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Title: Characteristics and Origin of the Quadruple System at Pluto
Authors: S.A. Stern, H.A. Weaver, A.J. Steffl, M.J. Mutchler, W.J. Merline, M.W. Buie, E.F. Young, L.A. Young, J.R. Spencer

The discovery of two new satellites of Pluto, designated S/2005 P 1 and S/2005 P 2, combined with the constraints on the absence of more distant satellites of Pluto, reveal that Pluto and its moons comprise an unusual, highly compact, quadruple system.
The two newly discovered satellites of Pluto have masses that are very small compared to both Pluto and Charon, creating a striking planet-satellite system architecture. These facts naturally raise the question of how this puzzling satellite system came to be.
Here researchers show that S/2005 P 1 and S/2005 P 2's proximity to Pluto and Charon, along with their apparent locations in high-order mean-motion resonances, likely result from their being constructed from Plutonian collisional ejecta.
They argue that variable optical depth dust-ice rings form sporadically in the Pluto system, and that rich satellite systems may be found, perhaps frequently, around other large Kuiper Belt objects.





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Mining old data from the Hubble Space Telescope, astronomers in Arizona and Colorado have determined the best orbits yet for two newly discovered moons around Pluto. These orbits confirm that Pluto is denser than its largest moon, Charon, suggesting the planet contains a greater proportion of rock.
Marc Buie and William Grundy of Lowell Observatory in Flagstaff, Arizona, and Eliot Young, Leslie Young, and S. Alan Stern of the Southwest Research Institute in Boulder, Colorado, have analysed Hubble images that captured the small moons back in 2002 and 2003. These images show the moons making complete orbits around Pluto.

Buie and his colleagues confirm that the orbital periods of P1, P2, and Charon are almost in a 6:4:1 resonance. That is, the outer small moon takes about six times longer than Charon to complete an orbit, and the inner small moon takes about four times longer. Charon orbits Pluto every 6.39 days, while P2 orbits the planet every 24.86 days and P1 revolves every 38.21 days.

Because the new moons feel the gravitational pull of both Pluto and Charon, their motion reveals both the total and individual masses of the planet and its large moon. The scientists find that Charon is 11.65 ± 0.55 percent as massive as Pluto, in agreement with previous work. For comparison, the Earth's Moon is 1.23 percent as massive as the Earth.
The masses also confirm previous signs that Pluto is denser than Charon. Density is important, because it helps reveal what a world is made of. Buie and his colleagues say Pluto is 2.03 ± 0.06 times denser than water, while Charon is 18 percent less dense than Pluto. It is only 1.66 ± 0.06 times denser than water.
Both Pluto and Charon probably consist of silicate rock and water ice. Since rock is denser than ice, and Pluto is denser than Charon, Pluto presumably contains a greater proportion of rock than Charon does.

Buie's team finds that the new moons are almost equally faint, which means they are equally small--if they reflect the same percentage of sunlight. In particular, if both moons reflect 35 percent of the sunlight hitting them, as Charon does, then they are 44 kilometres, across. But if they are blacker, reflecting only 4 percent of sunlight, they would be three times larger.
For comparison, Pluto is about 2,300 kilometres across; Charon's diameter, which is better known than Pluto's, is 1,205 kilometres.

Both Charon and P2 have circular orbits around Pluto. However, P1, the outermost moon, has a slightly elliptical orbit. Its orbital eccentricity is 0.5 percent. Still, that makes the orbit more circular than that of any planet in the solar system.

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The Pluto system as we understand it today, with Charon orbiting in the innermost position, and the two new satellites orbiting somewhat farther out. Positions and orbits are shown as seen from Earth for 15 May 2005.
Very little is known about these new bodies at this time. We know their brightness, but not their rotation periods, their colours, their surface reflectivity, or their surface compositions.

Both satellites appear to stray no farther than 3 arcseconds from Pluto; for reference, Charon orbits no farther than 0.9 arcseconds from Pluto.

By comparing the positions of the satellites in different images on different dates, we find that the positions are consistent with both S/2005 P 1 and S/2005 P 2 having orbits that are nearly circular and which lie in Pluto’s equatorial plane. This is a preliminary conclusion, but if it holds up, then the orbital semi-major axes of P1 and P2 are near 64,700 ±850 and 49,500 ±600 km, respectively. These semi-major axes in turn correspond to orbital periods of approximately 38.2 ±0.8 and 25.5 ±0.5 days, respectively.



The brighter satellite, S/2005 P 1, has a V-band magnitude of about 23.0. Because we do not yet know its surface reflectivity, we cannot definitively calculate its exact size. But by estimating surface reflectivity and using its distances from the Earth and Sun at the time of the HST observations, we can determine the range of plausible sizes it might have. Assuming surface reflectivity ranging from 4% (like the darkest known KBOs), to 15% (like many KBOs), to 35% (like Charon), this is what we find:

Assumed Reflectivity Approximate Diameter
04% 160 km
15% 80 km
35% 52 km

S/2005 P 2 is about 25% fainter than S/2005 P 1, so could be a 10% to 15% smaller than S/2005 P 1, assuming they have the same surface reflectivity.

