 Shape of Justitia as seen bystellar occultation observations on 31 August 2023 | |
| Discovery[1] | |
|---|---|
| Discovered by | Johann Palisa |
| Discovery site | Vienna Obs. |
| Discovery date | 21 September 1887 |
| Designations | |
| (269) Justitia | |
| Pronunciation | /dʒʌˈstɪʃiə/[2] |
Named after | Justitia |
| A887 SA 1942 XY | |
| Main belt (middle)[3] · background[4] | |
| Orbital characteristics[5] | |
| Epoch 5 May 2025 (JD 2460800.5) | |
| Uncertainty parameter 0 | |
| Aphelion | 3.179AU |
| Perihelion | 2.048 AU |
| 2.613 AU | |
| Eccentricity | 0.2164 |
| 4.22yr (1,543 d) | |
| 244.365° | |
| 0° 13m 59.88s / day | |
| Inclination | 5.477° |
| 156.503° | |
| 120.144° | |
| Physical characteristics | |
| Dimensions | 64.60 × 58.34 × 46.60 km ± (2.86 × 2.64 × 2.60 km)[6]: 6 [a] |
| 57.0–57.8 km (occultation)[7] 58±2 km (thermophysical)[8] | |
| 33.12962 ± 0.00001 h (1.38040083 ± 4.2×10−7 d)[8] | |
| 171°±15° (toecliptic)[b] | |
| −81°±15°[8][c] | |
| 73°±11°[8][d] | |
| 0.072±0.007 (occultation)[7] 0.058±0.006 (thermophysical)[8] | |
| RR or IR (TNO classification)[3] Z-type asteroid[9] Ld-type (SMASSII classification)[5][7] D-type (Bus–DeMeo classification)[7] | |
| 12–15[1] | |
| 9.82[7] | |
269 Justitia is anasteroid located in themiddlemain asteroid belt. It was discovered on 21 September 1887 by Austrian astronomerJohann Palisa atVienna Observatory and was named afterJustitia, theRoman goddess of justice. The asteroid is about 58 kilometres (36 mi) in diameter and rotates relatively slowly, with arotation period of 33.1 hours. Justitia is one of the targets of theUnited Arab Emirates' upcomingMBR Explorer mission, which will visit seven different asteroids in the asteroid belt during the 2030s. MBR Explorer is planned to enter orbit around Justitia viarendezvous in 2034 and will end its mission after dropping alander to the asteroid's surface in 2035.
Justitia is unusual in that it has a much redder color compared to any other asteroid in the asteroid belt.Spectroscopic observations show that Justitia's color and composition appears to resemble those ofcentaurs andtrans-Neptunian objects from the outer Solar System, whose surfaces are composed of ices and complexorganic compounds (tholins). Hence, researchers believe that Justitia originated from the outer Solar System and thenmigrated inward to its present-day location in the asteroid belt. Only a few other asteroids have been identified to exhibit very red colors like Justitia, with203 Pompeja and732 Tjilaki as examples from the main asteroid belt.
Justitia was discovered on 21 September 1887 by Austrian astronomerJohann Palisa, who at the time was mapping the sky and searching for asteroids by eye using theVienna Observatory's 12-inchrefractor telescope.[10][11] Justitia was the 60th asteroid discovered by Palisa, who discovered a total of 122 asteroids over his lifetime.[10] Palisa reported his discovery to theCentral Bureau for Astronomical Telegrams[12] and the asteroid was announced as the 269thminor planet discovery in a notice published in theAstronomische Nachrichten journal on 22 September 1887.[13]
The nameJustitia was given by August Biela, a private observatory owner and friend of Palisa.[14] Justitia is theRoman goddess of justice and the daughter of Jupiter andAstraea (who has an asteroid named5 Astraea).[14] TheGreek equivalent of Justitia isThemis, who also has an asteroid named24 Themis.[14]
Justitia is located in themiddle section of themain asteroid belt (2.50–2.82 AU), where it orbits the Sun at an average distance of 2.61 AU (390 million km; 243 million mi).[3]: 2 During its 4.2-yearorbital period, Justitia's distance from the Sun varies from 2.05 AU atperihelion to 3.18 AU ataphelion due to its moderatelyelliptical orbit, which has aneccentricity of 0.216.[5] Justitia has a loworbital inclination of 5.5° with respect to theecliptic plane.[8]: 3 Justitia is classified as abackground asteroid because it does not belong to any knownasteroid family.[4]
Justitia's size and shape has been directly measured via observations of astellar occultation on 31 August 2023, when the asteroid passed in front of a background star and briefly blocked out its light. Multiple observation locations were closely spaced to each other and spread over a large area to cover the entirety of the predicted width of the Justitia's shadow cast on Earth, allowing for the resolution of the asteroid's shape in high detail.[7]: 2 The occultation revealed that Justitia has a volume-equivalent diameter of 57.0 or 57.8 km (35.4 or 35.9 mi), with the larger value being preferred byMarc Buie and collaborators who analyzed the occultation observations.[7]: 10 Justitia was revealed to be highly irregular in shape, with at least three large facets 23–38 km (14–24 mi) in length being observed.[7]: 11
Observations of Justitia'slight curve, or brightness fluctuations over time, show that it has a relatively slowrotation period of about 33.1 hours, accurately measured to an uncertainty less than a tenth of a second.[8]: 2, 4, 7 Justitia has aretrograde rotation, meaning it rotates backwards relative to its orbit direction and has its rotational north pole pointed toward theecliptic south.[8]: 3 Although observations based on light curves alone suggested two possible rotation pole orientations for Justitia, the August 2023 occultation by Justitia eliminated this ambiguity.[8]: 5
The surface of Justitia is very dark, with ageometric albedo between 0.06 and 0.07.[8][7]Thermophysical modelling of the asteroid indicates it has a very rough surface with smallregolith grain sizes.[8]: 4 Spectroscopic observations of Justitia invisible andnear-infraredwavelengths of light indicate it has a very red color with a featureless spectrum lackingabsorption bands between0.5–2.5 μm wavelengths, which is unusual for a main-belt asteroid.[3][7]: 1 Very few asteroids are known to exhibit this very red color, with203 Pompeja and732 Tjilaki as examples from the main asteroid belt.[15]: 10
Early attempts at categorizing Justitia's visible color and spectrum were unable to find a suitableasteroid taxonomic class; theBus–DeMeo classification scheme classifies Justitia as aD-type asteroid whereas theSmall Main-belt Asteroid Spectroscopic Survey, Phase II (SMASSII) classification scheme classifies it as anLd-type asteroid.[7]: 1 In 2022, a team of astronomers led by Max Mahlke proposed a new Z-type classification within which Justitia alongside a few other very red asteroids would belong to.[9]: 16 Mahlke et al.'s Z-type asteroids are similar to D-type asteroids, but are distinguished by their strong spectrally red colors and distinct orbital characteristics.[9]: 16 Despite the new classification, Justitia and Pompeja still stand out as the reddest members of Mahlke et al.'s Z-type asteroids[9]: 16 [15]: 9 and are the reddest main-belt asteroids known.[16][6]: 1
The very red colors and spectra of Justitia and Pompeja most closely resemble those ofcentaurs andtrans-Neptunian objects (TNOs) from the outer Solar System, whose surfaces are composed ofvolatile ices (i.e.methanol andmethane) and complexorganic compounds (tholins).[3]: 1, 6 In particular, the spectra of the two asteroids resemble the IR (moderately red) and RR (very red)taxonomic classes for TNO spectra.[3] A team of astronomers led by Sunao Hasegawa argued in 2021 that Justitia and Pompeja may have originated from the primordialKuiper belt20–30 AU away from the Sun before latermigrating to the main asteroid belt—this would support theNice model scenario.[3]: 6 [17] Hasegawa et al. interpreted the featureless spectra of Justitia and Pompeja as a lack of volatile ices in their surface layers, possibly caused byspace weathering having broken down these surface volatile ices into tholins.[3]: 2–3 Polarimetric observations of Justitia show that it exhibits lightpolarization properties similar to those of icy Solar System bodies andF-type asteroids.[3]: 2 Observations by theNASA Infrared Telescope Facility in 2022 showed that Justitia's infrared spectrum at longer wavelengths resembles those ofCM chondrites, with an absorption feature at3.4 μm potentially attributed toaliphatic organic compounds.[16]
The small eccentricity of Justitia's present-day orbit suggests that the asteroid did not experience inwardgravitational scattering byNeptune during the planet's inward migration.[3]: 5 Given Justitia's relatively small size, it should have experienced at least one catastrophiccollision with another asteroid in the past. However, such collisions would have destroyed Justitia's red coloration, which suggests that either Justitia did not experience any destructive collisions or there may be an as-yet unknown mechanism that can retain the asteroid's red color even after a destructive collision.[3]: 6
The mass and density of Justitia has not been measured and thus remains unknown. A realistic range of possible densities is1–2 g/cm3,[6]: 6–7 which would correspond to a mass range of7.9×1016 kg to2.1×1017 kg.[e] Justitia's mass can be measured more accurately if an object such as anatural satellite or spacecraft is observed orbiting the asteroid. Occultation observations from August 2023 did not find any satellites or rings around Justitia.[7]: 4–5
Justitia is planned to be the seventh and final target of theUnited Arab Emirates Space Agency'sMBR Explorer mission, which will enter orbit around the asteroid viarendezvous in October 2034 and then deliver alander to its surface sometime in May 2035.[18]: 2–3 [6]: 5 [19] The asteroid was selected for exploration by the MBR Explorer due to its low orbital inclination making it an accessible rendezvous target,[18]: 6 as well as its very red spectrum which makes it a scientifically attractive target.[18]: 1 The MBR Explorer's rendezvous with Justitia will provide a measurement of the asteroid's mass, which will facilitate the spacecraft's next stage of entering orbit around the asteroid.[6]: 4 MBR Explorer is planned to enter a near-polar orbit around Justitia at an initial distance of 250 km (160 mi) for a duration of three weeks, before shrinking its orbital distance to 90–100 km (56–62 mi).[6]: 3, 9, 15–16 [20] During orbit, MBR Explorer will map Justitia's surface via imaging and will measure the asteroid'sgravity field.[6]: 3 [20]
