While the bulk of space exploration initially was directed at the earth-moon system, the focus
gradually shifted to other members of the solar system. The U.S.
Mariner program studied Venus and Mars, the two planets closest to
the earth; the Soviet Venera series also studied Venus. From 1962 to
1971, these probes confirmed the high surface temperature and thick
atmosphere of Venus, discovered signs of recent volcanism and
possible water erosion on Mars, and investigated Mercury. Between
1971 and 1973 the Soviet Union launched six successful probes as
part of its Mars program. Exploration of Mars continued with the
U.S. Viking landings on the Martian surface. Two Viking spacecraft
arrived on Mars in 1976. Their mechanical arms scooped up soil
samples for automated tests that searched for photosynthesis,
respiration, and metabolism by any microorganisms that might be
present; one test suggested at least the possibility of organic
activity. The Soviet Phobos 1 and
2 missions were unsuccessful in 1988. The U.S.
Magellan spacecraft succeeded in orbiting Venus
in 1990, returning a complete radar map of the planet's hidden
surface. The Japanese probes Sakigake and
Suisei and the European Space Agency's probe
Giotto both rendezvoused with Halley's comet in 1986, and
Giotto also came within 125 mi (200 km) of the
nucleus of the comet Grigg-Skjellerup in 1992. The U.S. probe
Ulysses returned data about the poles of the
sun in 1994, and the ESA Solar and Heliospheric
Observatory (SOHO) was put into orbit
in 1995. Launched in 1996 to study asteroids and comets, the
Near Earth Asteroid Rendezvous
(NEAR) probe made flybys of the asteroids
Mathilde (1997) and Eros (1999) and began orbiting the latter in
2000. The Mars Pathfinder and Mars Global
Surveyor, both of which reached Mars in 1997, were
highly successful, the former in analyzing the Martian surface and
the latter in mapping it. The ESA Mars Express,
launched in 2003, began orbiting Mars later that year, and although
its Beagle 2 lander failed to establish contact,
the orbiter has sent back data. Spirit and
Opportunity, NASA rovers, landed successfully
on Mars in 2004, as did the NASA rover Curiosity in
2012, and InSight in 2018, along with two flyby
satellites called MarCO. NASA launched an upper
atmosphere probe named MAVEN that reached the
planet in September 2014; that same month, India joined the
Russians, USA, and ESA by placing the probe
Mangalyaan into Mars’ orbit. The
Europeans and Russians joined together to launch the ExcoMars Trace
Gas Orbiter into orbit in October 1916, but its accompanying landing
vehicle was destroyed on impacting the planet’s surface. In
July 2020, Mars 2020 was launched by NASA carrying the
Perseverance rover, which landed on the
planet’s surface on February 18, 2021. Earlier in that month,
the Chinese mission Tianwen-1 began orbiting the planet with a
planned landing of its rover, Zhurong (the name of
the fire-god in Chinese folklore), in spring 2021, while the United
Arab Emirates’ Hope Mars Mission was also in orbit.
Messenger, also launched by NASA, became the
first space probe to orbit Mercury in 2011; its mission ended in
2015. In 2014 the ESA's Rosetta became the first
probe to orbit a comet (Comet 67P, which it then studied for two
years); prior to that rendezvous the space probe had made flybys of
Mars and two asteroids.
Space probes have also been aimed at the outer planets, with spectacular results. One such probe, Pioneer 10, passed through the asteroid belt in 1973, then became the first object made by human beings to move beyond the orbits of the planets. In 1974, Pioneer 11 photographed Jupiter's equatorial latitudes and its moons, and in 1979 it made the first direct observations of Saturn. Voyagers 1 and 2, which were launched in 1977, took advantage of a rare alignment of Jupiter, Saturn, Uranus, and Neptune to explore all four planets. Passing as close as 3,000 mi (4,800 km) to each planet's surface, the Voyagers discovered new rings, explored complex magnetic fields, and returned detailed photographs of the outer planets and their unique moons. They subsequently moved toward the heliopause, the boundary between the influence of the sun's magnetic field and the interstellar magnetic field, and in 2013 NASA reported that Voyager 1 most likely crossed the heliopause in 2012 and entered interstellar space, becoming the first spacecraft to do so. Voyager 2 entered interstellar space in 2018.
Launched in 1989, NASA's Galileo spacecraft followed a circuitous route that enabled it to return data about Venus (1990), the moon (1992), and the asteroids 951 Gaspra (1991) and 243 Ida (1993) before it orbited Jupiter (1995–2003); it also returned data about the Jupiter's atmosphere and its largest moons (Io, Ganymede, Europa, and Callisto). NASA returned to Jupiter in 2016 with Juno (launched 2011); its instruments are designed to examine the nature and properties of the planet. The joint U.S.-ESA Cassini mission, launched in 1997, explored Saturn, its rings, and some of its moons upon from its arrival in 2004 until it was crashed into the planet in 2017. It also deployed Huygens, which landed on the surface of Saturn's moon Titan in early 2005. Pluto was visited for the first time in 2015 by NASA's New Horizons (launched 2006); the space probe then flew by the Kuiper belt's Arrokoth, the most distant object to be visited by a space probe.
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