Despite the various uncertainties in what we know about P1 and P2 so far, and regardless of their surface reflectivity, these bodies are clearly very small compared to both Pluto (2284 km diameter) and Charon (1192 km diameter). Indeed, it is unlikely that either object has a mass larger than 3/10,000th of Pluto’s or 3/1000th of Charon’s.

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Using NASA's Hubble Space Telescope to view the ninth planet in our solar system, astronomers discovered Pluto may have not one, but three moons.

If confirmed, the discovery of the two new moons could offer insights into the nature and evolution of the Pluto system; Kuiper Belt Objects with satellite systems; and the early Kuiper Belt. The Kuiper
Belt is a vast region of icy, rocky bodies beyond Neptune's orbit.


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"If, as our new Hubble images indicate, Pluto has not one, but two or three moons, it will become the first body in the Kuiper Belt known to have more than one satellite" - Hal Weaver of the Johns
Hopkins Applied Physics Laboratory, Laurel, Md. He is co-leader of the team that made the discovery.

Pluto was discovered in 1930. Charon, Pluto's only confirmed moon, was discovered by ground-based observers in 1978. The planet resides about 3 billion miles from the sun in the heart of the Kuiper Belt.

"Our result suggests other bodies in the Kuiper Belt may have more than one moon. It also means planetary scientists will have to take these new moons into account when modelling the formation of the Pluto system" - Alan Stern of the Southwest Research Institute, Boulder, Colorado.
Stern was co-leader of the research team.

The candidate moons, provisionally designated S/2005 P1 and S/2005 P2, were observed approximately 27,000 miles away from Pluto. The objects are roughly two to three times as far from Pluto as Charon.

The team plans to make follow-up Hubble observations in February to confirm the newly discovered objects are truly Pluto's moons. Only after confirmation will the International Astronomical Union consider names for S/2005 P1 and S/2005 P2.

The Hubble's Advanced Camera for Surveys observed the two new candidate moons on May 15, 2005. The candidates are roughly 5,000 times fainter than Pluto. Three days later, Hubble looked at Pluto again. The two objects were still there and appeared to be moving in orbit around Pluto.
The team looked long and hard for other potential moons around Pluto.

"These Hubble images represent the most sensitive search yet for objects around Pluto. It is unlikely that there are any other moons larger than about 10 miles across in the Pluto system" - team member Andrew Steffl of the Southwest Research Institute.

The Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency. The Space Telescope Science Institute in Baltimore conducts Hubble science operations. The Institute is operated for NASA by the Association of Universities for Research in Astronomy, Inc., Washington.


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Astronomers have produced a new colour map of Pluto, the most distant planet in our Solar System, using images from the Hubble Space Telescope.
The detailed map shows areas likely to be methane frost and a bright spot perhaps made of frozen carbon monoxide.



And another team has obtained the most precise estimate yet for the size of its moon, Charon, with data gathered during the planet's eclipse of a star.
This figure could be used to calculate a more accurate size for Pluto itself.
The US space agency's New Horizons spacecraft will set off for an encounter with Pluto and Charon next year, but will not arrive until 2015 at the earliest.
Until then, astronomers say they will continue to seek insights into this mysterious world and its lone satellite.

The latest global map was produced using data obtained by Hubble's Advanced Camera for Surveys (ACS) between July 2002 and June 2003.
The telescope worked over 12 orbits and looked through two filters. Producing the map has taken two years of computer processing.

The researchers, led by Marc Buie of the Lowell Observatory, have found dark areas thought to be dirty water-ice and brighter ones indicating nitrogen frost. Red areas indicate methane ice and possibly other organics (carbon-based molecules).
The methane frost seems to be everywhere, running into dark and light areas on "a hemispheric level".

An unusual bright spot near the centre of the global map could indicate the presence of carbon monoxide. The Lowell Observatory researcher said he had asked members of the New Horizons team to investigate this area with their spacecraft.
An accurate measurement of Charon's radius and density were obtained from observations made during the moon's occultation on 11 July 2005. During this occultation, the moon eclipsed a star.

Bruno Sicardy, from the Paris Observatory, France, and colleagues used the data from this event to tie down the radius of Charon to 602.5km, ±1 km - the most precise figure yet obtained for its size.
Previous observations had given a lower limit for Charon's size, but could not say how big it might be.

From the new radius, Dr Sicardy's team was able to determine a very accurate density for Charon of 1.73 (± 0.08) grams per cubic centimetre.
Astronomers can now re-analyse data on Pluto gathered in the 1980s using the new figures for Charon's size and density to better constrain these values for Pluto itself.
Recent discoveries in the outer Solar System have cast doubt on Pluto's status as a planet. Some think it is simply the first historically recorded representative of a larger family of distant bodies known as Kuiper belt objects.

Pluto seemed to be very similar to Neptune's moon Triton, which is thought to be a Kuiper belt object captured by Neptune's gravity. This is despite the fact that the process of capture should have altered Triton's surface drastically through heating.

"I'm surprised Triton and Pluto aren't more different than they are,"- Marc Buie.

The primary launch window for the New Horizons mission runs from 11 January-14 February 2006. If it launches within that window, it will swing by Jupiter for a gravity assist and arrive at Pluto in 2015.

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