A space probe is a robotic spacecraft that does not orbit Earth, but instead, explores further into outer space. A space probe may approach the Moon; travel through interplanetary space; flyby, orbit, or land on other planetary bodies; or enter interstellar space.
The space agencies of the USSR (now Russia and Ukraine), the United States, the European Union, Japan, China,India, and Israel have collectively launched probes to several planets and moons of the Solar System, as well as to a number of asteroids and comets. Approximately 15 missions are currently operational.
Once a probe has left the vicinity of Earth, its trajectory will likely take it along an orbit around the Sun similar to the Earth's orbit. To reach another planet, the simplest practical method is a Hohmann transfer orbit. More complex techniques, such as gravitational slingshots, can be more fuel-efficient, though they may require the probe to spend more time in transit. Some high Delta-V missions (such as those with high inclination changes) can only be performed, within the limits of modern propulsion, using gravitational slingshots. A technique using very little propulsion, but requiring a considerable amount of time, is to follow a trajectory on the Interplanetary Transport Network.
Some notable probesEdit
First mission to photograph the far side of the Moon, launched in 1959.
First robotic sample return probe from the Moon.
First rover on Moon. It was sent to the Moon on November 10, 1970.
First probe to Mercury.
First successful in-place analysis of another planet. It may have also been the first space probe to impact the surface of another planet, although it is unclear whether it reached Venus' surface.
Spirit and OpportunityEdit
The Mars Exploration Rovers, Spirit and Opportunity landed on Mars to explore the Martian surface and geology, and searched for clues to past water activity on Mars. They were each launched in 2003 and landed in 2004. Communication with Spirit stopped on sol 2210 (March 22, 2010). JPL continued to attempt to regain contact until May 24, 2011, when NASA announced that efforts to communicate with the unresponsive rover had ended. Opportunity arrived at Endeavour crater on 9 August 2011, at a landmark called Spirit Point named after its rover twin, after traversing 13 miles (21 km) from Victoria crater, over a three-year period. After a planet wide dust storm in June 2018, the final communication was received on June 10, 2018, and Opportunity was declared dead on February 13, 2019. The rover lasted for almost fifteen years on Mars — although the rover was intended to last only three months. 
The first dedicated missions to a comet; in this case, to Halley's Comet during its 1985–86 journey through the inner Solar System. It was also the first massive international coordination of space probes on an interplanetary mission, with probes specifically launched by the Soviet (now Russian) Space Agency, European Space Agency, and Japan's ISAS (now integrated with NASDA to JAXA).
Originally a solar observatory in the International Sun-Earth Explorer series, it was sent into solar orbit to make the first close observations of a comet, Comet Giacobini–Zinner, in 1985 as a prelude to studies of Halley's Comet.
Two Russian/French spacecraft. They dropped landers and balloons (first weather balloons deployed on another planet) at Venus before their rendezvous with Halley's Comet.
This Japanese probe was the first non-US, non-Soviet interplanetary probe.
A second Japanese probe, it made ultraviolet wavelength observations of the comet.[clarification needed]
The first space probe to penetrate a comet's coma and take close-up images of its nucleus.
First sample return probe from a comet tail.
First probe to land on an asteroid.
First sample return probe to launch from an asteroid.
First probe to Jupiter. Radio communications were lost with Pioneer 10 on January 23, 2003, because of the loss of electric power for its radio transmitter, with the probe at a distance of 12 billion kilometers (80 AU) from Earth.
First probe to fly by Saturn. (Communications were later lost due to power constraints and vast distance.)
Voyager 1 is the farthest human-made object from Earth, traveling away from both the Earth and the Sun at a relatively faster speed than any other probe. As of September 12, 2013, Voyager 1 is about 12 billion miles (19 billion kilometers) from the Sun.
On August 25, 2012, Voyager 1 became the first human made object to enter interstellar space. Voyager 1 has not had a functioning plasma sensor since 1980, but a solar flare in 2012 allowed scientists from NASA to measure vibrations of the plasma surrounding the craft. The vibrations allowed scientists to measure the plasma to be much denser than measurements taken in the far layers of our heliosphere, thus concluding the craft had broken beyond the heliopause.
Voyager 2 was launched by NASA on August 20, 1977. The probe's primary mission was to visit the ice giants, Uranus and Neptune, which it completed on October 2, 1989. It is currently the only probe to have visited the ice giants. It is the fourth of five spacecraft to have left the solar system. It has been operational for 41 years and 2 months as of October 20, 2018.
Cassini–Huygens was a 5,712kg (12,593lb) space probe designed to study gas giant Saturn, along with its ringed system and moons. The NASA probe was launched with ESA lander Huygens on October 1, 1997 from Cape Canaveral. The Cassini probe entered Saturn orbit on July 1, 2004, and Huygens landed on Titan, Saturn's largest moon, on January 14, 2005. On September 15, 2017, the probe was de-orbited and burned up in Saturn's atmosphere, after almost 20 years in space.
First probe to be launched to Pluto. Launched on January 19, 2006, it flew by the Pluto–Charon system on July 14, 2015.
First spacecraft to visit and orbit a protoplanet (4 Vesta), entering orbit on July 16, 2011. Entered orbit around dwarf planet Ceres in early 2015. Currently orbiting Ceres as of February 2017.
Beyond the Solar SystemEdit
Along with Pioneer 10, Pioneer 11, and its sister space probe Voyager 2, Voyager 1 is now an interstellar probe. Voyager 1 and 2 have both achieved solar escape velocity, meaning that their trajectories will not return them to the Solar System.
Examples of space probe imaging telescope/cameras (focused on visible spectrum).
|Mars Reconnaissance Orbiter—HiRISE||50 cm (19.7″)||R/C||Mars orbit||2005|
|Mars Global Surveyor—MOC||35 cm (13.8″)||R/C||Mars orbit||1996–2006|
|New Horizons—LORRI||20.8 cm (8.2″)||R/C||Space (33+ AU from Earth)||2006|
|Lunar Reconnaissance Orbiter LROC-NAC||19.5 cm (7.68″)||Reflector||Lunar orbit||2009|
|Cassini—ISS-NAC||19 cm (7.5″)||Reflector||Saturn orbit||2004–2017|
|Galileo – Solid State Imager||17.65 cm (6.95″)||Reflector||Jupiter||1989–2003|
|Voyager 1/2, ISS-NAC||17.6 cm (6.92″)||Catadioptric||Space||1977|
|Mariner 10 – TV Photo Experiment (x2)||15 cm (5.9″)||Reflector||Space||1973–1975|
|Deep Space 1—MICAS||10 cm ( 3.94″)||Reflector||Solar orbit||1998–2001|
|Voyager 1/2, ISS-WAC||6 cm (2.36″)||Lens||Space||1977|
|Cassini—ISS-WAC||5.7 cm (2.2″)||Lens||Saturn orbit||2004–2017|
|MESSENGER MDIS-WAC||3 cm (1.18″)||Lens||Mercury orbit||2004–2015|
|MESSENGER MDIS-NAC||2.5 cm (0.98″)||R/C||Mercury orbit||2004–2015|
|Dawn Framing Camera (FC1/FC2)||2 cm (0.8″)||Lens||Asteroid belt||2007–2018|
Image forming systems on space probes typically have a multitude of specifications, but aperture can be useful because it constrains the best diffraction limit and light gathering area.
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