List of minor planets and comets visited by spacecraft
Proposals -- Recent
Table:
Name
Diameter
Year of
discovery
Agency
Proposed year
Notes

99942 Apophis
370 m
2004
CNAS
Launch: ~2022
Flyby[23]

169P/NEAT

1871
CNAS
Launch: ~2022
Flyby[23]

(175706) 1996 FG3
1,550 m
1996
CNAS
Launch: ~2022
Sample-return[23]

(172034) 2001 WR1
660 m
2001
JAXA
Flyby: 2023
Potential mission extension of Hayabusa2 spacecraft.[24]

(138971) 2001 CB21

2001
NASA
Flyby: 2022
Potential flyby during transit by DART spacecraft.[15]

67P/Churyumov–Gerasimenko
4.1×3.3×1.8 km
1969
NASA
Launch: 2024
CONDOR, a proposed asteroid sample-return mission.[25] Not selected for launch.

88P/Howell
4.4 km
1981
NASA
Launch: 2024
CORSAIR, a proposed comet sample-return mission.[26] Not selected for launch.

67P/Churyumov–Gerasimenko
4.1×3.3×1.8 km
1969
NASA
Launch: 2024
CAESAR, a proposed comet sample-return mission.[27]

65803 Didymos
170 m
1996
ESA-NASA
Launch: 2023
AIDA, a proposed asteroid impactor and orbiter.[28]

Trojan asteroids

1906
JAXA
Launch: 2026
OKEANOS, a proposed multiple flyby mission to Jupiter's Trojan asteroids using solar sail propulsion.[29]

2 Pallas
512 km
1779
NASA
Launch: 2022
Flyby: 2024
Athena, a proposed flyby of Pallas

10199 Chariklo
330 km
1997
NASA

Camilla, a mission concept for a flyby and impactor

By analysing the table together with the context snippets below.

Solve the question: What is the diameter of the small planet/comet that caused a brief period of concern in December 2004 ? Output only the correct answer as {"answer": "YOUR ANSWER"}. Use only the information given.

