486958 Arrokoth

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(486958) 2014 MU69
Color composite image of 2014 MU69[1][a]
Discovery [4][5][6]
Discovered byMarc Buie
Discovery siteHubble Space Telescope
Discovery date26 June 2014
Designations
(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
44.53998 AU
Eccentricity0.0417249±0.0000346
298 yr
316.55086°
0° 0m 11.92s / day
Inclination2.45116°±0.000012°
158.99773°±0.00045°
174.418°±0.037°
Physical characteristics
Dimensions35 km × 20 km × 10 km
± 1 km × 1 km × 3 km[10]
"Ultima" 22 km × 20 km × 7 km
± 0.6 km × 1 km × 2 km[10]
"Thule" 14 km × 14 km × 10 km
± 0.4 km × 0.7 km × 3 km[10]
Mean diameter
31.7±0.5 km long axis
"Ultima" 19.46±0.04 km[11]
"Thule" 14.24±0.12 km[11]
Equatorial surface gravity
~ 0.000101971621 g
~ 0.001 m/s2[12]
15.92±0.02 h[10]
98°[10]
North pole right ascension
311°[10]
North pole declination
−25°[10]
0.165±0.01 (geometric)
0.061±0.026 (Bond)
G−I=1.42±0.14[13]
G−R=0.95±0.14[13]
26.8[14]
11.1[5]

(486958) 2014 MU69, nicknamed Ultima Thule,[b] is a trans-Neptunian object located in the Kuiper belt. It is a contact binary 35 km (22 mi) long, composed of two planetesimals 22 km (14 mi) and 14 km (9 mi) across, nicknamed "Ultima" and "Thule", respectively, that are joined along their major axes. Ultima, which is flatter than Thule, appears to be an aggregate of 8 or so smaller units, each approximately 5 km (3 mi) across, that fused together before Ultima and Thule came into contact. Because there have been few to no disruptive impacts on 2014 MU69 since it formed, the details of its formation have been preserved. 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.[18][19][20]

2014 MU69 was discovered on 26 June 2014 by astronomer Marc Buie 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.[6] With an orbital period of 298 years and a low orbital inclination and eccentricity, 2014 MU69 is classified as a cold classical Kuiper belt object. Its nickname, a Greco-Latin term for a place beyond the known world, was chosen as part of a public competition in 2018.[21] The New Horizons team plans to submit a proper name to the International Astronomical Union once the nature of the object is better understood.

Nomenclature

When 2014 MU69 was first observed, it was labelled 1110113Y,[22] nicknamed "11" for short.[23][24] Its existence as a potential target of the New Horizons probe was announced by NASA in October 2014[25][26] and it was unofficially designated as "Potential Target 1", or PT1.[24] Its official designation, 2014 MU69 (a provisional designation indicating that it was the 1745th object assigned one for the second half of June 2014),[27] was assigned by the Minor Planet Center (MPC) in March 2015, after sufficient orbital information was gathered.[24] After further observations refining its orbit, it was given the permanent minor planet number 486958 on 12 March 2017.[28]

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.[29][21] In the interim, NASA invited suggestions from the public on a nickname to be used.[21] 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",[b] 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".[30]

The nickname was criticized due to its use by Nazi occultists as the supposed mythical origin of the Aryan race, although it is commonly used 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."[31][32]

Shape

(left) Shape model of 2014 MU69 viewed from multiple perspectives. The side view demonstrates the flattened shape of 2014 MU69.[12]
(right) Stereoscopic animation of two LORRI images. (3D version)

2014 MU69 is a contact binary consisting of two lobes attached by a bright, narrow neck.[30] Prior to the New Horizons flyby of 2014 MU69, stellar occultations of 2014 MU69 had provided evidence for its double-lobed shape.[33] The two lobes were likely once two objects that had merged in a slow collision.[34] The larger lobe, nicknamed Ultima, is measured at about 22 km (14 mi) across its longest axis[10] while the smaller lobe, Thule, is measured at 14.2 km (8.8 mi) across its longest axis.[35] The larger lobe is highly flattened and moderately elongated, being described as having a lenticular shape.[10] From shape models of 2014 MU69, the derived dimensions of the larger lobe are approximately 22 km × 20 km × 7 km (13.7 mi × 12.4 mi × 4.3 mi).[10] In contrast, the smaller lobe is less flattened, with derived dimensions of 14 km × 14 km × 10 km (8.7 mi × 8.7 mi × 6.2 mi).[10] Overall, the entire object is 35 km (22 mi) across its longest axis.[10] The centers of the lobes are separated 16 km (9.9 mi) from each other.[10]

