2003 EL61 is an object in the Kuiper belt, the region of space beyond Neptune that includes Pluto and the large planetoids Quaoar and Orcus, 2005 FY9, and the planet 2003 UB313 among others. 2003 EL61 is currently the third brightest object in this region after Pluto and 2005 FY9. It is so bright that it can readily be seen by high-end amateur telescopes equipped with CCD cameras. Other than being extremely bright, 2003 EL61 appeared at first to be typical of a type of Kuiper belt objects that astronomers call "scattered Kuiper belt objects." They are called "scattered" because it is believed that they once had a close encounter with Neptune which gravitationally "scattered" these objects onto more eccentric orbits. You can compare the orbits of the three bright newly discovered Kuiper belt objects below. 2003 EL61 and 2005 FY9 are both on similar eccentric orbits tilted by ~30 degrees from the rest of the solar system. The 10th planet, 2003 UB313, is even more eccentric and tilted by 45 degrees.
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 objcet 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 know anywhere in the solar system.
The first moon to 2003 EL61 was discovered on January 28th 2005 by observations at the Keck Observatory. The observations were obtained with a brand new high powered system at Keck which used a laser projecting out of the telescope to correct for the turbulence of the atmosphere. This system, called laser guide star adaptive optics, allows astronomers on the ground to take images with the sharpness of images obtained from space by the Hubble Space Telescope. When we observed 2003 EL61 with this new facility we immediately saw that there was a faint moon in the vicinity. From 5 observations over the span of 6 months we have precisely determined the orbit of the satellite to 2003 EL61.
About 10% of Kuiper belt objects have satellites, but until recently no other object in the Kuiper belt was known to have more than one satellite. Recently, however, 2 small satellites around Pluto were also discovered. It appears likely, now, that other Kuiper belt objects (at least large Kuiper belt objects) might also have multiple satellite systems.