In geometry , a diameter of a circle is any straight line segment that passes through the center of the circle and whose endpoints lie on the circle . It can also be defined as the longest chord of the circle . Both definitions are also valid for the diameter of a sphere . In more modern usage , the length of a diameter is also called the diameter . In this sense one speaks of the diameter rather than a diameter ( which refers to the line segment itself ) , because all diameters of a circle or sphere have the same length , this being twice the radius r .
99942 Apophis ( /əˈpɒfɪs/ , previously known by its provisional designation 2004 MN4 ) is a near-Earth asteroid with a diameter of 370 metres that caused a brief period of concern in December 2004 because initial observations indicated a probability of up to 2.7% that it would hit Earth on April 13 , 2029 . Additional observations provided improved predictions that eliminated the possibility of an impact on Earth or the Moon in 2029 . However , until 2006 , a possibility remained that during the 2029 close encounter with Earth , Apophis would pass through a gravitational keyhole , a small region no more than about 800 metres ( 1⁄2 mi ) in diameter , that would set up a future impact exactly seven years later on April 13 , 2036 . This possibility kept it at Level 1 on the Torino impact hazard scale until August 2006 , when the probability that Apophis would pass through the keyhole was determined to be very small and Apophis 's rating on the Torino scale was lowered to zero . By 2008 , the keyhole had been determined to be less than 1 km wide . During the short time when it had been of greatest concern , Apophis set the record for highest rating on the Torino scale , reaching level 4 on December 27 , 2004 . As of 2014 , the diameter of Apophis is estimated to be approximately 370 metres ( 1,210 ft ) . Preliminary observations by Goldstone radar in January 2013 effectively ruled out the possibility of an Earth impact by Apophis in 2036 . By May 6 , 2013 ( April 15 , 2013 observation arc ) , the probability of an impact on April 13 , 2036 had been eliminated . Using observations through February 26 , 2014 , the odds of an impact on April 12 , 2068 , as calculated by the JPL Sentry risk table are 1 in 150,000 . As of October 2019 [ update ] , there were two asteroids with a more notable cumulative Palermo Technical Impact Hazard Scale than Apophis , though none of them have a Torino level greater than 0 . On average , one asteroid the size of Apophis ( 370 metres ) can be expected to impact Earth once in about 80,000 years .
China National Space Administration ( CNSA ) is the national space agency of China . It is responsible for the national space program and for planning and development of space activities . CNSA and China Aerospace Corporation ( CASC ) assumed the authority [ when ? ] over space development efforts previously held by the Ministry of Aerospace Industry . It is a subordinate agency of the State Administration for Science , Technology and Industry for National Defence ( SASTIND ) , itself a subordinate agency of the Ministry of Industry and Information Technology ( MIIT ) . The headquarters are in Haidian District , Beijing .
169/NEAT is a periodic comet in the Solar System . It is the parent body of the alpha Capricornids meteor shower . 169/NEAT may be related to comet P/2003 T12 ( SOHO ) . 169P is a low activity comet roughly a few kilometers in diameter . 169P and the smaller body P/2003 T12 likely fragmented from a parent body roughly 2900 years ago . It is the parent body of the Alpha Capricornids meteor shower in late July .
( 175706 ) 1996 FG3 is a carbonaceous asteroid and binary system , classified as near-Earth object and potentially hazardous asteroid of the Apollo group , approximately 1.7 kilometers ( 1.1 miles ) in diameter . The primary has a spheroidal shape . Its minor-planet moon measures approximately 490 meters ( 1,600 feet ) in diameter . It was discovered on 24 March 1996 , by Australian astronomer Robert McNaught at Siding Spring Observatory in New South Wales , Australia . 1996 FG3 was also a target of a rejected mission-proposal for the Marco Polo spacecraft . In 2017 , scientist from the Chinese Academy of Sciences ' Purple Mountain Observatory revealed a plan to land a probe on this asteroid in 2029 , as part of an asteroid exploration mission .
( 172034 ) 2001 WR1 , provisional designation 2001 WR1 , is a sub-kilometer near-Earth object of the Amor group , approximately 650 meters ( 2,100 feet ) in diameter . The S-type asteroid has been identified as a potential flyby target of the Hayabusa2 mission . It was discovered on 17 November 2001 , by astronomers with the Lincoln Near-Earth Asteroid Research at the Lincoln Laboratory 's Experimental Test Site near Socorro , New Mexico , in the United States . The asteroid has a rotation period of 8.0 hours and possibly an elongated shape . It remains unnamed since its numbering in December 2007 .
