The Global Positioning System (GPS) is a space-based satellite navigation system that provideslocation and time information in all weather, anywhere on or near the Earth, where there is an unobstructed line of sight to four or more GPS satellites. It is maintained by the United Statesgovernment and is freely accessible by anyone with a GPS receiver with some technical limitations which are only removed for military users.

The GPS program provides critical capabilities to military, civil and commercial users around the world. It is an engine of economic growth and jobs, and has generated billions of dollars of economic activity. It maintains future warfighter advantage over opponents and is one of the four core military capabilities. In addition, GPS is the backbone for modernizing the global air traffic system.

The GPS project was developed in 1973 to overcome the limitations of previous navigation systems, integrating ideas from several predecessors, including a number of classified engineering design studies from the 1960s. GPS was created and realized by the U.S. Department of Defense (DoD) and was originally run with 24 satellites. It became fully operational in 1994.

Advances in technology and new demands on the existing system have now led to efforts to modernize the GPS system and implement the next generation of GPS III satellites and Next Generation Operational Control System (OCX). Announcements from the Vice President and the White House in 1998 initiated these changes. In 2000, U.S. Congress authorized the modernization effort, referred to as GPS III.

In addition to GPS, other systems are in use or under development. The Russian GLObal NAvigation Satellite System (GLONASS) was in use by only the Russian military, until it was made fully available to civilians in 2007. There are also the planned European Union Galileo positioning system, Chinese Compass navigation system, and Indian Regional Navigational Satellite System.

GLONASS is a radio-based satellite navigation system operated for the Russian government by the Russian Space Forces. It both complements and provides an alternative to the United States' Global Positioning System (GPS), the Chinese Compass navigation system and the planned Galileo positioning system of the European Union.

Development of GLONASS began in the Soviet Union in 1976. Beginning on 12 October 1982, numerous rocket launches added satellites to the system until the "constellation" was completed in 1995. In the first decade of the 21st century, under Vladimir Putin's presidency, the restoration of the system was made a top government priority and funding was substantially increased. GLONASS is currently the most expensive program of the Russian Federal Space Agency, consuming a third of its budget in 2010.

By 2010, GLONASS had achieved 100% coverage of Russia's territory and in October 2011, the full orbital constellation of 24 satellites was restored, enabling full global coverage. The GLONASS satellites designs have undergone several upgrades, with the latest version being GLONASS-K.

Sputnik, a Russian word meaning "companion" or "satellite", is a name applied to certain spacecraft launched under the Soviet space program. "Sputnik 1", "Sputnik 2" and "Sputnik 3" were the official Soviet names of those objects, while the remaining designations in the series ("Sputnik 4" and so on) were not official names, but were names applied in the West, to objects whose original Soviet names may not have been known at the time.

The Chandra X-ray Observatory is a satellite launched on STS-93 by NASA on July 23, 1999. It was named in honor of Indian-American physicist Subrahmanyan Chandrasekharwho is known for determining the maximum mass for white dwarfs. "Chandra" also means "moon" or "luminous" in Sanskrit.

Chandra Observatory is the third of NASA's four Great Observatories. The first was Hubble Space Telescope; second the Compton Gamma Ray Observatory, launched in 1991; and last is the Spitzer Space Telescope. Prior to successful launch, the Chandra Observatory was known as AXAF, the Advanced X-ray Astrophysics Facility. AXAF was assembled and tested by TRW (now Northrop Grumman Aerospace Systems) in Redondo Beach,California. Chandra is sensitive to X-ray sources 100 times fainter than any previous X-ray telescope, due primarily to the high angular resolution of the Chandra mirrors.

Since the Earth's atmosphere absorbs the vast majority of X-rays, they are not detectable from Earth-based telescopes, requiring a space-based telescope to make these observations.

The NASA's Space Shuttle program, officially called Space Transportation System (STS), was the United States government's manned launch vehicle program from 1981 to 2011. The winged Space Shuttle orbiter was launched vertically, usually carrying four to sevenastronauts (although eight have been carried) and up to 50,000 lb (22,700 kg) of payload intolow Earth orbit (LEO). When its mission was complete, the shuttle could independently move itself out of orbit using its Maneuvering System (it oriented itself appropriately and fired its main OMS engines, thus slowing it down) and re-enter the Earth's atmosphere. During descent and landing the orbiter acted as a re-entry vehicle and a glider, using its OMS system and flight surfaces to make adjustments.

The shuttle is the only winged manned spacecraft to have achieved orbit and land, and the only reusable space vehicle that has ever made multiple flights into orbit. Its missionsinvolved carrying large payloads to various orbits (including segments to be added to theInternational Space Station), provided crew rotation for the International Space Station, and performing service missions. The orbiter also recovered satellites and other payloads (e.g. from the ISS) from orbit and returned them to Earth, though its use in this capacity was rare. Each vehicle was designed with a projected lifespan of 100 launches, or 10 years' operational life.

