way. Its mirror will be twice the size of Hubble’s, allowing it “to detect objects 16 times fainter than Hubble” (18). Instead of orbiting around the earth the telescope will be placed in orbit at the Sun-Earth Lagrange point, allowing it to be orbiting the sun while shielded from its infrared light and heat by the earth. This, along with its specialized infrared telescope, will allow it to take the “clearest picture(s) ever of space objects that emit invisible radiation beyond the red end of the visible spectrum— early galaxies, infant stars, clouds of gas and dust, and much more” (19).

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