The first detailed image of 2014 MU69 confirmed its double-lobed appearance and was described as a "snowman" by New Horizons principal investigator Alan Stern, as the two lobes appeared nearly spherical.[36] On 8 February, one month after the New Horizons flyby, Stern announced that 2014 MU69 was more flattened than initially thought, based on additional images of 2014 MU69 taken by New Horizons after its closest approach. The flattened larger lobe of 2014 MU69 was described as a "pancake", while the smaller lobe was described as a "walnut" as it appeared less flattened compared to the larger lobe.[37] By observing how the unseen sections of 2014 MU69 occulted background stars, scientists were able to then outline the shapes of both lobes.[37] The cause of 2014 MU69's unexpectedly flattened shape is uncertain, though one explanation suggests that the two separate lobes were once rotating rapidly, causing them to become flattened due to centrifugal forces.[38][39]

The longest axes of the two lobes are nearly aligned toward their rotational axis, which is situated between the two lobes.[10] This alignment of the two lobes suggests that they were mutually locked to each other, likely due to tidal forces, before merging.[10][40] This alignment of the two lobes supports the idea that the two had individually formed from the coalescence of a cloud of icy particles.[41]

Geology

Surface and spectra

MVIC color and spectral images of 2014 MU69, showing subtle color variations across its surface. The image on the right is the same MVIC color image superimposed on the higher resolution black and white LORRI image.[c]

The surface color of 2014 MU69 is red.[42] Its color and spectrum was first measured by the Hubble Space Telescope, which revealed its red spectral slope, in which 2014 MU69 reflects more light in the red spectrum.[43] The color of 2014 MU69 closely matches the reddish colors of other known Kuiper belt objects, and it is more red than the dwarf planet Pluto.[40] Due to its strong red color, 2014 MU69 is considered an 'ultra red' object by the New Horizons team.[42] The observed spectrum of 2014 MU69 displays a strong red spectral slope extending from the red visible spectrum to the infrared at wavelengths from 1.2 to 2.5 μm.[10] Spectral measurements from the New Horizons LEISA instrument has revealed the presence of methanol, water ice, and organic molecules on the surface of 2014 MU69.[42][40] An unidentified absorption band was also found in 2014 MU69's spectrum at a wavelength of 1.8 μm.[10] The spectrum of 2014 MU69 shares similarities with that of 2002 VE95 and the centaur 5145 Pholus, as their spectra display strong red spectral slopes around similar wavelength ranges along with signs of methanol present on their surfaces.[10]

The red color of 2014 MU69 is caused by the presence of a mix of organic compounds (known as tholins) on the surface of 2014 MU69.[10] The tholins are thought to have been produced from ultraviolet solar radiation processing methane and other compounds.[44] The presence of tholins on the surface of 2014 MU69 suggests that other volatile materials such as methane and ammonia are also present on 2014 MU69 as well, though it is expected that they would be lost on short timescales due to the small size of 2014 MU69.[10][44] Less volatile materials such as methanol, acetylene, ethane, and hydrogen cyanide could be retained over a longer period of time, and could also account for the reddening and production of tholins on 2014 MU69 when irradiated by the Sun.[10] Hydrogen gas is also produced as a result from the photoionization of these materials, and it is slowly lost into space over time.[10] The escape of hydrogen gas forms a tenuous coma which is ionized though solar wind interactions. No signature of solar wind interaction with 2014 MU69 was detected by the New Horizons SWAP and PEPSSI instruments.[10]