The Japan Aerospace Exploration Agency ( JAXA ) ( 国立研究開発法人宇宙航空研究開発機構 , Kokuritsu-kenkyū-kaihatsu-hōjin Uchū Kōkū Kenkyū Kaihatsu Kikō , literally National Research and Development Agency on Aerospace Research and Development ) is the Japanese national aerospace and space agency . Through the merger of three previously independent organizations , JAXA was formed on 1 October 2003 . JAXA is responsible for research , technology development and launch of satellites into orbit , and is involved in many more advanced missions such as asteroid exploration and possible human exploration of the Moon . Its motto is One JAXA and its corporate slogan is Explore to Realize ( formerly Reaching for the skies , exploring space ) .
Hayabusa2 ( Japanese : はやぶさ2 , Peregrine falcon-2 ) is an asteroid sample-return mission operated by the Japanese space agency , JAXA . It follows on from the Hayabusa mission which returned asteroid samples in 2010 . Hayabusa2 was launched on 3 December 2014 and rendezvoused with near-Earth asteroid 162173 Ryugu on 27 June 2018 . It surveyed the asteroid for a year and a half and took samples . It left the asteroid in November 2019 and is expected to return to Earth in late 2020 . Hayabusa2 carries multiple science payloads for remote sensing , sampling , and four small rovers that investigated the asteroid surface to inform the environmental and geological context of the samples collected .
The National Aeronautics and Space Administration ( NASA , /ˈnæsə/ ) is an independent agency of the United States Federal Government responsible for the civilian space program , as well as aeronautics and aerospace research . [ note 1 ] NASA was established in 1958 , succeeding the National Advisory Committee for Aeronautics ( NACA ) . The new agency was to have a distinctly civilian orientation , encouraging peaceful applications in space science . Since its establishment , most US space exploration efforts have been led by NASA , including the Apollo Moon landing missions , the Skylab space station , and later the Space Shuttle . NASA is supporting the International Space Station and is overseeing the development of the Orion Multi-Purpose Crew Vehicle , the Space Launch System and Commercial Crew vehicles . The agency is also responsible for the Launch Services Program which provides oversight of launch operations and countdown management for uncrewed NASA launches . NASA science is focused on better understanding Earth through the Earth Observing System ; advancing heliophysics through the efforts of the Science Mission Directorate 's Heliophysics Research Program ; exploring bodies throughout the Solar System with advanced robotic spacecraft missions such as New Horizons ; and researching astrophysics topics , such as the Big Bang , through the Great Observatories and associated programs .
Double Asteroid Redirection Test ( DART ) is a planned space probe that will demonstrate the kinetic effects of crashing an impactor spacecraft into an asteroid moon for planetary defense purposes . The mission is intended to test whether a spacecraft impact could successfully deflect an asteroid on a collision course with Earth . A demonstration of an asteroid deflection is a key test that NASA and other agencies wish to perform before the actual need of planetary defense is present . DART is a joint project between NASA and the Johns Hopkins Applied Physics Laboratory ( APL ) , and it is being developed under the auspices of NASA 's Planetary Defense Coordination Office . In August 2018 , NASA approved the project to start the final design and assembly phase .
67P/Churyumov-Gerasimenko ( abbreviated as 67P or 67P/C-G ) is a Jupiter-family comet , originally from the Kuiper belt , with a current orbital period of 6.45 years , a rotation period of approximately 12.4 hours and a maximum velocity of 135,000 km/h ( 38 km/s ; 84,000 mph ) . Churyumov-Gerasimenko is approximately 4.3 by 4.1 km ( 2.7 by 2.5 mi ) at its longest and widest dimensions . It was first observed on photographic plates in 1969 by Soviet astronomers Klim Ivanovych Churyumov and Svetlana Ivanovna Gerasimenko , after whom it is named . It came to perihelion ( closest approach to the Sun ) on 13 August 2015 . Churyumov-Gerasimenko was the destination of the European Space Agency 's Rosetta mission , launched on 2 March 2004 . Rosetta rendezvoused with Churyumov-Gerasimenko on 6 August 2014 and entered orbit on 10 September 2014 . Rosetta 's lander , Philae , landed on the comet 's surface on 12 November 2014 , becoming the first spacecraft to land on a comet nucleus . On 30 September 2016 , the Rosetta spacecraft ended its mission by landing on the comet in its Ma'at region .
COmet Nucleus Dust and Organics Return ( CONDOR ) is a mission concept to retrieve a sample from comet 67P/Churyumov-Gerasimenko to test ideas regarding Solar System formation , and accretion of rocky planets with habitable surface environments . CONDOR is a concept by a NASA 's Jet Propulsion Laboratory team led by Christopher Guethe and M. Choukroun .
