2003 EL61: Encyclopedia - 2003 EL61

2003 EL61

2003 EL₆₁ (also written 2003 EL61), nicknamed "Santa" (non-official designation), is a very large and unusual Kuiper belt object recently discovered by Mike Brown et al. at Caltech in the United States.

The nickname stems from its discovery just after Christmas, on December 28, 2004, although the Caltech team had acquired images of it starting May 6, 2004. The nickname is only temporary and will not be the official name of the object. IAU guidelines dictate that the object will be named after a creation myth, unless it is in a Pluto-like resonance with Neptune which would require an underworld mythical name.

2003 EL61 - 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, 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 that Brown's team had made publicly available on the web.

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. [1]

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, 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.

2003 EL61 - Size and composition

The total mass of the 2003 EL₆₁ system can be determined from the orbits of its satellites by Kepler's third law. It turns out to be 4.2×10²¹ kg, 28% the mass of the Plutonian system.

If 2003 EL₆₁ were an ordinary body, its size could only be guessed at. However, it rotates extremely rapidly, faster than any known body larger than 100 km in diameter, and 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 surface (neighbouring Pluto shows the second-largest such albedo variation in the solar system, fluctuating in brightness by 35% during its six-day rotation period), it is thought that in the case of 2003 EL₆₁ the fluctuation is due to an elongated shape. This constrains the density of the object, because the desnser the object is, the less it would elongate for a given rotation. This suggests that 2003 EL₆₁ has the density of rock rather than ice, about 3 g/cm³, and from this plus the mass, the dimensions can be estimated.

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; third after 2003 UB₃₁₃ and Pluto and larger than 90377 Sedna, 90482 Orcus, and 50000 Quaoar. (Another newly discovered object, 2005 FY₉ may be slightly larger.) Isaac Asimov suggested the term mesoplanet be used for planetary objects intermediate in size between Mercury and 1 Ceres, which would include all seven of these objects.

The rotation period of 2003 EL₆₁ is less than four hours. It does not seem to have been cause by either spiralling in or out by its satellites, and may have been caused by the same impact that boiled off most of 2003 EL₆₁'s water, leaving a rocky body with only a crust of ice.

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. Methane ice has been detected on the surface of 2003 EL₆₁, which means it has never been very close to the Sun. Its reflectivity is reported being “almost that of pure snow”.

2003 EL61 - Orbit

The Minor Planet Center reports the object is about 51 astronomical units from the Sun. Its eccentric orbit brings it as close as 35 AU from the Sun, which is closer than Pluto's average distance of 39 AU. The reasons it was not discovered before are its very low speed and the plane of its orbit. That plane is tilted by 28° with respect to the orbital plane of most planets, even more than Pluto's, which is tilted by "merely" 17°. This probably means it is in orbital resonance with the planet Neptune and ended up in this orbit when Neptune migrated outwards.

2003 EL61 - Moons

2003 EL61 - S/2005 2003 EL₆₁ 1

S/2005 (2003 EL₆₁) 1 (provisional designation; nicknamed Rudolph by the Caltech team) is the first satellite discovered around 2003 EL₆₁. It is estimated as 1% of the mass of 2003 EL₆₁. The Caltech team measurements only retrieve the total mass of the system, but if one assumes the moon has the same density and albedo as 2003 EL₆₁, their magnitude difference (3.3) can be used to resolve the mass budget.

S/2005 (2003 EL₆₁) 1 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° [2]. Mutual occultations occurred in 1999 and will not occur again until 2138. Its diameter is about 22% of its primary, or in the range of 350 km. 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.

2003 EL61 - S/2005 2003 EL₆₁ 2

S/2005 (2003 EL₆₁) 2 is a smaller second satellite announced on November 29, 2005 in CBET 310 [3], with further details forthcoming. It orbits perhaps 39,300 km away in 34.1±0.1 days, and is inclined 35-45° from the larger moon. It has about 0.2% the mass of 2003 EL₆₁, and is perhaps 12% its size, maybe 170 km.

Adapted from the Wikipedia article "2003 EL61", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki