Exploration of the outer planets requires extreme patience. Our launch vehicles are not powerful enough to send massive spacecraft directly to the giant planets, so they must take circuitous paths returning to Earth or even traveling inward to Venus for gravity assists to boost momentum enough to send them beyond the asteroid belt. Rendezvousing with asteroids and comets can be even more challenging; they lack sufficient mass to brake fast-moving spacecraft into orbit, so the ships must perform years of orbit adjustment to match position and velocity with the tiniest worlds.
Power is also a problem. Located very far from the Sun, comets, outer planets, and most asteroids receive very little solar energy. Solar arrays must be very large to gather the little sunlight, like those of Rosetta, Dawn, and Juno; or else spacecraft must carry radioisotope thermoelectric generators. The successes of missions like Voyager, Pioneer, Galileo, Cassini-Huygens, and New Horizons require these nuclear power supplies, but Earth has run short of refined plutonium-238, preventing us from planning future missions.
The exceptions to these rules are the missions to near-Earth asteroids, which, by definition, have orbits very similar to our own planet's, permitting them to be explored relatively cheaply and quickly by solar-powered spacecraft like Hayabusa and NEAR. And sometimes, small craft like Deep Impact and Stardust can catch comets as they speed across the inner solar system.
Launch: September 2016 (planned) Arrival at Asteroid 101955 Bennu: Planned for 2018 Return to Earth: Planned for 2023
The Origins Spectral Interpretation Resource Indentification Security Regolith Explorer (OSIRIS-REx) mission is an asteroid study and sample return mission to the carbonaceous near-Earth asteroid Bennu. The goal of the mission is to learn more about the formation of the Solar System, planet formation, and the source of the organic compounds that led to the formation of life.
Launch: December 3, 2014 Arrival at asteroid 162173 Ryugu: 27 June 2018 Sample collection: 21 Feb 2019 Ryugu departure: Planned for late2019 Return to Earth: Planned for December 2020
The follow-on mission for Hayabusa, Hayabusa2 is planned is visiting asteroid Ryugu for a year and a half before returning to Earth with a sample. The mission includes a small lander called the Mobile Asteroid Surface Scout (MASCOT), built by the German Aerospace Center.
Launch: August 5, 2011 Jupiter arrival: July 4, 2016
Following a lengthy cruise and October 2013 Earth flyby, Juno is surveying Jupiter from a polar orbit, carrying a suite of instruments designed to study the planet's interior. It is investigating the existence of an ice-rock core; determining the amount of global water and ammonia present in the atmosphere; studying convection and deep wind profiles in the atmosphere; investigating the origin of the Jovian magnetic field; and exploring the polar magnetosphere. Its science mission does not require a camera, but it does carry one, specifically designed to capture unusual and beautiful views of Jupiter from its unusual polar perspective for public pleasure.
Launch: January 19, 2006 Jupiter flyby: January–May, 2007 Pluto encounter: January–August 2015, close approach July 14
New Horizons is the result of a long battle to take advantage of a once-in-a-lifetime opportunity for a Jupiter gravity assist trajectory to Pluto. It observed Jupiter over five months around the flyby in early 2007, with its closest approach on February 27. It was the first spacecraft to observe the newly formed Little Red Spot, and also caught Io's north polar volcano Tvashtar in the middle of a spectacular eruption. In July 2015 it traveled within 10,000 kilometers of Pluto before traveling onward to its second target out in the Kuiper Belt, 2014 MU69.
Launch: September 5, 1977 Jupiter encounter: January 4 to April 13, 1979 Saturn encounter: August 23 to December 15, 1980
Launched 16 days after Voyager 2, Voyager 1 was on the fast track to Jupiter and actually arrived four months ahead of the other spacecraft. Voyager 1 flew by Jupiter on March 5, 1979, taking more than 18,000 images of planet and its moons. The spacecraft flew by Saturn on November 12, 1980, coming within 64,200 kilometers of the planet's cloud tops. During the flyby, the spacecraft took almost 16,000 images of Saturn, its moons, and ring system. Voyager 2's path past Saturn and Titan directed it up and out of the plane of the ecliptic, allowing scientists to get an overhead view of the planet and rings. Voyager 1 is currently on an Interstellar Mission and is the most distant man-made object ever launched, taking that title from Pioneer 10 on February 17, 1998. It is now probing the boundaries of the heliosphere, where the solar system gives way to the interstellar medium.
Successful “Grand Tour” flybys of Jupiter, Saturn, Uranus, and Neptune (NASA)
Launch: August 20, 1977 Jupiter encounter: April 25 to August 5, 1979 Saturn encounter: June 5 to September 5, 1981 Uranus encounter: November 4, 1985 to February 25, 1986 Neptune encounter: June 5 through October 2, 1989
Even though Voyager 2 launched 16 days before Voyager 1, it took its time getting to Jupiter and arrived four months after Voyager 1. Ten months into the flight, well before the spacecraft reached the planet, Voyager 2's primary radio receiver failed. The backup receiver kicked in, but it proved to be somewhat unreliable. Controllers tried to revive the primary receiver, without any luck. They were forced to continue with the backup. Despite its irregularities, the backup receiver worked admirably during the Jupiter fly by. Voyager 2 flew by Jupiter on July 9, 1979, taking about the same number of images as Voyager 1 (18,000 images of Jupiter and its moons). Between the two spacecraft, three new moons were discovered as well as a thin, dark ring around Jupiter. Voyager images of Jupiter's moon Io revealed active volcanoes, the first ever discovered on another body besides Earth. Voyager 2 passed by the ringed world on August 26, 1981. It flew within 41,000 kilometers of the planet's cloud tops and provided scientists with almost 16,000 images of the planet, its moons and rings. While at Saturn, the two Voyager spacecraft discovered three new moons of Saturn, the intricate structure and spoke-like features of the ring system, and information about the planet's atmosphere and magnetic field. Voyager 2 flew by Uranus on January 24, 1986, coming within 81,500 kilometers of the planet's cloud tops. The spacecraft took almost 8,000 images of the planet, its moons and its dark ring system. The planet itself appeared as a vague, nearly featureless ball covered by a greenish blue methane haze. Although Voyager 2 performed a survey of Uranus’ moons, it passed by when tilted Uranus was at the height of southern summer, meaning that only the moons’ southern hemispheres were visible. Voyager 2 had to pass very close by Uranus to get the gravity assist necessary to send it on to Neptune. The close flyby altitude, combined with the vertical, “bull’s-eye” pattern of Uranus’ tilted system of rings and moons, meant that Voyager 2 saw only Miranda close-up; the rest of the moons were seen only distantly. The Voyager 2 images yielded the discoveries of 10 new moons. Voyager 2 flew by Neptune on August 25, 1989. Since Neptune was the final target for the spacecraft, scientists decided they could take risks they had avoided during previous planetary encounters. They programmed Voyager 2 to fly within 5,000 kilometers of the planet's cloud tops, closer than it had come to Jupiter, Saturn, or Uranus. The results were impressive. Even at such a great distance from the Sun, the 4-hour time lag in communications and low lighting conditions, the spacecraft returned 10,000 images of Neptune, its moons, and ring system. Voyager 2 discovered interesting cloud features on the planet and recorded some of the fastest winds in the solar system. The spacecraft also discovered the clumpiness of Neptune’s rings, as well as six new moons. The close approach to Neptune actually slowed Voyager 2’s speed with respect to the Sun, and sent the spacecraft on a trajectory diving below the plane of the solar system. Like Voyager 1, it is now probing the boundaries of the heliosphere, where the solar system gives way to the interstellar medium.
Launch: 2022 (planned) Arrival at Jupiter: Planned for 2030
The Jupiter Icy Moon Explorer (JUICE) will focus on three of Jupiter's moons: Ganymede, Callisto, and Europa. The goals of the mission include mapping the surfaces these moons, characterizing their interiors and potential subsurface oceans, and investigating their habitability.
Launch: 2022 (planned) Arrival at Jupiter: Planned for 2030
The Europa Multiple-Flyby Mission (formerly known as the Europa Clipper) is an interplanetary mission in development by NASA comprised of an orbiter and a lander. Planned to launch in the 2020s, the mission's goal is to study Europa through a lander and a series of flybys while in orbit around Jupiter.
Launch: October 2023 (planned) Arrival at Asteroid 16 Psyche: Planned for 2030
Psyche is an orbiter mission that will explore the origin of planetary cores by studying the metallic asteroid 16 Psyche. This asteroid may be the exposed iron core of a protoplanet, likely the remnant of a violent collision with another object that stripped off the outer crust.
Comet 67P/Churyumov-Gerasimenko orbiter and lander (ESA)
Launch: March 2, 2004 Churyumov-Gerasimenko arrival: May 2014 Philae Churyumov-Gerasimenko landing: November 12, 2014 Last signal from Philae: July 8, 2015 Rosetta deorbit: September 30, 2016 July 8, 2015
Rosetta's original goal was comet 46P/Wirtanen, but launch delays required a rerouting to 67P/Churyumov-Gerasimenko. The route is long, involving three Earth flybys (in 2005, 2007, and 2009) and one Mars flyby (February 25, 2007). It flew by asteroid 2867 Steins on September 5, 2008 and 21 Lutetia on July 10, 2010. Its long cruise will take it nearly to Jupiter's orbit before it travels inward again to rendezvous with the comet. Since Rosetta is solar-powered, ESA had to place it into a state of deep hibernation for this most distant period of its cruise. Rosetta went to sleep on June 8, 2011 and woke up again on January 20, 2014, four months before its arrival at Churyumov-Gerasimenko. Rosetta entered orbit on August 6, 2014, and dropped its small lander, Philae, to the surface of the comet on November 12, 2014. Philae exhausted its battery on November 15, but brief contact was made with the lander again when Churyumov-Gerasimenko approached perihelion in July 2015. Philae has remained silent since July 8, 2015. The Rosetta orbiter continued its science mission in orbit around Churyumov-Gerasimenko until it was intentionally deorbited on September 30, 2016.
Comet Giacobini-Zinner flyby and distant Halley observer (NASA)
(Formerly known as International Sun-Earth Explorer or ISEE-3) Launch: August 12, 1978 Flyby: September 11, 1985 Contact lost: September 16, 2014
Originally launched to explore Earth's magnetosphere and its interaction with the solar wind, the International Sun-Earth Explorer was renamed the International Cometary Explorer on December 22, 1983. On that date, a lunar gravity assist flyby launched the spacecraft onto a heliocentric orbit ahead of Earth to intercept comet Giacobini-Zinner. It flew through the tail of Giacobini-Zinner on September 11, 1985, and went on to transit between the Sun and Halley's comet in March 1986, becoming the first spacecraft to investigate two comets. There was no contact with ICE after the end of its mission in 1999 until September 18, 2008, when it was successfully re-contacted for a brief status check. On May 29, 2014, two-way communication with the spacecraft was reestablished by the ISEE-3 Reboot Project, and unofficial group with support from the Skycorp company. They successfully fired the thrusters on July 2, 2014, the first time they had fired since 1987. However, later firings of the thrusters failed, and contact with the spacecraft was lost on September 16, 2014. It is unknown whether contact can be reestablished as the probe's exact orbit is uncertain.
Launched with Hayabusa2, the PRoximate Object Close flYby with Optical Navigation (PROCYON) mission was a 50 kg-class microsatellite with two goals: Testing a microsatellite bus system for deep space exploration, and performing a close flyby observation (within 50 km) of an asteroid. The spacecraft entered its planned initial Earth-resonant orbit on December 4, 2014, where it commenced its initial operation phase. The mission returned to Earth near the end of 2015 with its solar electric propulsion system, where its ion engine failed to perform a deep space maneuver to change its trajectory toward its target asteroid 2000 DP107.
Asteroid 4 Vesta and dwarf planet 1 Ceres orbiter (NASA)
Launch: September 27, 2007 Vesta arrival: July 16, 2011 Ceres arrival: April 2015 End of mission: October 31, 2018
After flying past Mars on February 4, 2009, Dawn crept up on asteroid 4 Vesta, becoming the first orbiter of a main-belt asteroid. After surveying the asteroid from many altitudes, Dawn departed Vesta in the summer of 2012, embarking on a journey that ended with orbit insertion at 1 Ceres in April 2015. The mission observed Ceres until exhausting its fuel, and remains in a stable orbit.
Launch: January 12, 2005 Tempel 1 impact and flyby: July 4, 2005 Hartley 2 flyby: November 4, 2010 Last heard from: August 8, 2013
One day prior to its flyby of Tempel 1, Deep Impact released a 364-kilogram copper impactor onto a collision course with the comet. The impactor captured images all the way down to its 10.2 kilometer-per-second impact with Tempel 1. The flyby spacecraft captured amazing views of the impact from a safe distance as every large telescope on Earth was also pointed at the comet. Since the end of its primary mission, Deep Impact's blurred camera has been employed to study exoplanets (a project called the EPOXI mission), and the spacecraft has encountered a second comet, 103P/Hartley 2. The spacecraft is low on fuel but otherwise still functional and now being tested for future use as a deep-space astronomical observatory. It was then targeted for a 2020 flyby of asteroid (163249) 2002GT, but contact was lost after August 8, 2013.
Orbiter and sample return from asteroid Itokawa (1998 SF36) (ISAS/JAXA)
Launch: May 9, 2003 Itokawa arrival: September 2005 Earth sample return: June 13, 2010
Hayabusa's mission to and from asteroid Itokawa was one of the most thrilling adventures in modern space exploration, marked by numerous near-mission-ending disasters saved by the ingenuity of mission engineers, and culminating in the fiery death of the parent spacecraft on the night of the return of its sample capsule -- a story much too long for this space (and dramatic enough to be the subject of three feature-length films in Japan). Hayabusa rendezvoused with and touched down on a very small asteroid. It deployed a hopper named "Minerva" on November 12, 2005, but the hopper missed the asteroid. It did successfully drop a target marker containing 880,000 names to the surface, and then followed the marker down for two landing attempts. Upon the successful return of the sample capsule, a very small amount of asteroid dust was found inside, plenty for analysis by labs trained on the Stardust samples.
CONTOUR was lost August 15, 2002, when the spacecraft failed to contact Earth shortly after a scheduled firing of its main rocket motor. Investigation revealed that the spacecraft broke apart toward the end of the rocket motor firing. The spacecraft had been scheduled to fly by at least three comets: comet 2P/Encke in 2003, continuing with 29P/Schwassmann-Wachmann in 2006, and 6P/d'Arrest in 2008.
Flyby and coma sample return from comet P/Wild 2 (NASA)
Launch: February 7, 1999 Annefrank flyby: October 31, 2002 Wild 2 flyby: January 2, 2004 Sample return: January 15, 2006 Tempel 1 flyby: February 15, 2011 Propellant exhausted: March 24, 2011
Stardust flew past Earth on November 14, 2000, and then asteroid 5535 Annefrank. When Stardust flew by Wild 2, it collected samples of dust and volatiles from the comet's coma as well as images and other data. Other objectives of the mission included collecting samples of interstellar dust grains, imaging the comet nucleus, and conducting preliminary analysis of the composition of the cometary dust particles. It returned the samples to Earth on January 15, 2006. The aerogel collector plates proved to be full of cometary material, surpassing the science team's expectations. Following another Earth flyby on January 14, 2009, Stardust was sent onward to comet 9P/Tempel 1, which had been the target of the Deep Impact mission. With nearly no fuel left onboard, Stardust was commanded to burn the rest of it to depletion before powering down for good.
Flybys of asteroid 9969 Braille and comet 19P/Borrelly (NASA)
Launch: October 24, 1998 Braille flyby: July 28, 1999 Borrelly flyby: September 22, 2001 Engine shut down: December 18, 2001
Deep Space 1 was a demonstration probe designed to test new technologies such as ion propulsion. The spacecraft flew by asteroid 9969 Braille, which was named through a Planetary Society-run contest, within 15 kilometers of the asteroid's surface. With all systems still operating at the end of its primary mission in September 1999, engineers decided to extend the mission and attempt a flyby of comet 19P/Borrelly. By the time Deep Space 1 reached Borrelly, it had lasted three times longer than expected. It flew within 2,200 kilometers of the comet, providing the most detailed images of a comet's nucleus yet seen. With its fuel almost gone and its instruments in varying states of disrepair, communication with the spacecraft was terminated in December 2001. However, the spacecraft could, in theory, be re-contacted and returned to service, as ICE was.
