Photovoltaics/Crystalline Mono/NASAESA 01
NASA/ESA Collaborate for New Telescope
by Shirly Georgi
On March 4, spacewalkers John Grunsfeld and Richard Linnehan successfully installed the first of two new rigid solar arrays in seven hours and one minute. First, the connector was inserted into the Hubble and then the 25 foot wing was folded out. The astronauts successfully installed a second replacement solar array on the following day. (See front cover photo, courtesy of NASA.)
The Hubble SM3B space mission received more attention in the world news than any other scientific or technological endeavor in the current year. Although the general news media reported that the solar arrays were changed during this mission, there was little additional information about the solar panels that power this 24,500 lb. (earth weight) low earth orbiting (LEO) satellite located about 320 nautical miles above the surface of the earth.
New arrays -smaller, stronger and more stable
The Hubble’s two new wings, costing $19 million, are covered with eight panels having ultra-efficient, gallium arsenide solar cells of the same type that are used on the Iridium satellites. Each wing has four panels. These high-efficiency gallium arsenide solar panels can produce more electricity with 45 percent less surface. With these arrays, electrical output will be boosted by approximately 20 percent. The additional power from the new arrays will be especially useful for the new advanced camera, also installed on this mission, and will be needed in two more years when two more scientific instruments are launched. The new wings will also reduce the rate at which the Hubble sinks in orbit. The small surface area will lessen the Hubble’s atmospheric drag, allowing it to stay in orbit longer.
Upgrades made during the Hubble 3B mission. (Graphic is courtesy of NASA.)
The high efficiency solar panels have supporting frames made of aluminum-lithium, which is stronger and lighter than the type of aluminum commonly used in spacecraft construction. These supports are much less sensitive to the extreme temperature changes of Hubble’s harsh environment.
During each 97-minute orbit, Hubble spends about two-thirds of its time in searing sunlight and the other third in the frigid darkness of the Earth’s shadow. These brutal rapidly cycling conditions cause the temperature of the solar panels to fluctuate between minus 94 0F, (minus 70 0C) and 187 0F (86 0C). The solar arrays reach their hottest temperatures just ten minutes after leaving the chill of the Earth’s shadow.
These smaller, stiffer arrays, hinged in the middle and easily folded, are easier for the astronauts to work around during servicing missions - easier to fold up and move out of their way. Their smaller size decreases on-orbit drag and slows the rate at which Hubble’s orbit decays. Over time, all low-earth orbiting satellites feel the effects of atmospheric drag and lose altitude. These new arrays will slow that rate of altitude loss.
The achievement for managing design and construction of the solar panels goes to the European Space Agency (ESA). ESA built Hubble’s first two sets of solar arrays. For the newest pair, ESA designed, developed and tested the Solar Array Drive Mechanism, which maneuvers the arrays to keep them constantly pointed toward the sun. Based at NASA’s Goddard Space Flight Center, the “nerve center” for the Hubble operations, ESA and NASA focused their energies in a synergistic effort to successfully accomplish the exchange of the solar arrays.
ESA also provided the ability to test the new arrays in a unique never-before-done way. ESA’s world-class test facility in The Netherlands features a huge test chamber that can realistically simulate the extreme temperature cycles of Hubble’s orbit - including sunrise and sunset. This chamber, combined with the size of the new array and ESA’s vast Hubble experience, made this facility the only place in the world capable of performing this test.
ubble actually got a new look on this last mission with its newly installed solar wing. The third-generation solar arrays are two-thirds the size of the current arrays but will provide 20 percent more power to the telescope. Because of their small size, the new arrays will impart less atmospheric drag, slowing the rate at which Hubble’s orbit decays. Unlike their flexible predecessors, which could roll up like a window shade, the new set is rigid. Less suceptible to damage and the extreme temeprature variations of Hubble’s orit, these advanced arrays will provide enough extra power to run a new genreation of scientific instruments. (Graphics are courtesy of NASA.)
