486958- 2014 Mu69

(486958) 2014 MU69, nicknamed Ultima Thule, is a trans-Neptunian object located in the Kuiper belt. It is a contact binary 31 km (19 mi) long, composed of two joined bodies 19 km (12 mi) and 14 km (9 mi) across that are nicknamed "Ultima" and "Thule", respectively. With an orbital period of 298 years and a low inclination and eccentricity, it is classified as a classical Kuiper belt object. With the New Horizons space probe's flyby at 05:33 on 1 January 2019 (UTC time), 2014 MU69 became the farthest and most primitive object in the Solar System visited by a spacecraft, both bodies being planetesimal aggregates of much smaller building blocks.



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(486958) 2014 MU69
Greyscale image of 2014 MU69[1][lower-alpha 1]
Discovered byMarc Buie
Discovery siteHubble Space Telescope
Discovery date26 June 2014
MPC designation(486958) 2014 MU69
  • Ultima Thule (unofficial)[7]
Orbital characteristics[5]
Epoch 2019 April 27 (JD 2458600.5)
Uncertainty parameter 2
Observation arc851 days
Aphelion46.442 AU
Perihelion42.7212447±0.0014309 AU
44.5813998 AU
298 yr
 0m 11.92s / day
Physical characteristics
Dimensions31.7±0.5 km long axis. Ultima 19.5 km across, Thule 14.2 km[10]
15±1 h
North pole right ascension
~ 300°[11]
North pole declination
~ −21°[11]

    (486958) 2014 MU69, nicknamed Ultima Thule,[lower-alpha 2] is a trans-Neptunian object located in the Kuiper belt. It is a contact binary 31 km (19 mi) long, composed of two joined bodies 19 km (12 mi) and 14 km (9 mi) across that are nicknamed "Ultima" and "Thule", respectively. With an orbital period of 298 years and a low inclination and eccentricity, it is classified as a classical Kuiper belt object. With the New Horizons space probe's flyby at 05:33 on 1 January 2019 (UTC time), 2014 MU69 became the farthest and most primitive object in the Solar System visited by a spacecraft, both bodies being planetesimal aggregates of much smaller building blocks.[16]

    2014 MU69 was discovered on 26 June 2014 by astronomer Marc Buie[6] using the Hubble Space Telescope as part of a search for a Kuiper belt object for the New Horizons mission to target in its first extended mission; it was chosen over two other candidates to become the primary target of the mission. Its nickname, a Greco-Latin term for a place beyond the known world, was chosen as part of a public competition in 2018. The New Horizons team plans to submit a proper name to the International Astronomical Union when the nature of the object is better understood.


    The orbits of New Horizons potential targets 1 to 3. 2014 MU69 (PT1) is in blue, 2014 OS393 (PT2) is in red and 2014 PN70 is in green.

    When 2014 MU69 was first observed, it was labelled 1110113Y,[17] nicknamed "11" for short.[18][19] Its existence as a potential target of the New Horizons probe was announced by NASA in October 2014[20][21] and it was unofficially designated as "Potential Target 1", or PT1.[19] Its official designation, 2014 MU69 (a provisional designation indicating that it was the 1745th object assigned one for the second half of June 2014), was assigned by the Minor Planet Center (MPC) in March 2015, after sufficient orbital information was gathered.[19] After further observations pinning down its orbit, it was given the permanent minor planet number 486958 on 12 March 2017.[22]

    An official name for the object, consistent with the naming guidelines of the International Astronomical Union, will be proposed by the New Horizons team after the spacecraft's flyby, when the properties of (486958) 2014 MU69 are known well enough to choose a suitable name.[23][24] In the interim, NASA invited suggestions from the public on a nickname to be used.[24] The campaign involved 115,000 participants from around the world, who suggested some 34,000 names. Of those, 37 reached the ballot for voting and were evaluated for popularity this included eight names suggested by the New Horizons team and 29 suggested by the public. "Ultima Thule",[lower-alpha 2] which was selected on 13 March 2018,[7] was nominated by about 40 members of the public and obtained the seventh highest number of votes among the nominees.[7] It is named after the Latin phrase ultima Thule (literally "farthest Thule"), an expression referencing the most distant place beyond the borders of the known world.[7] Once it was determined the body was a contact binary, the New Horizons team nicknamed the larger body "Ultima" and the smaller "Thule".[25]

    The nickname was criticized due to its use by Nazi occultists as the supposed mythical origin of the Aryan race, aside from its use in ancient Greek and Latin literature. The Thule Society was a key sponsor of what became the Nazi Party, and some modern-day neo-Nazis and members of the alt-right continue to use the term. A few members of the New Horizons team were aware of that association when they selected the nickname, and have since defended their choice. Responding to a question at a press conference, principal investigator Alan Stern said, "Just because some bad guys once liked that term, we’re not going to let them hijack it."[26][27]