Launch: October 15, 1997 Jupiter flyby: December 30, 2000 Orbit insertion: July 1, 2004 Huygens probe descent: January 14, 2005 Saturn entry: September 15, 2017
Cassini-Huygens' path to Saturn required two flybys of Venus (on April 26, 1998, and June 24, 1999), one of Earth (on August 18, 1999), and one of Jupiter (on December 30, 2010). During the Jupiter encounter, Cassini conducted coordinated observations with Galileo. The Huygens probe descent was wildly successful, revealing a strange new world of channels and basins on Titan. Cassini shaped its orbit around Saturn with numerous gravity-assist flybys of Titan, occasionally surveying Saturn from above or below (with lovely perspectives on the rings) and occasionally from within the ring plane (affording frequent encounters with Saturn's other, smaller moons). Cassini's mission was extended twice, and ended on September 15, 2017, after 293 complete orbits of Saturn, with the spacecraft's plunge into the atmosphere.
Asteroid 433 Eros orbiter (eventually used as a lander!) (NASA)
Launch: February 17, 1996 Eros arrival: February 14, 2000 Eros landing: February 12, 2001 End of mission: February 28, 2001
During its yearlong mission, NEAR gathered 10 times more data than originally planned. On February 12, 2001, with its fuel and funding nearly depleted, mission planners tried the unprecedented maneuver of landing the orbiter on Eros. With fragile solar panels and protruding antennae, NEAR was never intended to be a lander. However, controllers successfully brought the spacecraft to a gentle 1.9 meter-per-second touchdown onto the rocky surface, taking 69 images during the final descent. The spacecraft continued to function even after it landed. NEAR was officially shut down on February 28, 2001.
Launch: October 6, 1990 Jupiter flyby: February 8, 1992 Mission end: June 30, 2009
The primary mission of Ulysses was to study the north and south pole of the Sun. However, getting to those solar poles required the spacecraft to perform some interplanetary gymnastics. The spacecraft first went to Jupiter, where the strong Jovian gravity helped redirect the spacecraft, placing it on its proper course. As Ulysses flew by the planet, instruments onboard the spacecraft studied Jupiter's strong magnetic field and radiation levels. The mission was long and productive, ending only in 2008 after the X-band transmitter failed and the fuel had nearly frozen.
Jupiter orbiter, with flybys of asteroids 951 Gaspra and 243 Ida/Dactyl (NASA)
Launch: October 18, 1989 Gaspra flyby: October 29, 1991 Ida/Dactyl flyby: August 28, 1993 Witnessed Shoemaker-Levy crash: July 1994 Jupiter probe descent: December 7, 1995 Jupiter orbit insertion: December 8, 1995 Plunge into Jupiter: September 22, 2003
After a long and troubled development process culminating with a launch from Space Shuttle Atlantis, Galileo traveled past Venus once (on February 10, 1990) and Earth twice (on December 8, 1992 and August 28, 1993). But it had suffered a crippling malfunction early in its mission when its high-gain antenna failed to open. Still, Galileo accomplished the first ever asteroid flybys as it traveled through the main belt on its way to Jupiter. It passed within 1,600 kilometers of Gaspra and 2,400 kilometers of Ida. Galileo made the surprising discovery that Ida has a tiny satellite, which was later named Dactyl. As Galileo approached its insertion into Jupiter orbit, it happened to be the right place in the right time to observe comet Shoemaker-Levy 9 break up and crash into Jupiter. Galileo was the only observatory that had a direct view of the impact, which happened on Jupiter’s night side; Earth-based telescopes had to wait until Jupiter’s rotation brought the impact zone into view hours later. Galileo was the first spacecraft to deploy a probe into an outer planet’s atmosphere. When the Jupiter Probe plunged into the Jovian clouds, it sent back information about the temperature, wind speeds, and pressure as it descended. It finally succumbed to the incredible pressure (24 times Earth's pressure at sea level) one hour after it began its descent. Galileo was also the first spacecraft to dwell in a giant planet's magnetosphere long enough to identify its global structure and investigate the dynamics of Jupiter's magnetic field. It revealed that Jupiter's ring system is formed by dust kicked up as interplanetary meteoroids smash into the planet's four small inner moons and that the planet's outermost ring is actually two rings, one embedded within the other. The spacecraft’s mission was extended three times in order to study the Galilean satellites Io, Europa, Ganymede, and Callisto. Galileo made many discoveries about these moons: Io's extensive volcanic activity is 100 times greater than that found on Earth; Europa harbors a salty ocean up to 100 kilometers (62 miles) underneath its frozen surface, containing about twice as much water as all the Earth's oceans; Callisto and Ganymede may also feature a liquid-saltwater layer; and Ganymede has an iron core, like Earth, and a magnetic field, making this moon the first satellite known to possess a magnetic field. In order to avoid any possibility of the spacecraft contaminating Europa’s salty ocean with material brought from Earth, the spacecraft was deliberately destroyed by sending it onto a collision course with Jupiter.