From color and spectral images of 2014 MU69, the surface displays subtle color variation among its surface features.[42][45] Spectral images of 2014 MU69 show that the neck region and lineation features appear less red while the central region of the smaller lobe appears more red.[12] The larger lobe also displays redder regions, which were nicknamed "thumbprints" by the New Horizons team.[12] The thumbprints are located near the larger lobe's limb.[12] The surface albedo or reflectivity of 2014 MU69 varies from 5 percent to 12 percent due to various bright features on its surface.[10] Its geometric albedo, the quantity of reflected light in visible spectrum, is measured at 16.5 percent, which is a typical value for most Kuiper belt objects.[10] The overall Bond albedo (the quantity of reflected light of any wavelength) of 2014 MU69 is measured at 6.1 percent.[10]

Craters

The surface of 2014 MU69 appears lightly cratered.[46][47] The occurrence of impact events on 2014 MU69 is thought to be uncommon, with a very low impact rate over the course of one billion years.[47] Due to the slower orbital speeds of Kuiper belt objects, the speed of objects impacting 2014 MU69 is expected to be low, with impact speeds being at least 2 km/s (1.2 mi/s).[47] At such slow impact speeds, large craters on 2014 MU69 are expected to be rare. With a low frequency of impact events along with the slow speeds of impacts, the surface of 2014 MU69 would remain preserved since its formation. The preserved surface of 2014 MU69 could possibly give hints to its formation process, as well as signs of accreted material.[47][30]

Numerous small pits on the surface of 2014 MU69 were identified in high resolution images from the New Horizons spacecraft.[48][2] The size of these pits are measured at about 700 m (2,300 ft) across.[48] The exact cause of these pits is unknown; several explanations for these pits include impact events, the collapse of material, the sublimation of volatile materials, or the venting and escape of volatile gases from the interior of 2014 MU69.[48][2][12]

Surface features

The geology of 2014 MU69, with comet 67P to scale. Notable surface features are highlighted. The eight subunits labeled ma to mh are rolling topography units that might be building blocks of the larger lobe.[10][12]

The surfaces of each lobe of 2014 MU69 display various regions of varying brightness.[49][10] Various geological features have been identified, including troughs and hills.[12] These geological features may have originated from the clumping of smaller subunits during the formation of 2014 MU69.[12] The surface gravity on the hilltops of 2014 MU69 is weaker compared to the surface gravity closer between the two lobes, thus material is likely to roll down the hills toward lower elevations, where surface gravity is stronger.[12] This could possibly account for the bright areas on its surface, especially bright lineation features where bright material may be deposited.[42][12][38]

The smaller lobe of 2014 MU69, Thule, bears a large depression feature dubbed "the Maryland Crater" by the New Horizons team.[42][12] The large depression feature is measured at 7 km (4.3 mi) to 8 km (5 mi) across.[10][42] The depth of the depression is estimated at around 2 km (1.2 mi), based on stereographic analysis.[10] It is likely an impact crater that was formed by an object 700 m (2,300 ft) in size.[46] Two bright streaks of similar size are notably present in the depression feature, and may be associated with avalanches where bright material rolls down into the depression.[10][12] Four troughs have been identified along the terminator of the small lobe, along with two possible kilometer-sized impact craters on the rim of the large depression feature.[10] The surface of the small lobe exhibits dark, broad regions (labeled dm in the right image) separating brighter mottled regions. These dark, broad regions are likely volatile ice deposits covered with a layer of darker material.[10] Another bright region (labeled rm in the right image), located at the equatorial end of the small lobe, exhibits rough terrain along with several identified features that may be pits, craters, or mounds.[10] Unlike the larger lobe, the small lobe does not appear to display subunits of rolling topography. This absence of subunits is thought to be the result of resurfacing caused by the same impact event that created the large depression feature of the small lobe.[10]