    Color MVIC image superimposed on higher resolution black and white LORRI image (polar view)[lower-alpha 3]
    Near-polar view of 2014 MU69's rotation over a period of seven hours.[3]

    On 2 January 2019, immediately after the flyby, New Horizons principal investigator Alan Stern described 2014 MU69's shape as a "snowman".[29][30] On 8 February, Stern announced that 2014 MU69 was more flattened than thought earlier based on additional images, describing it as being more like a large "pancake" (larger lobe), and a "walnut" (smaller lobe) than two spheres.[31] By observing how the unseen sections of 2014 MU69 were occulted, scientists were able to then outline the shapes of both lobes.[32] 2014 MU69 has a red color suggestive of a mix of organic compounds known informally as tholins.[33][34]

    Reflectivity variations on 2014 MU69. The area near the neck is the most reflective.[35]

    The 'neck' region connecting both lobes of 2014 MU69 is considerably brighter compared to the surfaces of each lobe.[16] The brighter region in the neck is likely composed of a more reflective material different from the surfaces of 2014 MU69's lobes. One hypothesis suggests the bright material in the neck region had likely originated from the deposition of small particles that had fallen from 2014 MU69's lobes over time.[10] Since 2014 MU69's center of gravity lies between the two lobes, small particles are likely to roll down the steep slopes toward the center between each lobe. [36] Another proposal suggests the bright material is produced by the deposition of ammonia ice. [37] Ammonia vapor present on the surface of 2014 MU69 would solidify around the neck region, where gases cannot escape due to the concave shape of the neck.

    2014 MU69's orbital period around the Sun is slightly under 300 years and it has a low inclination and low eccentricity compared to other objects in the Kuiper belt.[38] These orbital properties mean that it is a cold classical Kuiper belt object which is unlikely to have undergone significant perturbations.[8] Observations in May and July 2015 as well as in July and October 2016 greatly reduced the uncertainties in the orbit.[12][5] Results from Hubble Space Telescope observations[39] show that the brightness of 2014 MU69 varies by less than 20 percent as it rotates.[40] This placed significant constraints on the axis ratio of 2014 MU69 to <1.14, having assumed an equatorial view. Despite 2014 MU69's irregular shape, there is no detectable light curve amplitude, as its axis is oriented on its side, pointing towards the Sun.[41][42] Distant satellites of 2014 MU69 have been excluded to a depth of >29th magnitude.[43] The object has no detectable atmosphere, and no large rings or satellites larger than one mile in diameter.[44]


    Phases of formation of 2014 MU69.[45]

    2014 MU69 is thought to have originally been two objects, nicknamed "Ultima" and "Thule", that formed over time from a rotating cloud of small, icy bodies since the formation of the solar system 4.6 billion years ago. Eventually, loss of momentum, caused by momentum shifting to other bodies in the cloud, caused the pair to slowly spiral closer until they touched – where over time the joints fused together, forming its present double-lobed shape.[45] Over a period of at least four billion years since its formation, the frequency of impact events occurring on 2014 MU69 were uncommon due to the slower speeds of objects in the Kuiper Belt.[46] With the lack of frequent cratering events and perturbations of its orbit, the shape and appearance of 2014 MU69 would remain virtually pristine since the conjoining of two separate objects that formed its double-lobed shape.[46][47]



    Discovery images of 2014 MU69, cropped from five Wide Field Camera 3 images taken on 26 June 2014.

    2014 MU69 was discovered on 26 June 2014 using the Hubble Space Telescope during a preliminary survey to find a suitable Kuiper belt object for the New Horizons probe to fly by. Scientists were searching for an object in the Kuiper belt that the spacecraft could study after Pluto, and their next target had to be reachable on New Horizons' remaining fuel. Using large ground-based telescopes on Earth, researchers began looking in 2011 for candidate objects and searched multiple times per year for several years. But objects that would work for New Horizons were just too distant and faint to be seen through Earth's atmosphere. MU69 was first spotted by Hubble on 26 June 2014, discovered by astronomer Marc Buie, a member of the New Horizons team.[6]

    MU69 is too small and distant to be observed directly from Earth, but scientists have been able to take advantage of a special type of astronomical event called an occultation. This is when the object passes in front of a star from the vantage point of Earth. This event is only visible from certain parts of the Earth, however. The New Horizons team combined data from Hubble and the European Space Agency's Gaia space observatory to figure out exactly when and where on Earth's surface MU69 would cast a shadow. They determined that occultations would occur on June 3, July 10 and July 17, 2017, and set off for places around the world where they could see MU69 cover up a different star on each of these dates. Based on this string of three occultations, scientists were able to trace out the object's shape.

