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2003 EL61.
Computer illustration of 2003 EL61. Image credit: Mike Brown/Caltech.

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 foot ball that has too little air in it and has been stepped on. It spins on its axis 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. And it is surrounded by two satellites. Learn all about this strange object below! 2003 EL61 looks and spins approximately like this.

A small Dwarf planet out past the orbit of Neptune could eventually become one of the brightest comets ever seen. Caltech professor Mike Brown - the discoverer of Eris - presented his calculations during the annual meeting of the American Astronomical Society held in Seattle in early January.

Until it gets a fancier name, the object is called 2003 EL61. It’s a rapidly spinning, football shaped object roughly the size of Pluto. Its elliptical orbit could eventually bring it close enough to Neptune that it would get a gravitational yank into the inner Solar System. It would change from a Dwarf planet to a short period comet.

2003 EL61 spins rapidly, turning over and over like a badly thrown football. It completes a rotation once every 4 hours. What could give it such a strange shape and behaviour? Brown thinks it collided with another Kuiper belt object some time in its early history. The impact kicked it into fast spin, and elongated its shape. 2003 EL61 is even surrounded by a set of satellites, which could be the debris from this impact.

Don’t look to the skies just yet, though, 2003 EL61 might take millions of years before its interactions with Neptune shifts its orbit, and sends it into the inner Solar System.

The black circle shows the orbit of the outer giant planet, Neptune. At this scale, the earth's orbit would be inside of the yellow dot that depicts the sun!.
The black circle shows the orbit of the outer giant planet, Neptune. At this scale, the earth's orbit would be inside of the yellow dot that depicts the sun!

What is 2003 EL61?

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. The black circle shows the orbit of the outer giant planet, Neptune. At this scale, the earth's orbit would be inside of the yellow dot that depicts the sun!

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 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.

Here is an image of the Satellite from the night of 30 June 2005. 2003 EL 61 is the bright object in the center and the Satellite appears directly below about 0.5 arcseconds.
Here is an image of the satellite from the night of 30 June 2005. 2003 EL 61 is the bright object in the center and the satellite appears directly below about 0.5 arcseconds.

The moons of 2003 EL61

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.

A (technical) scientific paper describing the discovery of this satellite can be read in the October issue of the Astrophysical Journal Letters.

Here is an image of the satellite from the night of 30 June 2005. 2003 EL 61 is the bright object in the center and the satellite appears directly below about 0.5 arcseconds. To the south of 2003 EL61 you can also see a faint object which turns out to be a second satellite.

The second satellite is fainter than the first, but still detected in three of the five images of the first satellite. You can read the (again, technical) scientific paper describing the second satellite (and the satellite to 2003 UB313), too.

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.

From five observations like the one above we can mathematically solve for the precise orbit of the brighter satellite. You can see the orbit of the satellite along with the five observations below. Three observations of the fainter satellite are not sufficient to rigorously solve for the orbit of the satellite, but, if we assume it is on a circular orbit, we come up with a solution like the one seen below.

The orbits of the two satellites are seen almost edge on, which is why they look so elongated. The orbits are tilted to each other by about 40 degrees.

The fainter Satellite appears to circle 2003 EL61 once every 34 days, though more observations are required to confirm this orbit.
The fainter satellite appears to circle 2003 EL61 once every 34 days, though more observations are required to confirm this orbit.

From the two satellites, we have learned several important things about the 2003 EL61 system. First, both satellites appear to be quite small. The larger one is perhaps 1% of the mass of the primary while the smaller is only 0.2% of the mass of the primary.

The brighter satellite is on a long period orbit, circling 2003 EL61 every 49 days, in contrast to Pluto's large satellite Charon which orbits in just 6 days. The longer orbital period of the satellite is a consequence both of the more distant orbit of the satellite from 2003 EL61 and the lower mass of 2003 EL61. The orbit is almost circular but not quite.

The fainter satellite appears to circle 2003 EL61 once every 34 days, though more observations are required to confirm this orbit.

Seen from the Earth the satellite reaches a maximum distance from 2003 EL61 of about 1.5 arcseconds. This distance is just enough to enable to the satellite to be seen with Telescopes even without high power laser adaptive optics systems. This fact will enable detailed studies of this fascinating small body. The fainter satellite reaches a distance of 1.0 arcseconds. While it is theoretically possible for conventional Telescopes to separate the satellite from the primary we suspect that the satellite is too faint for detailed study without special techniques.

The biggest scientific benefit of the large satellite is that it allows us to calculate the mass of the system. Once we know the distance od the satellite from the primary and we know the time it take to orbit we can simply calculate the mass. A higher mass object would pull the satellite more tightly and cause it to orbit faster. Knowing the mass allows us to figure out the size, as discussed above.

Why is 2003 EL61 spinning fast, shaped like a football, made out of ice-covered rock, and surrounded by tiny satellites?

Good question. We have constructed a hypothesis which attempts to explain everything going on on 2003 EL61, but we must emphasize that this is just a hypothesis. No one has done rigorous scientific calculations to verify that the hypothesis actually works in detail, but our scientific instinct suggests that it should. This method is one of the ways in which science progresses. It is entirely possible that when detailed calculations are done it will be found that our hypothesis doesn't work, in which case we will have to start over. Even if the calculations show that our hypothesis does work it doesn't prove that this is what happened for 2003 EL61, but it begins the process of creating an understanding of the early history of the outer solar system.

The hypothesis goes like this: 4.5 billion years ago, when the solar system was in the process of forming, the object that is now 2003 EL61 was a ball made half of ice and half of rock and about the size of Pluto (much like what we think Pluto is today). Sometime early in its history, another large Kuiper belt object colllided with 2003 EL61 obliquely. This collision heated 2003 EL61 enough that most or all of the initial water was boiled off, leaving just a rock. The oblique impact caused 2003 EL61 to spin rapidly. The rapid spin elongated 2003 EL61 into the football shape we see today. The debris from the impact coalesced into the satellites, which would have been initially much closer but which have been evolving outward with time.

This story contains some elements of how we think the Earth and Moon formed: a Mars-sized body hit the proto-earth and the Moon coalesced from the debris following the impact. There is also an analogy to the formation of Charon around Pluto which was thought to have been from a Kuiper belt impact at a similar time.

One of the interesting predictions of this hypothesis is that the debris from the collision, from which the satellites formed, should have been composed predominantly of ice, rather than rock. The satellites themselves, then, should be almost purely ice. The best way to determine the composition of the satellites would be if we could determine their density. Unfortunately, the satellites are too small to measure the densities any time soon. We can, however, measure the composition of the surface of the brighter satellite. We find that it, like 2003 EL61 itself, is composed of water ice. On the satellite the water ice appears perhaps 100% pure. While we don't know for certain that there is not rock under the surface, we think that such pure water ice is a likely indication that there is water ice all the way down, which is consistent with our hypothesis.

Why is it called 2003 EL61

Very soon 2003 EL61 will get a real name, much like Sedna and Quaoar and Orcus. But when first discovered these objects are only given "preliminary designations" until they are confirmed and well know. There is little need of confirmation of 2003 EL61 as it has now been seen after the fact in images from more than 50 years ago! We will thus propose a name to the International Astronomical Union and that name will become the permanent - and with luck more melodious - name of 2003 EL61.

Original Source: Mike Brown's 2003 EL61 page.

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