The larger lobe of 2014 MU69, Ultima, is thought to consist of eight smaller subunits of rolling topography, each similarly sized at around 5 km (3.1 mi).[10] Each distinctive subunit appears to be separated by boundary regions with high reflectivities.[10] The similar sizes of the subunits of the large lobe suggests that each subunit were individual small planetesimals that accreted to form the large lobe of 2014 MU69.[10] These planetesimal units are expected to have accreted very slowly (at speeds of several meters per second), though they must be very weak in strength in order to merge and form compact bodies at these speeds.[10] The central subunit of the large lobe bears a bright ring-shaped feature informally called "The Road to Nowhere".[2][12] From stereographic analysis, the central feature appears to be relatively flat compared to other topography units of the large lobe.[10] Stereographic analysis also shows that one particular subunit located at the limb of the large lobe (labeled md in the right image) appears to have a higher elevation and tilt compared to other subunits.[10] Similarly to the smaller lobe, troughs and pit crater chains are present along the terminator of the large lobe.[10]

The 'neck' region connecting both lobes of 2014 MU69 is considerably brighter and less red compared to the surfaces of each lobe.[50] 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.[35] 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.[50] Another proposal suggests the bright material is produced by the deposition of ammonia ice.[51] 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.[51]

Internal structure

Topography variations at the limb of 2014 MU69 suggest that its interior is likely composed of mechanically strong material consisting of mostly amorphous water ice and rocky material.[35][52] Trace amounts of methane and other volatile gases in the form of vapors may be also present in the interior of 2014 MU69, trapped in water ice.[52] Under the assumption that 2014 MU69 has a low comet-like density of around 0.5 g/cm3, its internal structure is expected be porous, as volatile gases trapped in the interior of 2014 MU69 are thought to escape from the interior to the surface.[10][52] Assuming that 2014 MU69 may have an internal heat source caused by the radioactive decay of radionuclides, the trapped volatile gases inside 2014 MU69 would migrate outward and escape from the surface, similarly to the scenario of outgassing of comets.[52] The escaped gases may subsequently freeze and deposit on the surface of 2014 MU69, and could possibly account for the presence of ices and tholins on its surface.[52][44]

Orbit and rotation

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.
Near-polar view of 2014 MU69's rotation over a period of seven hours.[3]

2014 MU69 orbits the Sun at a distance of 44.6 astronomical units (AU) and completes a full orbit around the Sun in 298 years.[5] Its orbit has a low orbital eccentricity, as its perihelion (closest distance to the Sun) and aphelion (farthest distance from the Sun) differs by only about 2 AU.[5] It has a low orbital inclination and eccentricity compared to other objects in the Kuiper belt.[53] 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.[14][5]

Results from photometric Hubble Space Telescope observations show that the brightness of 2014 MU69 varies by around 0.3 magnitudes as it rotates.[54][55] Though the rotation period and light curve amplitude of 2014 MU69 could not be determined from Hubble observations, the subtle brightness variations suggests that 2014 MU69's rotational axis is either pointed toward the Earth or is being viewed at a equator-on configuration, which in turn implies that 2014 MU69 does not have a very elongated shape, with a constrained a/b ellipsoidal aspect ratio of 1–1.15.[55][54]

Upon the New Horizons spacecraft's approach to 2014 MU69, no rotational light curve amplitude was detected by the spacecraft despite the irregular shape of 2014 MU69.[56] To explain the lack of its rotational light curve, scientists surmised that 2014 MU69 is rotating on its side, with its rotational axis pointing nearly directly at the approaching New Horizons spacecraft.[56] Subsequent images of 2014 MU69 from New Horizons upon approach confirmed that its rotation is tilted, with its south pole facing towards the Sun.[18][20] Derived from images from New Horizons, the rotational axis of 2014 MU69 is estimated to be tilted an at angle of 98 degrees to its orbit.[10] From the first images of 2014 MU69 by New Horizons, the rotation period of 2014 MU69 was estimated at roughly 15 to 30 hours.[18] The rotation period of 2014 MU69 was later refined to a value of 15.92 hours, consistent with the more accurately constrained estimate of 15 to 16 hours.[10][35]

