polished, but at 94.5 inches (2.4 m) in polished, butat94.5inches(2.4m)in Hubble’s primarymirrorisnotonlyexquisitely before ithitstheirdetectors. (cameras andspectrographs)forfinalfocus primary whereitentersHubble’sinstruments reflects thelightbackthroughaholein above theprimary.Thesecondary,inturn, a smaller,secondarymirrorsuspended main, orprimary,mirrorisreflectedto optical design.Lighthittingthetelescope’s was amongthefirsttosuggestthisbasic named aftera15thcenturyFrenchclericwho Hubble isclassifiedasaCassegrainreflector, The Telescope in about10minutes. fast enoughtotravelacrosstheUnitedStates the speedofabout5miles(8km)persecond, one every95minutes.Thesatellitemovesat completes 15orbitsperday—approximately 340 milesaboveEarth’ssurfacewhereit Discovery, Hubbleiscurrentlylocatedabout on April24,1990,aboardtheSpaceShuttle from ultraviolettonear-infrared.Launched images inwavelengthsoflightspanning orbit aroundEarthwiththeabilitytorecord astronomical observatorytobeplacedinto NASA’s HubbleSpaceTelescopewasthefirst Hubble SpaceTelescope –An Overview National Aeronautics andSpaceAdministration radiation seenandmeasured. the energeticprocessesthatcreate astronomers usingHubbleafullerviewinto at variouswavelengths.Thisgives which arelimitedbyatmosphericabsorption spectrum thanground-basedtelescopes, across awiderrangeoftheelectromagnetic Hubble canviewastronomicalobjects Also becauseitcirclesabovetheatmosphere, extremely distantgalaxies. disks aroundstarsortheglowingnucleiof enables Hubbletolocatesuchobjectsasdust ground-based telescopes.Highresolution than thebesttypicallyattainedbyevenlarger, miles. Thisresolutionisabout10timesbetter the widthofadimefromdistance86 0.05 arcsecond—theequivalenttodiscerning objects withanangulardiameterofamere telescope candistinguishastronomical telescopes locatedontheground.Thespace much clearerviewofthecosmosthando of Earth’satmosphere,Hubblealsogetsa can see.Highabovetheblurringeffects billion timesfainterthantheunaidedeye . Hubblecandetectobjectsthatare10 diameter, collectsanimmenseamountof

NASAfacts Finally, Hubble’s observations are predictably Commands and data are transmitted between the consistent. The telescope’s seeing conditions do spacecraft and the control center through two high- not change from to day or even orbit to orbit. gain antennas that communicate through NASA’s Astronomers can revisit targets with the expectation Tracking and Data Relay Satellite System, which is that they will be imaged at the same high quality each in geosynchronous orbit. The science data is then time. This optical stability is critical for detecting tiny forwarded from the control center to the Space motions or other small variations in celestial objects. Telescope Science Institute via a wide- network for Such is not the case for ground-based observatories, processing, dissemination, and archiving. where observing conditions vary with weather and directly affect the quality of the images acquired. NASA has conducted five astronaut-servicing missions to repair and upgrade Hubble. These refurbishments, The Spacecraft along with the redundancies originally designed into Hubble is 43.5 feet long and 14 feet wide at the back, the observatory’s critical subsystems, should keep where the scientific instruments are housed. Weighing Hubble running for years to come. Based on formal about 27,000 pounds, the telescope is approximately the reliability studies, engineers believe that there is a high same and as a school bus. The observatory probability that Hubble’s instruments and primary is powered by two arrays that convert sunlight into spacecraft subsystems (gyros, reaction wheels, solar electrical , which is stored in six large batteries. arrays, batteries, etc.) will continue to operate until at The batteries allow the observatory to operate during the least the year 2020. NASA’s goal is to operate Hubble shadowed portions of Hubble’s orbit when Earth blocks concurrently with the James Webb , the the satellite’s view of the Sun. agency’s new and capable infrared observatory, planned for launch in 2021. In the middle of the spacecraft, near its center of , are four 100- reaction wheels used to reorient the observatory. Based upon Sir Isaac ’s Third Law of Motion—for every there is an equal and opposite reaction—turning a reaction wheel in one direction causes Hubble to react by turning the opposite way. The satellite knows where and when it should turn based on a target schedule uploaded from the control center. Hubble’s main computer then calculates which wheels should slow and which ones faster to most efficiently maneuver the spacecraft to the new target.

The observatory uses high-precision (gyros) to detect its rate and direction of motion. Hubble’s typical operating mode uses three gyros, but it has six from which to choose. The others serve as backups, as gyros eventually wear out and fail. Backup operating modes also exist that enable Hubble to continue collecting science data with only one gyro, if necessary, but with slightly less efficiency.

In addition to gyros, Hubble has three Fine Guidance Sensors (FGSs) that act within the spacecraft’s overall pointing and control system to keep the telescope Astronauts F. Story Musgrave (anchored on the end of the Remote Manipulator virtually motionless while observing. Hubble jitters less System arm) and Jeffrey A. Hoffman (bottom of frame) on Hubble during than 7 milliarcseconds in a 24- period when locked the first servicing mission in December 1993. on its target. This is equivalent to shining a laser on a dime 200 miles away for this period. For more information, contact:

National Aeronautics and Space Administration Claire Andreoli Office of Communications Goddard Space Center 301-286- 40 8800 Greenbelt Road 19 Greenbelt, MD 20771 claire.g.desaravia@.gov www.nasa.gov Or visit the Hubble website at: www.nasa.gov/hubble NASA Facts

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