This Hubble image of the Antennae galaxies is the sharpest yet of this merging pair of galaxies. Billions of stars will be formed during their collision. The brightest and most compact of these star-birth regions are called super star clusters. The image allows astronomers to better distinguish between the stars and super star clusters created in the collision of two spiral galaxies. Courtesy of NASA, ESA, and the Hubble Heritage Team. Acknowledgment: B. Whitmore (Space Telescope Science Institute).

NASA's Greatest Observatory—The Hubble Space Telescope

Allan R. Cohen and David A. Bearden

The Aerospace Corporation is assisting the Missile Defense Agency with defining requirements, identifying new technologies, and integrating existing sensor capabilities for the Ballistic Missile Defense System. Aerospace is also applying rigorous testing and systems engineering expertise to the development and operation of the system.

The Hubble Space Telescope is an orbiting astronomical observatory developed by NASA's Marshall Space Flight Center in Huntsville, Alabama, and Goddard Space Flight Center in Greenbelt, Maryland, and operated by the Space Telescope Science Institute in Baltimore. One of NASA's most successful missions, Hubble is arguably the most significant scientific instrument ever devised and fielded. But beyond that, its images are so spectacular that they have become part of popular culture everywhere. The spacecraft is enormous, often compared to a Greyhound bus in size and weight. Not only a marvel of space systems design, Hubble can also be periodically maintained and upgraded through on-orbit servicing by shuttle astronauts. It has been used by scientists worldwide for more than 18 years to make amazing discoveries about the universe in the fields of astronomy and physics (see sidebar, Hubble's Amazing Discoveries). The Aerospace Corporation has played an important role in Hubble's evolution: Since 1993, it has performed numerous independent assessments to assist NASA in ensuring the Hubble keeps flying and providing its groundbreaking science.

Vision for a Space Telescope

Space-based telescopes were first envisioned in 1946 by Lyman Spitzer of Yale University, who realized that astronomical observations could be significantly improved if the telescope performing the observations was beyond the distortions and spectral absorptions of Earth's atmosphere. Early design studies of a modern Earth-orbiting telescope were conducted in the late 1960s and early 1970s. Finally, upon the intense lobbying of noted scientists and despite its predicted high cost, the project—then called the Large Space Telescope Program—was approved by Congress in 1977.

Hubble Statistics
	Length	43.5 feet
	Maximum diameter	14 feet
	Weight	24,500 pounds
	Power	3 kilowatts (average)
	Orbit altitude	380 statute miles
	Orbit inclination	28.5 degrees
	Orbit period	97 minutes
	Number of orbits	more than 100,000
	Speed	17,500 miles per hour
	Wavelengths	Ultraviolet through near infrared
	Pointing accuracy	7/1000 of an arc-second (the width of human hair at one mile)
	Number of observations	20,000 per year

Requirements called for a stable optical telescope incorporating a 2.4-meter primary mirror with a 15-year lifetime (assuming change-out of instruments every 3 to 4 years) to be launched on the space shuttle into low Earth orbit. The launch date was planned for 1983, and contracts were awarded for the primary mirror and optical assembly, the spacecraft, and the solar arrays. After program delays as well as the 1986 space shuttle Challenger disaster (following which NASA grounded the shuttle fleet), the Hubble Space Telescope, renamed after the noted astronomer Edwin Hubble, was launched by space shuttle Discovery (mission STS-31) April 24, 1990. Shuttle astronauts deployed the observatory into a 612-kilometer orbit, with a complement of instruments that included the Wide Field/Planetary Camera, Goddard High Resolution Spectrograph, Faint Object Camera, and High-Speed Photometer.