Due to its high axial tilt of its rotation, the solar irradiance of the northern and southern hemispheres of 2014 MU69 varies greatly over the course of its orbit around the Sun.[10] As it orbits around the Sun, one polar region of 2014 MU69 faces the Sun continuously while the other faces away. The solar irradiance of 2014 MU69 varies by 17 percent due to the low eccentricity of its orbit.[10] The maximum equilibrium temperature of the illuminated face of 2014 MU69 is expected to reach 60 K (−213.2 °C). The temperature of the unilluminated face was measured through radiometry by the REX instrument aboard the New Horizons spacecraft. Radiometric data at radio wavelengths of 4.2 cm from the REX instrument implies that the temperature of the unilluminated face of 2014 MU69 ranges from 20 K (−253.2 °C) to 35 K (−238.2 °C).[10]

Mass and density

The mass and density of 2014 MU69 is unknown. A definite mass and density estimate cannot be given as the two lobes of 2014 MU69 are in contact rather than orbiting each other.[40] Although a possible natural satellite orbiting 2014 MU69 could help determine its mass,[50] no satellites were found orbiting 2014 MU69.[40] Under the assumption that both lobes of 2014 MU69 are bound by self-gravity, the object is measured to have a very low density similar to that of comets, with the minimum estimate of 0.28 g/cm3.[10] With this minimum density estimate, the mutual gravity of the two lobes would overcome centrifugal forces that would otherwise separate the lobes, indicating that the neck region between the two lobes is compressed by their mutual gravity.[10] Under an alternative comet-like density of around 0.5 g/cm3, the lobes would separate due to centrifugal forces if the entire object had a rotation period of at least 12 hours.[10]

Formation

Illustration depicting the formation sequence of 2014 MU69.[38]

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.[34][42] Icy particles experienced streaming instability, in which they slowed down due to drag against the surrounding gas and dust, and gravitationally coalesced into clumps of larger particles.[40] Based on the differing appearances of the two lobes, each were likely individual objects that had accreted separately and remained in a mutual orbit around each other after their formation.[42][45] Both objects are believed to have formed from a single source of material as they appear to be homogeneous in albedo, color, and composition.[10] The presence of rolling topography units on the larger object, Ultima, indicates that it had likely formed from the coalescence of smaller planetesimal units prior to merging with the smaller object.[45][10]

Although it is unclear how the two constituents of 2014 MU69 became flattened during its formation, one explanation by the New Horizons team suggests that the two objects were rotating rapidly, causing their shapes to become flattened due to centrifugal forces.[38] Over time, the rotation rates of the two objects gradually slowed down as they transferred their angular momentum to other orbiting debris left over from their formation.[38] 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.[34] The present appearance of 2014 MU69 does not display any indication of deformation or compression fractures, suggesting that the two objects had merged very slowly at a speed of 2 m/s (6.6 ft/s)—comparable to the average walking speed of a person.[10][45] There is a possible indication of shearing of the surface and terrain caused by the merging of the two objects.[42]

The frequency of impact events occurring on 2014 MU69 over a period of at least four billion years were uncommon due to the slower speeds of objects in the Kuiper Belt.[47] At such timescales since its formation, the effect of photon-induced sputtering of water ice on the surface of 2014 MU69 is estimated to very minimal; over a period of 4.5 billion years, the amount of water ice lost by sputtering would reduce the size of 2014 MU69 by 1 cm (0.39 in).[10] 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.[47][19]

Observation

Discovery

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. 2014 MU69 was first spotted by Hubble on 26 June 2014, discovered by astronomer Marc Buie, a member of the New Horizons team.[6]

2014 MU69 is too small and distant for its shape to be observed directly from Earth, but scientists were 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 2014 MU69 would cast a shadow.[57] They determined that occultations would occur on 3 June, 10 July, and 17 July in 2017, and set off for places around the world where they could see 2014 MU69 cover up a different star on each of these dates.[57] Based on this string of three occultations, scientists were able to trace out the object's shape.[57]

In the summer of 2018, nearly fifty New Horizons team members headed to Senegal and Colombia for another occultation event. They obtained information about 2014 MU69 for the spacecraft's flyby.[58][59]

In August 2018, New Horizons recorded its first images of 2014 MU69 for navigation purposes.[60][14][61]

2017 occultations

Results of the 2017 occultation campaign
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.[30]