The program started in the late 1960s and dominated NASA's manned operations since the mid-1970s. According to the Vision for Space Exploration, use of the space shuttle was to be focused on completing assembly of the ISS by 2011, after which it was retired. NASA planned to replace the shuttle with the Orion spacecraft, but budget cuts have placed full development of the Orion craft in doubt. The program commenced on April 12, 1981, withColumbia and STS-1, the first shuttle orbital flight. The space shuttle program finished with its last mission, STS-135 flown by Atlantis, in July 2011, retiring the final shuttle in the fleet. The Space Shuttle program formally ended on August 31, 2011.

Mars Pathfinder (MESUR Pathfinder) was an American spacecraft that landed a base station with roving probe onMars in 1997. It consisted of a lander, renamed the Carl Sagan Memorial Station, and a lightweight (10.6 kilograms/23 pounds) wheeled robotic rover named Sojourner.

Launched on December 4, 1996 by NASA aboard a Delta II booster a month after the Mars Global Surveyor was launched, it landed on July 4, 1997 on Mars' Ares Vallis, in a region called Chryse Planitia in the Oxia Palus quadrangle. The lander then opened, exposing the rover which conducted many experiments on the Martian surface. The mission carried a series of scientific instruments to analyze the Martian atmosphere, climate, geology and the composition of its rocks and soil. It was the second project from NASA's Discovery Program, which promotes the use of low-cost spacecraft and frequent launches under the motto "cheaper, faster and better" promoted by the then administrator, Daniel Goldin. The mission was directed by the Jet Propulsion Laboratory (JPL), a division of the California Institute of Technology, responsible for NASA's Mars Exploration Program.

NASA's Discovery Program (as compared to New Frontiers, Explorers, or Flagship Programs) is a series of lower-cost, highly-focused American scientific space missions that are exploring the Solar System. It was founded in 1992 to implement then-NASA Administrator Daniel S. Goldin's vision of "faster, better, cheaper" planetary missions. Discovery missions differ from traditional NASA missions where targets and objectives are pre-specified. Instead, these cost-capped missions are proposed and led by a scientist called the Principal Investigator (PI). Proposing teams may include people from industry, small businesses, government laboratories, and universities. Proposals are selected through a competitive peer review process. All of the completed Discovery missions are accomplishing ground-breaking science and adding significantly to the body of knowledge about the Solar System.

NASA also accepts proposals for competitively selected Discovery Program Missions of Opportunity. This provides opportunities to participate in non-NASA missions by providing funding for a science instrument or hardware components of a science instrument or to re-purpose an existing NASA spacecraft. These opportunities are currently offered through NASA's Stand Alone Mission of Opportunity program.

The Voyager program is a U.S program that launched two unmanned space missions, scientific probes Voyager 1 and Voyager 2. They were launched in 1977 to take advantage of a favorable planetary alignment of the late 1970s.

Although officially designated to study just Jupiter and Saturn, the probes were able to continue their mission into the outer solar system, and are as of December 2011 on course to exit the solar system. These probes were built at JPL and were funded by NASA. Voyager 1 is currently the farthest human-made object from Earth.

Both missions have gathered large amounts of data about the gas giants of the solar system, of which little was previously known. In addition, the spacecraft trajectories have been used to place limits on the existence of a hypothetical trans-Plutonian Planet X.

The Solar and Heliospheric Observatory (SOHO) mission purpose is to study the Sun from a point of gravitational balance.

The SOHO spacecraft has been built by a European industrial consortium led by Matra Marconi Space (now Astrium) that was launched on a Lockheed MartinAtlas IIAS launch vehicle on December 2, 1995 to study the Sun, and has discovered over 2100 comets. It began normal operations in May 1996.

It is a joint project of international cooperation between the European Space Agency (ESA) and NASA. Originally planned as a two-year mission,SOHO currently continues to operate after over fifteen years in space. In October 2009, a mission extension lasting until December 2012 was approved.

In addition to its scientific mission, it is currently the main source of near-real time solar data for space weather prediction. Along with the GGS Wind and Advanced Composition Explorer (ACE),SOHO is one of three spacecraft currently in the vicinity of the Earth-Sun L1 point, a point of gravitational balance located approximately 0.99 astronomical unit (AU)s from the Sun and 0.01 AU from the Earth. In addition to its scientific contributions, SOHO is distinguished by being the first three-axis-stabilized spacecraft to use its reaction wheels as a kind of virtual gyroscope; the technique was adopted after an on-board emergency in 1998 that nearly resulted in the loss of the spacecraft.

The Kepler spacecraft is an American space observatory, the space-based portion of NASA's Kepler mission to discover Earth-like planets orbiting other stars. The spacecraft is named in honor of the 17th-century German astronomer Johannes Kepler. The spacecraft was launched on March 7, 2009, with a planned mission lifetime of at least 3.5 years.

The Kepler mission is "specifically designed to survey a portion of our region of theMilky Way galaxy to discover dozens of Earth-size planets in or near the habitable zone and determine how many of the billions of stars in our galaxy have such planets." Kepler's only instrument is a photometer that continuously monitors the brightness of over 145,000 main sequence stars in a fixed field of view. This data isanalyzed to detect periodic fluctuations that indicate the presence of extrasolar planets that are in the process of crossing the face of other stars.