    In the summer of 2018, nearly fifty New Horizons team members headed to Senegal and Colombia for another occultation event. They reported success in obtaining valuable information about MU69 that will be useful for the spacecraft's flyby.[48][49]

    In August 2018, New Horizons recorded its first images of MU69 for navigation purposes.[50][12][51][52]

    2017 occultations

    In June and July 2017, 2014 MU69 occulted three background stars.[53] The team behind New Horizons formed a specialised "KBO Chasers" team to observe these stellar occultations from South America, Africa, and the Pacific Ocean.[54][55][56] On 3 June 2017, two teams of NASA scientists tried to detect the shadow of 2014 MU69 from Argentina and South Africa.[57] When they found that none of their telescopes had observed the object's shadow, it was initially speculated that 2014 MU69 might be neither as large nor as dark as previously expected, and that it might be highly reflective or even a swarm.[58][59] Additional data taken with the Hubble Space Telescope in June and July 2017 revealed that the telescopes had been placed in the wrong location, and that these estimations were incorrect.[60][61]

    2014 MU69 briefly blocked the light from an unnamed star in Sagittarius during an occultation on 17 July 2017. Data from 24 telescopes that captured this event revealed 2014 MU69's possible double-lobed or binary shape. Later, after the flyby in January 2019, the results from the occultation were shown to precisely fit the observed size and shape of the object.[62]

    On 10 July 2017, the airborne telescope SOFIA was successfully placed close to the predicted centerline for the second occultation while flying over the Pacific Ocean from Christchurch, New Zealand. The main purpose of those observations was the search for hazardous material like rings or dust near 2014 MU69 that could threaten the New Horizons spacecraft during its flyby in 2019. Data collection was successful. A preliminary analysis suggested that the central shadow was missed;[63] only in January 2018 was it realized that SOFIA had indeed observed a very brief dip from the central shadow.[64] The data collected by SOFIA will also be valuable to put constraints on dust near 2014 MU69.[65][66] Detailed results of the search for hazardous material were presented on the 49th Meeting of the AAS Division for Planetary Sciences, on 20 October 2017.[67]

    On 17 July 2017, the Hubble Space Telescope was used to check for debris around 2014 MU69, setting constraints on rings and debris within the Hill sphere of 2014 MU69 at distances of up to 75,000 km from the main body.[68] For the third and final occultation, team members set up another ground-based "fence line" of 24 mobile telescopes along the predicted ground track of the occultation shadow in southern Argentina (Chubut and Santa Cruz provinces) to better constrain the size of 2014 MU69.[55][69] The average spacing between these telescopes was 4.5 km (2.8 mi).[70] Using the latest observations from Hubble, the position of 2014 MU69 was known with much better precision than for the June 3 occultation, and this time the shadow of 2014 MU69 was successfully observed by at least five of the mobile telescopes.[69] Combined with the SOFIA observations, this will put good constraints on possible debris near 2014 MU69.[66][61]

    Results from the occultation on 17 July showed that 2014 MU69 could have had a very oblong, irregular shape or be a close or contact binary.[71][72] According to the duration of the observed chords, 2014 MU69 was shown to have two "lobes", with diameters of approximately 20 km and 18 km, respectively.[40] A preliminary analysis of all collected data suggested that 2014 MU69 was accompanied by an orbiting moonlet about 200–300 km away. [73][74] It was later realized, however, that an error with the data processing software resulted in a shift in the apparent location of the target. After accounting for the bug, the short dip observed on 10 July is now considered to be a detection of the primary body.[64]

    By combining data about its light curve,[75] spectra (e.g. colour), and stellar occultation data,[76] illustrations could rely on known data to create a concept of what it might look like prior to spacecraft flyby.

    Pre-flyby conceptual art for Ultima Thule (artist concepts)
    Contact binary type Kuiper object[77]
    Non-contact binary type, with NH flyby art

    2018 occultations

    Path of 2014 MU69's shadow on Earth during its 4 August 2018 occultation of an unnamed star in Sagittarius. This event was successfully observed from locations in Senegal and Colombia.

    There were two potentially useful 2014 MU69 occultations predicted for 2018: one on 16 July and one on 4 August. Neither of these was as good as the three 2017 events.[53] No attempts were made to observe the 16 July 2018 occultation, which took place over the South Atlantic and the Indian Ocean. For the 4 August 2018 event, two teams, consisting of about 50 researchers in total, went to locations in Senegal and Colombia.[48] The event gathered media attention in Senegal, where it was used as an opportunity for science outreach.[78] Despite some stations being affected by bad weather, the event was successfully observed, as reported by the New Horizons team.[79] Initially, it was unclear whether a chord on the target had been recorded. On 6 September 2018, NASA confirmed that the star had indeed been seen to dip by at least one observer, providing important information about the size and shape of 2014 MU69.[80]

