Image Credit: Ann Feild, STScI, ESA, NASA

A family of objects, including 2003 EL61 and its two moons, has been discovered in the Edgeworth-Kuiper Belt (more)

2003 EL61

Initally nicknamed "Santa" and currently designated as 2003 EL61, a team of astronomers (including Mike Brown, Chad Trujillo, and David Rabinowitz) first discovered the movement of a relatively large planetary body outside the orbit of Neptune just after Christmas, on December 28, 2004. Brown's team had been acquiring and analyzing images of EL61 since May 6, 2004 -- with the 1.3-meter, Small and Moderate Aperture Research Telescope System (SMARTS) -- and had sent abstracts of a report to the American Astronomical Society (AAS) on July 20, 2005 that was intended to support the official announcement of its discovery at an upcoming meeting of the AAS Division of Planetary Sciences. However, the team did not make its data public while waiting to refine Santa's orbital and physical characteristics with additional observations using NASA's Hubble Space Telescope. As a result, controversy subsequently developed over which astronomer team should receive "official" attribution for EL61's discovery 

SOURCE: http://www.solstation.com/stars/el61fam.htm

Along elongated and slanted, the orbits of 2003 EL61 and 2005 FY9 objects are still considerably less eccentric than that of dwarf planet Eris. 

With a spin period of just 3.9 hours, 2003 EL61 is the fastest rotating known body in our Solar System larger than 100 kilometers (60 miles) across (David Tytell, Sky and Telescope, April 20, 2006; and (Brown et al, 2005 preprint). Shaped like a rugby or American football because of its fast rotation, 2003 EL61 must be as dense as basalt and be made almost entirely of rock or else its very fast spin would have stretched it out even more than it is already. This suggests it has lost most of its low-density icy mantle -- unlike many other known EKOs -- probably due to an ancient cataclysmic collision which could have broken up 2003 EL61's ice mantle, set it reeling, and produced it's two small moons. Probably spherical and 20 percent larger before the collision, EL61 was about about 1,000 miles wide (almost 1,610 km), or about two-thirds the width of Pluto, and made of roughly half ice and half rock, like other EKOs. It is thought to have collided with an object that was perhaps 700 miles wide (roughly 1,130 km) and traveling at nearly 7,000 miles per hour (over 11,000 km per hour). The impact produced at least seven other large icy objects with diameters ranging from 6 to 250 miles (10 to 400 km), and its discovery team have grouped the scattered objects into a family based on their matching gray color and evidence of surface water ice derived from spectral analyses. 

The bright pure-ice surface of 2003 EL61 and its moons was unexpected. Observations of the primary body made by astronomer Chadwick A. Trujillo and his colleagues revealed the strong spectral signature of crystalline water ice. While crystalline ice forms at temperatures above 110° Kelvins (-163° Celsius), the ambient temperature of space around 2003 EL61 is much colder, at below 50° Kelvins. In addition, since crystalline water ice should turn dark and ruddy in less than 100 million years from cosmic rays and micrometeoroid impacts in a process known as space weathering. Given the orbital spread of the EL61 family of fragments, however, the collision that produced them must have taken place billions of years ago. Hence, the object may have experienced resurfacing, perhaps by micrometeorite impacts that convert surface ices to crystalline form by flash-heating. 

Spectra of EL61's outer satellite obtained by astronomers Kristina M. Barkume, Michael E. Brown, and Emily L. Schaller also reveal the signature of almost pure water ice (Barkume et al, 2006). While the observations were too low in resolution to distinguish the type of water ice, it seems that nearly all of the moon is coated in highly reflective frost, like the three moons of Pluto which may also have formed from a violent collision. Hence, the moons of the largest KBOs may differ in origin from those of ordinary KBOs, possibly because their satellite systems formed from the remains of violent impacts instead of by delicate gravitational capture (Noll et al, 2007; Stern et al, 2006 and 2005; and Brown et al, 2005). 

SOURCE: http://www.solstation.com/stars/el61fam.htm

2003 EL61 is one of the strangest known objects in the solar system. It is a big across as Pluto, but shaped like a cigar. Or perhaps like a football [American-style]. Or, most accurately, a football that has too little air in it and has been stepped on. It spins end over end every 4 hours like a football that has been kicked. It appears to be made almost entirely of rock, but with a glaze of ice over the surface. It is surrounded by two tiny satellites and is followed in its orbit by a swarm of other small icy bodies. Everything that we know about this body appears to tell us that in its past another object slammed into it at high speed and cracked it into pieces which flew all around the outer solar system and left what we see today. Follow the story below....

What is 2003 EL61?
2003 EL61 is the third-largest known dwarf planet in the Kuiper belt, the region of space beyond Neptune that contains the larger dwarf planets Eris and Pluto as well as thousands of smaller objects.

