(136108) 2003 EL61
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- The correct title of this article is (136108) 2003 EL61. It features superscript or subscript characters that are substituted or omitted because of technical limitations.
Artist's conception of (136108) 2003 EL61
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Discovery
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| Discovered by | Ortiz et al. / Brown et al. |
| Discovery date | December 28, 2004 |
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Designations
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| MPC designation | (136108) 2003 EL61 |
| Alternative names | none |
| Minor planet category |
TNO (cubewano) |
| Epoch 2005-08-18 (JD 2453600.5) | |
| Aphelion | 7,708 Gm (51.526 AU) |
| Perihelion | 5,260 Gm (35.164 AU) |
| Semi-major axis | 6,484 Gm (43.335 AU) |
| Eccentricity | 0.18874 |
| Orbital period | 104,234 d (285.4 a) |
| Average orbital speed | 4.484 km/s |
| Mean anomaly | 198.07° |
| Inclination | 28.19° |
| Longitude of ascending node | 121.90° |
| Argument of perihelion | 239.51° |
| Satellites | 2 |
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Physical characteristics
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| Dimensions | ~1,960×1,518×996 km (~1,500 km) |
| Mass | (4.2±0.1)×1021 kg |
| Mean density | 2.6–3.3 g/cm³ |
| Equatorial surface gravity | 0.44 m/s² |
| Escape velocity | 0.84 km/s |
| Sidereal rotation period |
0.16314±0.00001 d (3.9154±0.0002 h) |
| Albedo | 0.7±0.1 |
| Temperature | 32±3 K |
| Spectral type | ? |
| Absolute magnitude | 0.1 |
(136108) 2003 EL61 (also written (136108) 2003 EL61), nicknamed "Santa", is a large Kuiper belt object, roughly one-third the mass of Pluto, discovered by J. L. Ortiz et al. of the Instituto de Astrofísica de Andalucía at Sierra Nevada Observatory in Spain and Mike Brown's group at Caltech in the United States. The MPC currently gives formal discovery credit to Ortiz's group, who were first to announce the object.
Its two moons, rapid rotation, extreme elongation, and high albedo due to crystalline water ice on the surface, make it exceptional among the known cubewanos. It is thought to be the largest member of a collisional family, created in a single break-up event that is responsible for its other unusual characteristics.
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[edit] Name
Before the discovery of the object was published and designated, the Caltech team used the nickname "Santa", which stems from its discovery just after Christmas, on December 28, 2004, although the team had acquired images of it from May 6, 2004. Following IAU guidelines, the object should be formally named after a deity related to a creation myth. The Caltech team submitted formal names from Hawaiian mythology in September 2006 for (136108) 2003 EL61 and both of its satellites "to pay homage to the place where the satellites were discovered." However, since the discoverers of a body are generally given the right to name it, and the Spanish team now has formal credit for (136108) 2003 EL61 itself, while the Caltech team has credit for the moons, it is not clear if the submitted names will be accepted.
[edit] Discovery controversy
José Luis Ortiz Moreno, an astronomer at the Sierra Nevada Observatory in Spain, and colleagues Francisco José Aceituno Castro and Pablo Santos-Sanz announced the discovery of the object on July 25, 2005, when they re-analysed observations they had made on March 7, 2003. They then scoured older archives (a process known as precovery) and found the object in images dating back to 1955. Ortiz's group announced their discovery on July 27, 2005, and it was published two days later by the MPC.
A Caltech team consisting of Michael E. Brown, Chad Trujillo, and David Rabinowitz had been observing the object for half a year with the 1.3 m SMARTS Telescope, but had not yet made the data public. Brown and his collaborators initially supported giving Ortiz and his group credit for the discovery, but withdrew support when they found reason to suspect that Ortiz may have used discovery data from Brown's team, which had inadvertently been made publicly available on the web.