88P/Howell is a periodic comet in the Solar System . It was discovered on 29 August 1981 . In 1975 the comet 's perihelion ( closest approach to the Sun ) was 1.9 AU , But a close approach to Jupiter in 1978 perturbed the perihelion distance closer to the Sun . It last came to perihelion on 6 April 2015 ; the next occurrence will be in 2020 . In 14 September 2031 the comet will pass 0.074 AU ( 11,100,000 km ; 6,900,000 mi ) from Mars . In response to New Frontiers program call for Mission 4 , a team from Johns Hopkins University Applied Physics Laboratory ( JHUAPL ) submitted a mission concept proposal called Comet Rendezvous , Sample Acquisition , Investigation , and Return ( CORSAIR ) that would perform a sample return from comet 88P/Howell .
Comet Rendezvous , Sample Acquisition , Investigation , and Return ( CORSAIR ) is a concept mission to return comet nucleus samples to Earth for detailed analysis . The mission concept was submitted in May 2017 by a team from NASA 's Goddard Space Flight Center in response to the New Frontiers program call for mission 4 , but did not pass the initial down selection . As a comet sample return mission was not ultimately selected for mission 4 ( Dragonfly , a Titan probe was selected ) , the CORSAIR team may re-submit the concept to a future New Frontiers program call .
CAESAR ( Comet Astrobiology Exploration Sample Return ) is a sample-return mission concept to comet 67P/Churyumov-Gerasimenko . The mission was proposed in 2017 to NASA 's New Frontiers program mission 4 , and on 20 December 2017 it was one of two finalists selected for further concept development . On 27 June 2019 , the other finalist , the Dragonfly mission , was chosen instead . Had it been selected in June 2019 , it would have launched between 2024 and 2025 , with a capsule delivering a sample back to Earth in 2038 . The Principal Investigator is Steve Squyres of Cornell University in Ithaca , New York . CAESAR would be managed by NASA 's Goddard Space Flight Center in Greenbelt , Maryland . Curation of the returned sample would take place at NASA 's Astromaterials Research and Exploration Science Directorate , based at Johnson Space Center in Houston , Texas . The CAESAR team chose comet 67P over other cometary targets in part because the data collected by the Rosetta mission , which studied the comet from 2014 to 2016 , allows the spacecraft to be designed to the conditions there , increasing the mission 's chance of success . The Rosetta mission also provides a vast geologic context for this mission 's sample-return analysis .
65803 Didymos , provisional designation 1996 GT , is a sub-kilometer asteroid and synchronous binary system , classified as potentially hazardous asteroid and near-Earth object of both the Apollo and Amor group . The asteroid was discovered in 1996 , by the Spacewatch survey at Kitt Peak , and its small 170-meter minor-planet moon was discovered in 2003 . Due to its binary nature , it was then named Didymos , the Greek word for twin . Didymos is the target of the proposed AIDA asteroid-mission to test the viability of asteroid impact avoidance by collision with a spacecraft .
The Asteroid Impact and Deflection Assessment ( AIDA ) mission is a proposed pair of space probes which will study and demonstrate the kinetic effects of crashing an impactor spacecraft into an asteroid moon . The mission is intended to test and validate impact models of whether a spacecraft could successfully deflect an asteroid on a collision course with Earth . The original plan called for a European spacecraft , the Asteroid Impact Mission ( AIM ) , to operate in synergy with a large NASA impactor called Double Asteroid Redirection Test ( DART ) and observe the immediate effects of the impact . AIM was cancelled in 2016 when Germany was unable to fund its portion , and after some backlash within ESA , AIM was replaced in 2018 with a smaller spacecraft called Hera that will launch five years after DART to orbit and study the crater on the asteroid . DART is currently planned to impact in 2022 on the small moon of asteroid Didymos , while Hera will arrive at Didymos in 2027 , five years after DART 's impact .
In astronomy , a trojan is a small celestial body that shares the orbit of a larger one , remaining in a stable orbit approximately 60° ahead or behind the main body near one of its Lagrangian points L4 and L5 . Trojans can share the orbits of planets or of large moons . Trojans are one type of co-orbital object . In this arrangement , a star and a planet orbit about their common barycenter , which is close to the center of the star because it is usually much more massive than the orbiting planet . In turn , a much smaller mass than both the star and the planet , located at one of the Lagrangian points of the star-planet system , is subject to a combined gravitational force that acts through this barycenter . Hence the smallest object orbits around the barycenter with the same orbital period as the planet , and the arrangement can remain stable over time . In the Solar System , most known trojans share the orbit of Jupiter . They are divided into the Greek camp at L4 ( ahead of Jupiter ) and the Trojan camp at L5 ( trailing Jupiter ) . More than a million Jupiter trojans larger than one kilometer are thought to exist , of which more than 7,000 are currently catalogued . Only nine Mars trojans , 22 Neptune trojans , two Uranus trojans , and a single Earth trojan , have been found to date . A temporary Venus Trojan is also known . Numerical orbital dynamics stability simulations indicate that Saturn and Uranus probably do not have any primordial trojans .