In June and July 2017, 2014 MU69 occulted three background stars.[57] The team behind New Horizons formed a specialised "KBO Chasers" team to observe these stellar occultations from South America, Africa, and the Pacific Ocean.[62][63] On 3 June 2017, two teams of NASA scientists tried to detect the shadow of 2014 MU69 from Argentina and South Africa.[64] 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.[64][65] 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.[65]

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;[66] only in January 2018 was it realized that SOFIA had indeed observed a very brief dip from the central shadow.[67] The data collected by SOFIA will also be valuable to put constraints on dust near 2014 MU69.[68][69] 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.[70]

Pre-flyby conceptual art, based on occultation data
Artist concept of 2014 MU69 as a contact binary, illustrating the understanding as of August 2017
Artist concept of an ellipsoid shape for 2014 MU69, a shape that could not be ruled out prior to the flyby in 2019

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 (47,000 mi) from the main body.[71] 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.[63][72] The average spacing between these telescopes was around 4 km (2.5 mi).[73] 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.[72] Combined with the SOFIA observations, this will put constraints on possible debris near 2014 MU69.[69]

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.[73][33] According to the duration of the observed chords, 2014 MU69 was shown to have two "lobes", with diameters of approximately 20 km (12 mi) and 18 km (11 mi), respectively.[55] A preliminary analysis of all collected data suggested that 2014 MU69 was accompanied by an orbiting moonlet about 200–300 km (120–190 mi) away from the primary.[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.[67]

By combining data about its light curve,[54] spectra (e.g. color), and stellar occultation data,[73] illustrations could rely on known data to create a concept of what it might look like prior to spacecraft flyby.

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 were as good as the three 2017 events.[57] 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.[58] The event gathered media attention in Senegal, where it was used as an opportunity for science outreach.[75] Despite some stations being affected by bad weather, the event was successfully observed, as reported by the New Horizons team.[76] 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.[77]

Hubble observations were carried out on 4 August 2018, to support the occultation campaign.[78][58] 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.[77] 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.[79]

Exploration

See also: New Horizons § Encounter with (486958) 2014 MU69
2014 MU69 among the stars of Sagittarius—with and without background star omission (apparent magnitude 20 to 15; New Horizons; late 2018).[80]
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 made 33 different observations of 2014 MU69 with the LORRI imaging camera. During the first week after the flyby, only three of these were sent back to Earth.[81]
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.[82] It is the first object to be targeted for a flyby that was discovered after the visiting spacecraft was launched,[14][83] and is the farthest object in the Solar System ever to be visited by a spacecraft.[24][84][85] 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][86] at which point it was 43.4 AU from the Sun in the direction of the constellation Sagittarius.[87][88][89][90] 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 showing details of 30 m (98 ft) to 70 m (230 ft) are expected.[74][91] From Hubble observations, faint, small satellites orbiting 2014 MU69 at distances greater than 2,000 km (1,200 mi) have been excluded to a depth of >29th magnitude.[54] The object has no detectable atmosphere, and no large rings or satellites larger than 1.6 km (1 mi) in diameter.[92] 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, informally known as "Ultima" and "Thule".[34]

New Horizons made its first detection of 2014 MU69 on 16 August 2018, from a distance of 172 million km (107 million mi).[93] At that time, 2014 MU69 was visible at magnitude 20, in the direction of the constellation Sagittarius.[94] 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.[80] 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][94] High-resolution images from New Horizons were taken on 1 January. The first images with medium resolution arrived on the next day.[95] The downlink of data collected from the flyby is expected to last 20 months, through to September 2020.[86]

See also

Notes

  1. ^ Color MVIC image superimposed over a higher-resolution 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 nicknamed "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. ^ a b Normally pronounced /ˈθjuːliː/ THEW-lee [US /ˈθuːliː/ THOO-lee].[15] The New Horizons team use this classical pronunciation, the pseudo-Latin pronunciation /ˈtuːleɪ/ TOO-lay, and the hybrid pronunciation /ˈtuːliː/ TOO-lee.[16][17]
  3. ^ Composite of black and while and color photographs taken respectively by the LORRI and MVIC instruments aboard New Horizons on 1 January 2019.

References

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External links

Media related to 2014 MU69 at Wikimedia Commons

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