A week before Ortiz's discovery announcement, on July 20, Brown's team had published an abstract of a report they intended to use to announce the discovery, in which the object was referred to by the internal code name K40506A. Typing this code into internet search engines allowed anyone to find the observation logs of Brown's group, including the observed positions of the object. Third-party web server logs indicated that the page in question had been accessed by an IP address used by computers at the Instituto de Astrofísica de Andalucía where Ortiz's group worked.[1] Brown's group accused Ortiz's group of a serious breach of scientific ethics and asked the Minor Planet Center to strip them of discovery status.[2]
Ortiz later admitted he accessed the internet telescope logs with the relevant information a day before making his announcement, but denied any wrongdoing.[3] According to him they did not use the data, other than checking them out of curiosity whether it could be the same object they had found in their 2003 images that same month. This after they realized the object in the abstract by Brown et al. seemed to be an object with similar characteristics. Googling the informal designation mentioned in the abstract, they ended up at the telescope log.
The ambiguity in who discovered the object stems from the fact that the Caltech group of Brown did not submit their discovery to the Minor Planet Center for a year after detecting it in their images. Standing protocol is that the one who first does submit a report to the MPC with enough positional data for a decent orbit determination, gets discovery credit. This is what Ortiz' group did, thus following correct protocol, using their 2003 imagery, 2005 follow-up imagery, and "precovery" positions from historic archives.
The Minor Planet Center's discovery circumstances database gives the group of Ortiz et al. as the formal discoverers of the object.
On July 29, 2005, shortly after the Ortiz discovery announcement, Brown's group announced the discovery of another Kuiper belt object, Eris, which is more distant and is thought to be larger than the dwarf planet Pluto. The announcement was made earlier than planned, at the urging of the Minor Planet Center, to forestall the possibility of that discovery leaking out as well.
[edit] Size and composition
The only method to estimate the size of a small trans-Neptunian object is its magnitude assuming a value for the albedo. For larger objects, thermal emission can provide a size directly. (136108) 2003 EL61 is exceptional because its two moons provide the means to determine directly the mass of the system from Kepler's third law. The estimated mass is 4.2 × 1021 kg, 28% the mass of the Plutonian system.[4] Because (136108) 2003 EL61 rotates roughly once every four hours, faster than any other known body in the solar system larger than 100 km in diameter, it should be distorted into a triaxial ellipsoid. (136108) 2003 EL61 displays large fluctuations in brightness. Although these fluctuations could be due to a mottled surface, it is thought that this fluctuation is due to an elongated shape. Rapid rotation and elongated shape result in constraints on the density (the denser the object, the less elongated), estimated at 2.6–3.3 g/cm³, suggesting substantial non-ice content (compare with Pluto's density of 2.0 g/cm³ and Moon's density of 3.3 g/cm³). These limits on the density, together with the known mass, give another way to constrain the dimensions of the object.[5] (136108) 2003 EL61 has approximately the diameter of Pluto along its longest dimension, and half that along its shortest. This would make it one of the largest trans-Neptunian objects discovered so far; possibly fourth after Eris, Pluto and arguably (136472) 2005 FY9, larger than Sedna, Orcus, and Quaoar.
The short rotation period of (136108) 2003 EL61 may have been caused by a giant impact, which also created its satellites. (136108) 2003 EL61 may not be the only elongated, rapidly rotating, large object in the Kuiper Belt. In 2002, Jewitt and Sheppard suggested that Varuna should be elongate, based on its rapid rotation (see the references there).
2003 EL61, 2002 TX300 and four smaller Kuiper belt objects are traveling in similar orbits and all have a similar color and proportion of water ice. Mike Brown and his team have postulated that they are the remnants of a past impact and their surfaces were once ejected from the mantle of the original object.[6] (See Collisional family below).
[edit] Surface
Gemini telescope obtained spectra of (136108) 2003 EL61, which show strong water ice features similar to the surface of Pluto's moon Charon. Trujillo, Brown, et al. report crystalline water ice.[7]
Water ice has been reported on many trans-Neptunian objects but typically in the form of amorphous ice. Crystalline ice is unstable on timescales of 10 million years under conditions in the Kuiper Belt. This discovery hints at resurfacing processes producing fresh ice. As surprising as the crystalline form is the inferred amount of ice. Following the report, the surface of (136108) 2003 EL61 appears to be 66% to 80% pure ice, with the remainder of the surface material of unknown composition.