OKEANOS ( Oversize Kite-craft for Exploration and Astronautics in the Outer Solar System ) is a proposed mission concept to Trojan asteroids , which share Jupiter 's orbit , using a hybrid solar sail for propulsion ; the sail is covered with thin solar panels to power an ion engine . In-situ analysis of the collected samples would be performed by either direct contact or using a lander carrying a high-resolution mass spectrometer . A sample-return to Earth is an option under study . OKEANOS was a finalist for Japan 's ISAS ' 2nd Large-class mission to be launched in 2026 , and possibly return Trojan asteroid samples to Earth in the 2050s . The winning mission was LiteBIRD .
Solar sails ( also called light sails or photon sails ) are a method of spacecraft propulsion using radiation pressure exerted by sunlight on large mirrors . Based on the physics , a number of spaceflight missions to test solar propulsion and navigation have been proposed since the 1980s . A useful analogy to solar sailing may be a sailing boat ; the light exerting a force on the mirrors is akin to a sail being blown by the wind . High-energy laser beams could be used as an alternative light source to exert much greater force than would be possible using sunlight , a concept known as beam sailing . Solar sail craft offer the possibility of low-cost operations combined with long operating lifetimes . Since they have few moving parts and use no propellant , they can potentially be used numerous times for delivery of payloads . Solar sails use a phenomenon that has a proven , measured effect on aerodynamics . Solar pressure affects all spacecraft , whether in interplanetary space or in orbit around a planet or small body . A typical spacecraft going to Mars , for example , will be displaced thousands of kilometers by solar pressure , so the effects must be accounted for in trajectory planning , which has been done since the time of the earliest interplanetary spacecraft of the 1960s . Solar pressure also affects the orientation of a spacecraft , a factor that must be included in spacecraft design . The total force exerted on an 800 by 800 meter solar sail , for example , is about 5 newtons ( 1.1 lbf ) at Earth 's distance from the Sun , making it a low-thrust propulsion system , similar to spacecraft propelled by electric engines , but as it uses no propellant , that force is exerted almost constantly and the collective effect over time is great enough to be considered a potential manner of propelling spacecraft .
Pallas [ d ] ( minor-planet designation : 2 Pallas ) is the second asteroid to have been discovered ( after Ceres ) , and is one of the largest asteroids in the Solar System . With an estimated 7% of the mass of the asteroid belt , it is the third-most-massive asteroid , being 10-30% less massive than Vesta . It is 512 kilometers ( 318 mi ) in diameter , somewhat smaller than 4 Vesta . It is likely a remnant protoplanet . When Pallas was discovered by the German astronomer Heinrich Wilhelm Matthäus Olbers on 28 March 1802 , it was counted as a planet , as were other asteroids in the early 19th century . The discovery of many more asteroids after 1845 eventually led to their reclassification . Pallas 's surface is most likely composed of a silicate material ; its spectrum and estimated density resemble carbonaceous chondrite meteorites . With an orbital inclination of 34.8° , Pallas 's orbit is unusually highly inclined to the plane of the asteroid belt , and its orbital eccentricity is nearly as large as that of Pluto , making Pallas relatively inaccessible to spacecraft . It was considered as a potential planet in 2006 , but it has since been determined that its shape departs significantly from an ellipsoid .
Athena is a proposed space mission that would perform a single flyby of asteroid 2 Pallas , the third largest asteroid in the Solar System . If Athena is funded , it would share the launch vehicle with the Psyche spacecraft and fly its own trajectory for a Mars gravity assist to slingshot into the asteroid belt . It would take about two years to reach Pallas . The mission 's Principal Investigator is Joseph O'Rourke , at Arizona State University .
10199 Chariklo ( /ˈkærɪkloʊ/ or /kəˈrɪkloʊ/ ; Ancient Greek : Χαρικλώ ; provisional designation 1997 CU26 ) is the largest confirmed centaur ( small body of the outer Solar System ) . It orbits the Sun between Saturn and Uranus , grazing the orbit of Uranus . On 26 March 2014 , astronomers announced the discovery of two rings ( nicknamed Oiapoque and Chuí ) around Chariklo by observing a stellar occultation , making it the first minor planet known to have rings . Chariklo was discovered by James V. Scotti of the Spacewatch program on February 15 , 1997 . Chariklo is named after the nymph Chariclo ( Χαρικλώ ) , the wife of Chiron and the daughter of Apollo . A photometric study in 2001 was unable to find a definite period of rotation . Infrared observations of Chariklo indicate the presence of water ice , which may in fact be located in its rings . Michael Brown 's website lists it as possibly a dwarf planet with a measured diameter of 232 km .