Surprisingly, initial images received from Hubble were far more distorted than expected. A subsequent failure investigation revealed a spherical aberration of the primary mirror, which had been ground—more precisely than any object in history—to the wrong shape at the outer edge. Since servicing of the vehicle was built into its design, NASA incorporated a fix for the optics problem into the first servicing mission. The resolution devised by NASA was to incorporate optical correction into Wide Field/Planetary Camera-2, a revised Wide Field/Planetary Camera, and replace the High-Speed Photometer with a new device, the Corrective Optics Space Telescope Axial Replacement, to refocus the other instruments. Solar arrays, solar array drive electronics, four gyroscopes, electrical control units, and magnetometers were also replaced.

Aerospace Establishes Civil Office

In late 1993 just prior to Servicing Mission 1 (SM1), Edward "Pete" Aldridge Jr., Aerospace president and CEO at the time, had been discussing with NASA Administrator Daniel Goldin a new office at Aerospace called the Civil and International Space Programs, the forerunner of today's Civil and Commercial Operations. This office would allow non-DOD customers to take advantage of Aerospace's unique capabilities. Out of those discussions came a task for a team of Aerospace experts to perform an independent assessment of SM1. Since the rescue of a valuable national asset was on the line, NASA wanted to ensure that the risk of SM1 was sufficiently low. Goldin knew that Aerospace could provide him with a high-quality, unbiased assessment. A team of 20 engineers had just three weeks to review the nine subsystems on SM1.

SM1 was launched on space shuttle Endeavor (STS-61) on December 2, 1993, and succeeded flawlessly. Shuttle astronauts performed five lengthy extravehicular activities, restoring the Hubble to its originally envisioned glory just in time for it to capture spectacular images of the collision of Comet Shoemaker-Levy 9 with Jupiter and forging a relationship between Aerospace and NASA that continues to this day.

Stellar Spire in the Eagle Nebula. Hubble captured this image, which appears like a winged fairy-tale creature poised on a pedestal. Deep within the Eagle Nebula, stars are being forged in this tower of gas and dust 57 trillion miles (91 trillion kilometers) high. The tower is thought to be a giant incubator for newborn stars. (NASA)

The Orbiter, Aerospace's corporate newspaper, reported December 21 NASA's response to the team's assessment: "NASA was impressed by the Aerospace contribution to the review, commenting particularly on the unique perspective brought to the mission. Although no "show-stoppers" were found, several recommendations were made for this and future missions. Recommendations included additional visual checks to verify successful operation; additional anomaly investigation, including ground test and analysis; verification of predictions with flight data; and modification of sequence of service operations to ensure adequacy of the fix."

Hubble was again serviced in February 1997 (SM2) and December 1999 (SM3A—the original SM3 mission was divided into two missions: SM3A and SM3B). SM2 replaced two older instruments with the Space Telescope Imaging Spectrograph and the Near-Infrared Camera and Multi-Object Spectrometer. One of three fine-guidance sensors (instruments used to lock on guide stars for performing observations) was also replaced. SM3A again replaced the gyros and fine guidance sensor, and added a new flight computer.

In 2001, prior to SM3B, NASA looked to Aerospace to upgrade its reliability models and perform extensive "what if?" scenarios to determine what science could be performed and for how long once SM3B was completed. Aerospace now maintains the reliability model for Hubble and performs all reliability analyses. This model, which is used to generate the probability of failure as a function of time, estimates reliabilities of individual components or groups of components, taking into account complex configurations, such as rate gyroscopes, with random and "wear-out" failure modes. The reliability model provided a framework for NASA managers to make decisions on when and how to maintain Hubble and extend its life as well as guide more frequent decisions on how to operate the observatory.

Gyroscopes, which provide attitude information essential to pointing the telescope at the desired point in the heavens, have been a particular source of frustration throughout the Hubble mission. Hubble carries a complement of six gyros. Initially, at least three of the six gyros were required for science operations. Component failures necessitated the replacement of four gyros on SM1 and all six on SM3A (gyros are always replaced in pairs). Aerospace reliability analyses have examined numerous cases of one or more gyros having failed and the affect this has on science and spacecraft operations.