(136108) 2003 EL61 has an albedo approaching that of pure snow, consistent with crystalline ice on the surface. This very high albedo does not appear to be unique among large TNOs. Recent measurements of Eris imply an even higher (inferred) albedo (0.86) for that object.
[edit] Orbit
(136108) 2003 EL61 is classified as a classical trans-Neptunian object with an orbit common for large cubewanos: the perihelion is close to 35 AU and significantly inclined. The diagram shows a view of its orbit in yellow, (Pluto in red, Neptune in grey) and position (as of April 2006). The object passed its aphelion (Q) in 1991, and is currently more than 50 AU from the Sun and takes 285 Earth Years for a complete orbit.
The inclination of its orbit (~28° to compare with 17° for Pluto) and its current position, far from the ecliptic where most of the early surveys took place, combined with a slow mean motion explain why (136108) 2003 EL61 was only discovered recently, in spite of its magnitude.
[edit] Moons
Two small satellites have been discovered orbiting (136108) 2003 EL61 currently named S/2005 (136108) 1 and S/2005 (136108) 2.
[edit] S/2005 (136108) 1
S/2005 (2003 EL61) 1 (provisional designation; nicknamed "Rudolph" by the Caltech team), renamed S/2005 (136108) 1 once its primary was numbered, was the first satellite discovered around (136108) 2003 EL61. It orbits once every 49.12 ± 0.03 days with semimajor axis 49,500 ± 400 km and eccentricity 0.050 ± 0.003[1]. Mutual occultations of the moon and the primary, as seen from Earth, occurred in 1999 and will not occur again until 2138.
Only the total mass of the system is known, but assuming the moon has the same density and albedo as the primary, the mass of the satellite is 1% of the mass of (136108) 2003 EL61 and it has a diameter of ~310 km.[8]
Strong absorption features at 1.5 and 2 micrometres discovered in the infrared spectrum are consistent with absorption due to water ice. Their depth suggests that much of the satellite’s surface is covered with ice.[9]
[edit] S/2005 (136108) 2
S/2005 (2003 EL61) 2 (provisional designation; nicknamed "Blitzen" by the Caltech team.[10]), later renamed S/2005 (136108) 2, is the smaller inner satellite of (136108) 2003 EL61.
Its discovery was announced on November 29, 2005. It was found 39,300 km away and, with the assumption of a circular orbit, it orbits the primary in 34.7 ± 0.1 days, and is inclined 39 ± 6° from the larger moon.
The measured brightness implies a diameter 12% that of (136108) 2003 EL61, ~170 km, assuming similar albedo.
[edit] Collisional family
EL61 is the largest member of a collisional family, similar to asteroid families: a group of objects with similar orbital parameters and common physical characteristics, presumably with a common origin in a disruptive impact of the progenitor object of EL61.[11]
The family, the first to be identified among TNOs, includes EL61 and its moons, 2002 TX300, (24835) 1995 SM55, (19308) 1996 TO66, (120178) 2003 OP32 and (145453) 2005 RR43. The dispersion of the proper orbital elements of the members is a few percent or less (5% for semi-major axis, 1.4° for the inclination and 0.08 for the eccentricity). The diagram illustrates the orbital elements of the members of the family in relation to other TNOs.
The objects' common physical characteristics include neutral colours and deep infrared absorption features (at 1.5 and 2.0 μm) typical of water ice.[12]
Collisional formation of the family requires a progenitor some 1660 km in diameter, with a density of ~2.0 g/cm³, similar to Pluto and Eris. During the formational collision, EL61 lost roughly 20% of its mass, mostly ice, and became denser.[11]
The current orbits of the members of the family cannot be accounted for by the formational collision alone. To explain the spread of the orbital elements, an initial velocity dispersion of ~400 m/s is required, but such a velocity spread should have dispersed the fragments much further. This problem applies only to 2003 EL61 itself; the orbital elements of all the other objects in the family require an initial velocity dispersion of ~140 m/s. To explain this mis-match in the required velocity dispersion, Brown et al. suggest that 2003 EL61 initially had orbital elements closer to those of the other members of the family and its orbit (especially the orbital eccentricity), changed after the collision. Unlike the other members of the family, EL61 is in a chaotic orbit, near the 7:12 resonance with Neptune, which would increase EL61's eccentricity to its current value.[11]
[edit] References
- ^ Brown, Michael. The electronic trail of the discovery of (136108) 2003 EL61. Retrieved on 2006-08-16.