Team lead, project manager Ton Linssen from ESA’s Science Directorate at the European Space Research and Technology Center (ESTEC), stated, “The new arrays are yet another step in the long-standing, international partnership between ESA and NASA. ESA provided the first two sets of solar arrays, and for the third pair, ESA and European industry designed, developed and tested the drive mechanisms which manoeuver the arrays so that they stay pointed at the sun.”
The new solar arrays were manufactured by Lockheed Martin. The Hubble program bought these solar panels from the production line of the commercial arrays used in communications satellites.
Why new solar arrays ?
- The set of solar panels (Hubble’s second set comprised of silicon solar cells) which were removed on this mission have deteriorated dramatically due to the impact of space debris and the effects of hard radiation from the Sun. The first set of solar panels was replaced in December 1993 during the first service mission. This second set served seven years until they were replaced in the March 2002 mission. Radiation and debris have taken their toll on the sensitive electronics on the old wings. New solar wings will help to ensure uninterrupted service for the remainder of the mission.
- In addition to needing more power for the advanced camera, the new solar arrays will have to provide power for scientific instruments being added in two years. In 2004, a Cosmic Origins Spectrograph and a Wide Field Camera will be installed on the Hubble.
- During the last Hubble Servicing Mission in 1999, the team inspected in very close detail photographic surveys of the solar arrays which NASA had made for ESA. Suddenly, they found something that was not supposed to be there. A “hinge pin” protruded from one of the sides of the arrays. Later, more protruding pins were found. These hinge pins were 2.4 meter long strings of piano wire that held the segments of wire together. The team believed that these pins wandered back and forth due to the differences in the thermal expansion between the solar blanket and the pins themselves when Hubble comes in and out of the Sun. During this cycling, the temperature varies between -60 0C. to +85 0C. During the life time of the second set of arrays, the cycling occurred 45,000 times. Because there was a possibility that the solar blanket segments could fall apart at a later time if the pins fell out completely, it was considered best to change the arrays. The solution, proven by ground tests, was to create a solid construction of the blanket which would be implemented in the new solar arrays.
In October 2000, one of the new arrays was shipped to the ESA test site, located at ESTEC in Noordwijk, The Netherlands. Here the combined ESA/NASA team conducted the special thermal test to measure the amount of movement produced within the solar array due to harsh extremes of Hubble’s environment. After extensive evaluation, the team verified that Hubble’s new arrays would stay steady throughout the extreme temperature cycles of each orbit.
- Analysis of the second generation arrays are needed since analysis of the previous arrays, “retired” in 1993, has proved to be crucial. When ESA’s first set of solar arrays were brought back to Earth in 1993, scientists and engineers examined the effects of exposure to the harsh environment at 600 km. altitude. They found more than 80,000 particle impacts of varying sizes on the arrays. Lothar Gerlach, who is the central person in these postflight activities, says, “I am very excited to see our arrays having performed so fabulously, even after this many impacts! The big question is: what caused all these impacts? Some of the impacts are from micrometeorites coming from the birth of the Solar system, and some can be attributed to paint flakes from spacecraft or other space debris. The analyses on the first arrays have shown that they are hit daily by as many as 1000 small (0.001 mm or 1 micron-sized) objects. Now that the Space Shuttle has landed, Lothar Gerlach will be hoisted into the Shuttle’s cargo bay. Here he will take samples of the second generation arrays before they are ‘contaminated’ by our atmosphere. Then, the arrays will be sent back to Europe and will await further analysis. The scientific contribution of ESA’s solar arrays will in this way continue for a number of years into the future, no longer for observational astronomy, but for material science and space debris research.
Celebration and New Beginnings
The technical skill in building the old arrays should be applauded. The second generation arrays just “retired” far exceeded the five year lifetime they were built for; they have worked flawlessly for more than eight years and even exceeded the power output they were specified to deliver by about ten percent. However, the with the knowledge gained over the last 20 years, the new solar arrays should outshine their predecessor’s record. Although NASA plans to stop serving the Hubble in 2004, the space agency hopes to keep this observatory working until 2010. That means the new gallium arsenide solar arrays are scheduled to power the Hubble for another eight years. And, our expectations go along with what Anne Kinney, NASA’s astronomy director, said, “We really hope they’ll give us nice, long science lifetime on the Hubble Space Telescope.”