    Hubble observations were carried out on 4 August 2018, to support the occultation campaign.[81][48] Hubble could not be placed in the narrow path of the occultation, but due to the favourable location of Hubble at the time of the event, the space telescope was able to probe the region down to 1,600 km (990 mi) from 2014 MU69. This is much closer than the 20,000 km (12,000 mi) region that could be observed during the 17 July 2017 occultation. No brightness changes of the target star have been seen by Hubble, ruling out any optically thick rings or debris down to 1,600 km (990 mi) from 2014 MU69.[80] Results of the 2017 and 2018 occultation campaigns were presented at the 50th meeting of the AAS Division for Planetary Sciences, on 26 October 2018.[82]


    Heliocentric positions of the five interstellar probes (squares) and other bodies (circles) until 2020, with launch and flyby dates. Markers denote positions on 1 January of each year, with every fifth year labelled.
    Plot 1 is viewed from the north ecliptic pole, to scale; plots 2 to 4 are third-angle projections at 20% scale.
    In the SVG file, hover over a trajectory or orbit to highlight it and its associated launches and flybys.
    2014 MU69 among the stars of Sagittarius – with and without background star omission (apparent magnitude 20 to 15; New Horizons; late 2018).[83]
    View of 2014 MU69 by New Horizons after closest approach. The silhouette of 2014 MU69's shape can be seen among the background stars.

    Having completed its flyby of Pluto in July 2015, the New Horizons spacecraft made four course changes in October and November 2015 to place itself on a trajectory towards 2014 MU69.[84][85] It is the first object to be targeted for a flyby that was discovered after the visiting spacecraft was launched,[12] and is the farthest object in the Solar System ever to be visited by a spacecraft.[19][86][87] New Horizons came within 3,500 km (2,200 mi) of 2014 MU69, three times closer than the spacecraft's earlier encounter with Pluto. Closest approach occurred on January 1, 2019, at 05:33 UTC (Spacecraft Event Time – SCET)[74][88] at which point it was 43.4 AU from the Sun in the direction of the constellation Sagittarius.[89][90][91][92] At this distance, the one-way transit time for radio signals between Earth and New Horizons was 6 hours.[74] The science objectives of the flyby include characterizing the geology and morphology of 2014 MU69, mapping the surface composition (searching for ammonia, carbon monoxide, methane, and water ice). Surveys of the surrounding environment to detect possible orbiting moonlets, a coma, or rings, were conducted.[74] Images with resolutions as fine as 30 m (98 ft) to 70 m (230 ft) are expected.[74][93] Nonetheless, a search for a related moon (or moons) continues, which may help better explain the formation of 2014 MU69 from two free-flying objects, known as "Ultima" and "Thule".[45]

    New Horizons made its first detection of 2014 MU69 on 16 August 2018, from a distance of 107 million mi (172 million km).[94] At that time, 2014 MU69 was visible at magnitude 20, in the direction of the constellation Sagittarius.[95] 2014 MU69 was expected to be magnitude 18 by mid-November, and magnitude 15 by mid-December. It reached naked eye brightness (magnitude 6) from the spacecraft's point of view just 3–4 hours before closest approach.[83] If obstacles were detected, the spacecraft had the option of diverting to a more distant rendezvous, though no moons, rings or other hazards were seen.[74][96] High-resolution images from New Horizons were taken on January 1. The first images with medium resolution arrived on the next day.[97] The downlink of data collected from the flyby is expected to last 20 months, through to September 2020.[88]

    The download of data was paused from 4 to 10 January 2019 as the spacecraft entered solar conjunction.[98][99]  

    Observations of 2014 MU69 during closest approach
    As shown in this computer simulation provided by NASA's Eyes on the Solar System application, between the morning of 31 December 2018 and 1 January 2019, the New Horizons spacecraft has made 33 different observations of 2014 MU69 with the LORRI imaging camera. In the week after the flyby, only three of these have already been sent back to Earth.[100]
    Animated simulation of 2014 MU69, modelled as two ellipsoids, compared to 12 photos taken during the flyby

    See also


    1. Greyscale composite of nine 0.025 second exposures taken by the Long Range Reconnaissance Imager (LORRI) aboard New Horizons on 1 January 2019, from a distance of 6,628 kilometres (4,118 mi) at a resolution of 33 metres (108 ft) per pixel.[2] The contact binary object is made up of two lobes named "Ultima" (right) and "Thule" (left). Its axis of rotation is located near the bright "neck" of the object and spins clockwise from this viewpoint.[3]
    2. 1 2 Normally pronounced /ˈθjl/ THEW-lee [US /ˈθl/ THOO-lee].[13] The New Horizons team use this classical pronunciation, the pseudo-Latin pronunciation /ˈtl/ TOO-lay, and the hybrid pronunciation /ˈtl/ TOO-lee.[14][15]
    3. Composite of black and while and color photographs taken respectively by the LORRI and MVIC instruments aboard New Horizons on 1 January 2019.[28]


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