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2003 EL61

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 2003 EL61
Orbital elements
Eccentricity (orbit)>e
Inclination>i °
Perihelion>q AU
Argument of perihelion>ω °
Semi-major axis>a AU
Longitude of the ascending node>ω °
Aphelion>Q AU
Mean anomaly>M °
Orbital period>P years
Mean motion>n °/day
Time of perihelion passage>TP

2003 EL61 (also written 2003 EL61), is a large and very unusual Kuiper belt object discovered by Mike Brown et al. at Caltech in the United States. Its two moons, rapid rotation, and high albedo due to crystalline water ice on the surface, make it exceptional among the known cubewanos.

Before the discovery of the object was published and designated, the discovery team used the nickname "Santa", which stems from its discovery just after Christmas, on December 28, 2004, although the Caltech team had acquired images of it starting May 6, 2004. Following IAU guidelines, the object should be named after a deity related to a creation myth. Santa is famous in popular fiction for the creation of presents, and is derived from ancient Germanic myths of the Green Man and a winter feast deity, so the name suits guidelines.

Contents

Discovery controversy

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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 1.3m SMARTS Telescope, but had not yet made the data public. Brown et al. 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 was publicly available on the web through the SMARTS consortium.

A week before Ortiz's discovery, on July 20, Brown et al. 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. 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. [link]

Ortiz later admitted he accessed the internet telescope logs, downloading the relevant information a day before making his announcement, but [denied] any wrongdoing. He concedes that it was Brown's team that had discovered the object.

On July 29, 2005, shortly after the Ortiz discovery announcement, Brown's group announced the discovery of another Kuiper belt object, 2003 UB313 "Xena", which is more distant and is thought to be larger than the planet Pluto. The announcement was made earlier than planned, at the urging of Brian Marsden of the Minor Planet Center, to forestall the possibility of that discovery leaking out as well.

Size and composition

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9}}, Sedna, Quaoar, and Earth

Too often, the only element to estimate the size of a trans-Neptunian object is its magnitude assuming the albedo. For larger objects, thermal emissions can be measured providing an independent measure for the albedo. 2003 EL61 is exceptional as 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. 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] Moreover, as the object rotates extremely rapidly, faster than any known body larger than 100 km in diameter, this rotation should distort its shape into an oblate spheroid. Indeed, 2003 EL61 displays large fluctuations in brightness. Although these fluctuations could be due to a mottled surface1, it is thought that this fluctuation is due to an elongated shape. Rapid rotation and elongated shape result in constraints on the density (denser object would be 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 make then possible to estimate the dimensions. 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)] If these assumptions are correct, 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 2003 UB313, Pluto and arguably 2005 FY9, larger than 90377 Sedna, 90482 Orcus, and 50000 Quaoar.

The rotation period of 2003 EL61 is less than four hours, possibly caused by the impact which created its satellites. 2003 EL61 is possibly not the only elongated, rapidly rotating large object. As early as in 2002, Jewitt & Sheppard suggested a shape of elongated spheroid for 20000 Varuna to explain its rapid rotation (see the references there).

1Pluto shows such albedo variation, fluctuating in brightness by 35% during its six-day rotation period

Surface

Gemini telescope was able to obtain the spectra of 2003 EL61, which show strong water ice features similar to what is seen on the surface of Pluto's moon Charon. Brown, Trujillo et al. report strong presence of crystalline water ice, known so far only on Quaoar.C. A. Trujillo, Brown M.E., Barkume K., Shaller E., Rabinowitz D. The Surface of 2003 EL61 in the Near Infrared. To appear in The Astronomical Journal (2006); [Preprint on arXiv (pdf)] Water ice signature has been reported on many trans-Neptunian objects but typically in the form of amorphous ice. The crystalline ice is unstable within the timescale of 10 million years under the low temperatures and (lack of) pressure conditions encountered in the Kuiper Belt. This discovery hints on the resurfacing processes producing fresh ice, similar to the one observed on icy satellites in the solar system. As surprising as the crystalline form is the inferred amount of ice. Following the report, the surface of 2003 EL61 appears to be covered with 2/3 to 4/5 of pure ice, the remaining component appearing blue in near infrared could be hydrogen cyanide or kaolinite. However, their presence is conjectural as other components could fit the observed spectra.

These findings concur with the high albedo, giving 2003 EL61 the reflectivity of almost that of pure snow. This very high albedo does not appear to be unique among TNOs. The recent measurements of 2003 UB313 imply an even higher (inferred) albedo (0.86) for that object.

Orbit

2003 EL61 is classified as a classical trans-Neptunian object with an orbit common for large cubewanos: the perihelion close to 35AU and a significant inclination. The diagram shows a view of its orbit in yellow (Pluto in red, Neptunein grey) and position (as of April 2006). The perihelion (q) of 2003 EL61 is below the ecliptic. The object passed its aphelion (Q, in 1991) and is currently more than 50AU from the Sun but remains very bright (m =17.4, H =0.2) due to its size and albedo.

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 (due to the distance) explain why 2003 EL61 was only discovered recently, in spite of its magnitude.

Moons

Two small satellites have been discovered orbiting 2003 EL61.

S/2005 (2003 EL61) 1

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S/2005 (2003 EL61) 1 (provisional designation; nicknamed "Rudolph" by the Caltech team) is the first satellite discovered around 2003 EL61. It orbits 2003 EL61 once every 49.12±0.03 days at a distance of 49,500±400 km, with an eccentricity of 0.050±0.003 and an inclination of 234.8±0.3° [1]. Mutual occultations occurred in 1999 and will not occur again until 2138.

Measured brightness is 5.9±0.5% translating into the diameter about 22% of its primary, or in the range of 350 km, assuming similar albedo. To put this in perspective, this moon would be the fifth largest asteroid after 1 Ceres, 2 Pallas, 4 Vesta, and 10 Hygiea if it were in the asteroid belt.

Only the total mass of the system is known, but assuming the moon has the same density and albedo as the primary, their magnitude difference (3.3) can be used to estimate the mass of the satellite as 1% of the mass of 2003 EL61.

S/2005 (2003 EL61) 2

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S/2005 (2003 EL61) 2 is a smaller second, inner satellite of 2003 EL61.

It 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 34.1±0.1 days, and is inclined 39±6° from the larger moon.

Measured brightness is 1.5±0.5% of the primary what could translate, with the arguably unsafe assumption of an albedo similar to that of the primary, to perhaps 12% of the size of 2003 EL61, maybe 170 km.

External links

References

Large trans-Neptunian objects[http://encycl.opentopia.com/ edit ]
Kuiper belt: Pluto (Charon) | Orcus | Ixion | 2002 UX25 | Varuna | 2002 TX300 | 2003 EL61 | Quaoar | 2005 FY9 | 2002 AW197
Scattered disc: 2002 TC302 | 2003 UB313 | 2004 XR190 | Sedna†
 See also Triton, astronomical objects and the solar system's list of objects, sorted by radius or mass. 
For pronunciation, see: Centaur and TNO pronunciation.
† Current MPC classification. Some consider Sedna an Oort cloud object.
The minor planets
Vulcanoids | Near-Earth asteroids | Main belt | Jupiter Trojans | Centaurs | Damocloids | Comets | Trans-Neptunians (Kuiper belt · Scattered disc · Oort cloud)
For other objects and regions, see: , , asteroid moons and the Solar system
For a complete listing, see: List of asteroids. See also Pronunciation of asteroid names and Meanings of asteroid names.

 

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