The reliability model is so accurate it can assist NASA in developing strategies for dealing with failures. For example, after concern about gyro life reached a critical state, NASA developed an innovative technique to point the vehicle to an acceptable accuracy with only two gyros (or even one under dire circumstances) and asked Aerospace to determine how many gyros should actually be running to preserve the greatest life. Aerospace's analyses indicated that it was better to operate two gyros and keep one in reserve than to operate three gyros simultaneously. This enabled NASA to "buy back" an additional six months of science operations.

Batteries are another potential mission-altering component during Hubble's lifetime. Aerospace analysis concluded that Hubble's nickel-hydrogen batteries are likely to last well beyond previous expectations, and once they do fail, the failure mechanism will be a gradual loss of capacity rather than a cell short circuit if power budgets are effectively managed (see sidebar, Hubble Battery Life). But the capac-ity reduction findings did set off alarm bells, warning that if no actions were taken, operational life would begin to be affected in 2009–2010. To mitigate this consequence, battery capacity tests, which ironically were themselves robbing the cells of capacity, were ceased until a modified charge technique could be implemented. Additionally, in independent tests of portions of the nickel electrodes of spare Hubble batteries, Aerospace looked for any unusual imperfections that might shorten the life of batteries in orbit. Tests found the plaque portion of the electrodes to be normal.

The figure above shows the solar radio flux measured at a 10.7 centimeter wavelength as a function of time for future solar cycles as predicted by the MSAFE and Shatten models. The atmospheric density at high altitudes is directly correlated with the F10.7 value. During peak periods of solar activity, low altitude satellites will experience more drag force and their orbits will decay more rapidly. Aerospace's analysis reconciled the differing atmospheric models, predicting that the Hubble orbit would not decay to the point of reentry until at least 2036.

Columbia Loss

The loss of space shuttle Columbia during reentry in February 2003 had far-reaching ramifications for NASA's ability to access space and placed the Hubble in a precarious state. In response to perceptions of increased risk, NASA canceled Hubble SM4. Only missions where the International Space Station could be used as a safe haven were continued. Once at Hubble's orbit, the shuttle no longer has sufficient performance to reach the Space Station. Aerospace studies gained added significance as NASA made every effort to squeeze as much life as possible from the Hubble in its current configuration. With each period of extended operating time or component failure, Aerospace generated revised predictions on the observatory's lifetime.

If not serviced, Hubble would eventually lose attitude control. It would reenter the Earth's atmosphere uncontrolled since the spacecraft has no onboard propulsion, posing a potential danger to people and property. Aerospace was asked to evaluate two separate implications of this end-of-life scenario: uncontrolled Hubble deorbit and its resulting debris, or controlled reentry by means of an auxiliary propulsion system that would dock with the observatory and "drive" it into open ocean.

Estimating the time when uncontrolled reentry of a large spacecraft will occur is by no means straightforward because of the uncertainty of factors such as solar activity, atmospheric density, orientation of the spacecraft, and others. Aerospace's independent calculation of orbital lifetime showed that the telescope was likely to remain in orbit for longer than originally assumed.

To perform a controlled reentry, a propulsion stage would be launched that must dock with the spacecraft. This would be implemented upon the Hubble's demise, when power would be lost along with any ability to control or communicate with the spacecraft. The spacecraft would lapse into a stable, almost motionless state stabilized by gravitational forces (called "gravity-gradient"). Below a certain altitude, aero-torques would cause the gravity-gradient to swing like a pendulum, making it difficult for a propulsion stage to dock. At still lower altitudes, Hubble would begin to tumble, making docking impossible. To understand this phenomenon, Aerospace performed orbital decay analyses to determine altitude as a function of time and probability simulations to assess the likelihood of the spacecraft residing at an unacceptable altitude at the time of a docking mission. These analyses helped NASA in planning a prospective propulsion stage docking mission.