How big is 2003 EL61?
Many times when objects like this are discovered we don't actually know how big we are, just how bright they are. How bright they are tells us how much sunlight they reflect. But they could be bright and reflect a lot of sunlight because they are large or they could be bright because they are highly reflective, like a ball of snow. In the case of 2003 EL61, however, we have gotten lucky, because soon after the discovery of the object we discovered a moon orbiting it. By following the orbit of the moon over the course of 6 months we are able to precisely determine the mass of 2003 EL61 and its moon. The mass is about 32% that of Pluto. Unfortunately, the mass does not directly tell us it's size. An object that weighs a lot can either be small and dense, like something made out of rock, or large and less dense, like something made out of ice.

The fast spin of 2003 EL61, however, gives away its size in a somewhat complicated way. When an object spins quickly, it stretches out, much like a pizza crust tossed into the air. A denser rockier object stretches out less than a less dense lighter object. By seeing just how much 2003 EL61 stretches out due to its spin we can tell how dense it is. We find that 2003 EL61 must be made almost entirely of rock or else its very fast 4 hour spin would stretch it out even more than it is already stretched!

Once we know how much 2003 EL61 weighs and we know what 2003 EL61 is made of we can figure out  how big it is. The answer is that it is as big as Pluto -- along its longest dimension.  Nothing else so large and so elongated or so quickly rotating is known anywhere in the solar system.

What is 2003 EL61 made out of?
From the spin we see that the density of 2003 EL61 is almost that of rock. By looking at sunlight reflected from the surface of 2003 EL61 and carefully analyzing the spectrum of the light we have found, however, that the surface appears to be pure ice  (again, a technical paper). It seems that 2003 EL61 is a large rocky body with a thin film of ice. A cross-section would look something like this:

SOURCE: Caltech

While the odd characteristics of 2003 EL61 and its moons initially led us to the hypothesis of a giant impact, the smoking gun came when we found the other icy debris left over from the collision in orbits similar to 2003 EL61 itself. In a paper just published in Nature we show the existence of five small objects in the Kuiper belt which look just like the moons of 2003 EL61 and appear to be additional chunks of the icy outer layer of 2003 EL61 that got blasted off the surface. These chunks appear to be about 10% of the total amount of material blasted off the surface. Some of the rest probably evaporated into space, but some smaller ones are probably still in the vicinity waiting to be found. We recognized these chunks as debris from 2003 EL61 because we first noticed that a small number of Kuiper belt objects appeared to have unique surfaces composed of almost pure ice. Most of the other Kuiper belt objects have much more complex surfaces that are difficult to understand, but 2003 EL61, its satellites, and these five new objects all appeared unique. While trying to understand why these we different it suddenly became glaringly obvious that all of these objects were on very similar orbits, and that if you traced those orbits back in time you would be able to connect them to a single location where they were once part of a larger body (this is an oversimplification, but gets the main point).

This is pretty exciting stuff! We had never before had evidence for such catastrophic disruptions in the outer solar system, though they appear moderately common within the asteroid belt in the inner solar system. What is even more interesting is that the collision occurred near a region of space where Kuiper belt objects don't live for long without having their orbits become unstable (for the curious: the orbits become unstable because they go around the sun precisely 7 times for  every 12 times that Neptune goes around the sun, and, over time, the fact that this pattern repeats gives the Kuiper belt objects slight perturbations every orbit which eventually build up enough to cause the object to be unstable).  When the orbits become unstable, the objects can eventually work their way in towards the inner solar system where we would call them comments. It is clear that the giant impact that made the 2003 EL61 family must have created many many tiny fragments that have lit up the earth's skies in the past. Even more interesting, 2003 EL61 is on an unstable orbit and will possibly become a comet itself. When it does it will probably be 10,000 times brighter than the spectacular comet Hale-Bopp, making it something like the brightness of the full moon and easily visible in the daytime sky. The only catch is that all of this will happen in perhaps 1 billion years, so you have a little bit of waiting to do to see it!

SOURCE: Caltech

Image Credit: NASA, ESA, A. Feild (STScI)

• 2003 EL61; the strangest known object in the Kuiper belt

PAPERS

• THE SURFACE OF 2003 EL61 IN THE NEAR-INFRARED
• KECK OBSERVATORY LASER GUIDE STAR ADAPTIVE OPTICS DISCOVERY AND CHARACTERIZATIONOF A SATELLITE TO THE LARGE KUIPER BELT OBJECT 2003 EL61
• WATER ICE ON THE SATELLITE OF KUIPER BELT OBJECT 2003 EL61

• Astronomer David C. Hewitt - on Edgeworth-Kuiper objects at his Kuiper Belt Page.