- ^ Overbye, Dennis. "One Find, Two Astronomers: An Ethical Brawl", New York Times, September 13, 2005. Retrieved on 2006-08-16.
- ^ Hecht, Jeff. "Astronomer denies improper use of web data", NewScientist.com, 21 September 2005. Retrieved on 2006-08-16.
- ^ M. E. Brown, A. H. Bouchez, D. L. Rabinowitz, R. Sari, C. A. Trujillo, M. A. van Dam, R. Campbell, J. Chin, S. Hartman, E. Johansson, R. Lafon, D. LeMignant, P. Stomski, D. Summers, P. L. Wizinowich Keck Observatory laser guide star adaptive optics discovery and characterization of a satellite to large Kuiper belt object 2003 EL61, The Astrophysical Journal Letters, 632, L45 (October 2005) Full text from Caltech
- ^ D. L. Rabinowitz, K. M. Barkume, M. E. Brown, H. G. Roe, M. Schwartz, S. W. Tourtellotte, C. A. Trujillo (2005), Photometric Observations Constraining the Size, Shape, and Albedo of 2003 EL61, a Rapidly Rotating, Pluto-Sized Object in the Kuiper Belt, The Astrophysical Journal (2006), 639, Issue 2, pp. 1238-1251 Preprint on arXiv (pdf)
- ^ "Icy chips off the old asteroid block date", New Scientist. Retrieved on 2007-03-15.
- ^ C. A. Trujillo, Brown M.E., Barkume K., Shaller E., Rabinowitz D. The Surface of 2003 EL61 in the Near Infrared. The Astrophysical Journal, 655 (Feb. 2007), pp. 1172-1178 Preprint
- ^ List of known trans-Neptunian objects
- ^ K. M Barkume, M. E. Brown, and E. L. Schaller Water Ice on the Satellite of Kuiper Belt Object 2003 EL61,The Astrophysical Journal, 640 (March 2006), pp. L87-L89. Preprint
- ^ New York Times: Piecing Together the Clues of an Old Collision, Iceball by Iceball
- ^ a b c Michael E. Brown, Kristina M. Barkume, Darin Ragozzine & Emily L. Schaller, A collisional family of icy objects in the Kuiper belt, Nature, 446, (March 2007), pp 294-296.
- ^ e.g. N. Pinilla-Alonso, J. Licandro, R. Gil-Hutton and R. Brunetto The water ice rich surface of (145453) 2005 RR43: a case for a population of carbon-depleted TNOs?, A&A 468, L25-L28 (2007) [1]
[edit] External links
- NASA visualization of the orbit
- Updated data
- MPEC listing for 2003 EL61
- Space.com story: Large new world discovered beyond Neptune
- New Scientist story: New world found in outer solar system
- BBC story: Distant object found orbiting Sun
- Astronomers Discover "10th Planet" - Sky & Telescope article describing the discovery of (136108) 2003 EL61 and 136199 Eris.
- Michael Brown's webpage
- New Scientist story: Distant solar system body may be cigar-shaped
- BBC story: Record spin rate for cosmic body
- news @ nature.com story: Cosmic 'cigar' spins at astonishing pace; Pluto's neighbour has a very unexpected shape, 9 September 2005.
- New York Times story: "One find, two astronomers", September 13, 2005.
- S.C. Tegler, W. Grundy, W. Romanishin, G. Consolmagno, K. Mogren, F. Vilas: Optical Spectroscopy of the Large Kuiper Belt Objects 136472 (2005 FY9) and 136108 (2003 EL61). Preprint
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