But the scientific community, political representatives, and, especially, the general public refused to accept the demise of Hubble. Strenuous protests compelled NASA to look for other ways to keep Hubble operating while its replacement, the James Webb Space Telescope (JWST), could be built. JWST will be larger than Hubble, sit farther from Earth, and have a much larger mirror than Hubble's to enable it to look deeper into space and even further back at the origins of the universe. But how was NASA to keep Hubble operating until the new telescope was ready without risking the lives of a shuttle crew?

Is Robotic Servicing an Option?

One approach under consideration was the performance of a servicing mission not by astronauts but by robots, operating under ground control or autonomously. Under the plan, an uncrewed vehicle would be sent to rendezvous and dock with the orbiting spacecraft. One part of the Hubble remote servicing vehicle would contain a highly sophisticated robot capable of grabbing serviceable components, extracting and storing them, then replacing them with new boxes, just as had previously been done by shuttle astronauts. Once the robot's work was done, it would be jettisoned, leaving behind the second portion of a servicing vehicle that contained a propulsion stage that would eventually deorbit the spacecraft when its life was truly over. Aerospace was asked by the Hubble program office to evaluate the risk of every aspect of the robotic servicing mission and report its findings to NASA. A team of a dozen engineers thoroughly reviewed system engineering, discipline engineering, and mission operations aspects, categorizing the source and severity of technical and schedule risks.

Because of the far-reaching implications of the cancellation of SM4 and replacement with a robotic servicer, NASA requested a different type of evaluation of the consequences. So, Aerospace conducted an analysis of alternatives. The analysis involved 50 engineers and scientists examining available possibilities for preserving Hubble or its science, including robotic servicing and rehosting of SM4 instruments on other platforms. One critical conclusion of the analysis was that a robotic servicing mission could not, in all likelihood, be developed in time and within the available budget before Hubble would lapse into an unrecoverable state.

Once public, the work Aerospace performed received intense visibility, scrutiny, and political interest at the highest levels of the government and culminated in testimony before Congress. The National Research Council's committee examining the Hubble life-extension options declared that Aerospace's analysis was "the only quantitative analysis" of the problem. The report, coupled with congressional testimony on its findings, is often cited as having a major impact on the reinstatement of shuttle servicing of the Hubble. NASA Administrator Michael Griffin announced the decision in October 2006:

"What we have learned has convinced us that we are able to conduct a safe and effective servicing mission to Hubble. While there is an inherent risk in all spaceflight activities, the desire to preserve a truly international asset like the Hubble Space Telescope makes doing this mission the right course of action."

Conclusion

SM4, scheduled for launch in May 2009, will likely be the last Hubble retrofit because the space shuttle fleet is planned for retirement in 2010. Along with gyros, batteries, and other components, two new cutting-edge science instruments to enhance Hubble's capabilities by large factors—Wide Field Camera-3 and Cosmic Origins Spectrograph-will be installed.

When anomalies were experienced on electronic components to be installed on SM4, Aerospace evaluated the parts to be used, and based on its analogous experience with these parts in national security space systems, helped clear them for continued processing. In addition, two other vital instruments that have previously failed, the Space Telescope Imaging Spectrograph and the Advanced Camera for Surveys, the most widely used instrument on the Hubble, will be brought back to life.

But this repair is different. Astronauts will execute a repair in place, which has never before been attempted. Without removal of the components from the spacecraft, astronauts will remove tiny screws, open the box cover, and extract and replace individual circuit boards. Hubble was never designed for this purpose, and it is risky; for example, the edges of the board could be sharp, a threat to the integrity of the astronaut's gloves and in turn their entire pressure suit. Astronauts will also install a docking ring on the aft end of the spacecraft to allow a prospective deorbit stage to dock with Hubble in the future.

SM4 is a new and final chapter for the Hubble Space Telescope. Aerospace continues to support this invaluable mission and is proud to be part of its phenomenal success.