Encyclopedia of Computer Science and Technology

space exploration and computers 449and compactness that is comparable to that of standardaudio CDs and suitable for most listeners. The compressionalgorithm relies upon psychoacoustics (the study ofhow people perceive the components of sound) to identifyfrequencies that humans can’t hear, and thus may be safelydiscarded. The digitized sound on a CD is compressed upto 1/12 or less of its original size, so a 630 MB CD becomesabout 50 MB in MP3 files.Since most PC users now have hard drives rated in thehundreds of gigabytes (GB), it is easy to store an extensivemusic library in MP3 form. Most PCs now come withsoftware that can play MP3 files (such as Windows MediaPlayer), and there are also free and shareware programsfrom a variety of sources, as well as plug-ins for playingsound files directly from the Web browser.Since MP3 is much more compact than “raw” CD format,users with inexpensive CD-RW drives can “burn”large amounts of music in MP3 form onto a single CD. Thisis typically done using software that “rips” the raw tracksfrom an audio CD and converts them to an MP3 file, whichcan then be stored on the PC’s hard drive.In recent years portable media players such as the iPodhave become ubiquitous (see music and video players,digital). MP3 is the most popular format for music that isnot digitally protected from copying (see digital rightsmanagement). However, because MP3 involves a numberof patents, it is not included by default in Linux distributions,which instead provide Ogg, a “container” that can beused for a variety of formats (see codecs).Further ReadingAudio File Types. Available online. URL: http://www.fileinfo.net/filetypes/audio. Accessed August 22, 2007.Johnson, Dave, and Rick Broida. How to Do Everything with MP3and Digital Music. New York: McGraw Hill Professional, 2001.Young, Robert. The MIDI Files. 2nd ed. New York: Prentice Hall,2001.space exploration and computersIt might have been barely possible to put a satellite (or person)in orbit without the use of computers, but any moreextensive exploration of space requires many types of computerapplications.Human Space ExplorationFlying to the Moon required precisely calculated and controlled“burns” to inject the Apollo spacecraft from orbitinto its arcing trajectory to the Moon. The detachableLunar Excursion Module (LEM) also had a computer onboard (roughly comparable in power to something foundin today’s programmable calculators). Although the pilotcontrolled the final landing manually, the computer interpretedradar data to fix the lander’s position, monitored fuelconsumption, and provided other key data.The Space Shuttle, the most complex vehicle ever builtby human beings, has five onboard computer systems thatcontrol flight maneuvers (including rendezvous and dockingoperations), monitor and control environmental conditions,keep track of fuel, batteries, life support, and otherconsumables, and provide many other functions to supportthe crew’s tasks and experiments.Automated Space ExplorationThus far, human explorers have flown no farther than theMoon. However, in the last 40 years an extensive survey ofmost of the solar system has been carried out by robot (thatis to say, computerized) probes and landers. These probeshave landed on Mars and visited every planet except Pluto,as well as making close approaches to asteroids and comets.The control computer aboard a space probe has severaljobs. It must keep the probe oriented in such a waythat its solar panels can receive energy from the Sun, aswell as keeping an antenna pointed toward Earth so it canreceive commands and return data from the probe’s scientificinstruments.Starting with Voyager 2 (a probe that is still returningdata from more than 7 billion miles from Earth), space probecomputers have been more autonomous, able to make attitudecorrections and course corrections as needed. The onboardcomputer can even be reprogrammed with new instructionssent from Earth. Space probes have returned incrediblydetailed pictures of the surface of the Moon and planets, preparingthe way for human missions or robot landers.Landers reach a fixed point on a planetary surface andtransmit photographs, temperature, radiation, and otherreadings. Probes can survive only for minutes on the hostilesurface of Venus, but have functioned for many monthson Mars. In a remarkably ambitious mission beginning in1976, the two Viking Mars landers were able to carry outexperiments on soil samples in an unsuccessful attempt tofind evidence of life while a third probe mapped the planet’ssurface from orbit. Besides demonstrating remarkable reliability(Viking 2 was still operating in 1982 when it wasaccidentally turned off by a remote command), the missionalso demonstrated the ability to coordinate surface andorbital exploration.In July 1997, the Mars Pathfinder probe landed on the redplanet, rolling and bouncing to a stop inside a sort of giantairbag. After deflating, the Pathfinder base station deployedthe Sojourner mobile robot. This vehicle (see robotics)was controlled by operators on Earth, but because of the10–15-minute time delay in signals arriving from Earth,the Sojourner had some autonomous ability to avoid collisionsor other hazards. The onboard computer also had tocompress and transmit images and other data. The followonMars Exploration Rover (MER) program began in 2003with the launching of two larger surface rovers dubbedSpirit and Opportunity. Landing in January 2004, the rovershave shown remarkable durability, still functioning in early2008, far beyond their original three-month mission life.The need to build compact computers and other electronicsfor space exploration helped spur the development of techniquesnow found in garden-variety consumer electronics.Space computers are also important for demonstrating thereliability and robustness that is necessary for applicationson Earth (such as in the military). Space electronics must beshielded and “hardened” to withstand the intense solar radiation,extreme changes in temperature, and electromagnetic