Comets 1P/Halley and 26P/Grigg-Skjellerup flyby (ESA0
Launch: July 2, 1985 Halley flyby: March 13, 1986 Grigg-Skjellerup flyby: July 10, 1992 End of mission: July 23, 1992
Giotto flew by Halley at a distance 596 kilometers. All experiments performed well and returned a wealth of new scientific results, of which perhaps the most important was the clear identification of the cometary nucleus. During an extended mission, the spacecraft successfully encountered comet Grigg-Skjellerup at a distance of 200 kilometers.
Launch: March 18, 1985 Flyby: March 8, 1986 Fuel depleted: February 22, 1991
Suisei (which translates to ‘Comet') was identical to Sakigake apart from its payload: an ultraviolet (UV) imaging system and a solar wind instrument. Suisei began UV observations in November 1985, generating up to 6 images per day. The spacecraft encountered Comet 1P/Halley at a distance of 151,000 kilometers. ISAS had decided during 1987 to guide Suisei to a November 24, 1998 encounter with 21P/Giacobini-Zinner, but due to depletion of the hydrazine, this, as well as plans to fly within several million kilometers of comet 55P/Tempel-Tuttle on February 28, 1998, were canceled.
Launch: January 8, 1985 Flyby: March 11, 1986 Contact lost: November 15, 1995
Sakigake (which translates to 'Pioneer') was a prototype spacecraft launched by the Japanese space agency ISAS. It successfully flew within 7 million kilometers of Halley's comet. The spacecraft was equipped with 3 instruments to measure plasma wave spectra, solar wind ions, and interplanetary magnetic fields. An extended mission was planned, including flybys of comet 45P/Honda-Mrkos-Pajdusakova in 1996 and comet 21P/Giacobini-Zinner in 1998. Unfortunately, controllers lost contact with the spacecraft.
Comet 1P/Halley flybys (Soviet Academy of Sciences)
Launch: December 15 and 21, 1984 Flyby: March 6 and 9, 1986
The identical Vega 1 and Vega 2 combined Venus swingbys with flybys of comet 1P/Halley. It is estimated that Vega 1 flew by at a distance of 10,000 kilometers (6,000 miles), and Vega 2 at 3,000 kilometers (1,800 miles).
Launch: April 5, 1973 Jupiter flyby: December 2, 1974
Pioneer 11 was the second spacecraft to explore the outer solar system (the first being Pioneer 10). Pioneer 11 flew within 34,000 kilometers (21,100 miles) of the Jovian cloud tops. The spacecraft studied the planet's magnetic field and atmosphere and took pictures of the planet and some of its moons. It then flew by Saturn on September 1, 1979 and continued on out of the solar system. Instruments were finally shut down on September 9, 1995, when there was no longer enough power.
Launch: March 2, 1972 Jupiter flyby: December 3, 1973
Pioneer 10 was the first spacecraft to pass through the Asteroid Belt and explore the outer solar system. It flew within 200,000 kilometers of the Jovian cloud tops. Scientists were surprised at the tremendous radiation levels experienced by the spacecraft as it passed the gas giant planet. Once past Jupiter, the spacecraft headed out of the solar system. Routine communication with Pioneer 10 ended on March 31, 1997, but controllers occasionally checked in with it until contact was lost on April 28, 2001. It is now heading in the general direction of Aldebaran, the red giant star in the constellation of Taurus. At its current speed, it would take about 2 million years to get to Aldebaran.