Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

Measuring the Size of a Small, Frost World

05.01.2006


Stellar occultation allows VLT to determine Charon’s size and to put upper limit on its atmosphere



Observing a very rare occultation of a star by Pluto’s satellite Charon from three different sites, including Paranal, home of the VLT, astronomers were able to determine with great accuracy the radius and density of the satellite to the farthest planet. The density, 1.71 that of water, is indicative of an icy body with about slightly more than half of rocks. The observations also put strong constraints on the existence of an atmosphere around Charon.

Since its discovery in 1978, Charon and Pluto have appeared to form a double planet, rather than a planet-satellite couple. Actually, Charon is about twice as small as Pluto in size, and about eight times less massive. However, there have been considerable discussions concerning the precise radii of Pluto and Charon, as well as about the presence of a tenuous atmosphere around Charon.


In August 2004, Australian amateur astronomer Dave Herald predicted that the 15-magnitude star UCAC2 26257135 should be occulted by Charon on 11 July 2005. The occultation would be observable from some parts of South America, including Cerro Paranal, in the northern Atacama Desert, the location of ESO’s Very Large Telescope (VLT).

Stellar occultations have proved to be powerful tools to both measure sizes - at km-level accuracy, i.e. a factor ten better than what is feasible with other techniques - and detect very tenuous atmosphere - at microbar levels or less. Unfortunately, in the case of Charon, such occultations are extremely rare, owing to the very small angular diameter of the satellite on the sky: 55 milli-arcsec, i.e. the size of a one Euro coin observed from 100 km away!

This explains why only one occultation by Charon was ever observed before 2005, namely on 7 April 1980 by Alistair Walker, from the South Africa Astronomical Observatory.

Similarly, only in 1985, 1988 and 2002 could astronomers observe stellar occultations by Pluto. Quite surprisingly, the 2002 event showed that Pluto’s atmospheric pressure had increased by a factor of two in four years (ESO PHOT 21/02).

"Several factors, however, have boosted our odds for witnessing occultations of Charon," said Bruno Sicardy, from Paris Observatory (France) and lead author of the paper reporting the results. "First, larger telescopes now give access to fainter stars, thus multiplying the candidates for occultations. Secondly, stellar catalogues have become much more precise, allowing us to do better predictions. And, finally, the Pluto-Charon system is presently crossing the Milky Way, thereby increasing the likelihood of an occultation."

The July 2005 event was eventually observed from Paranal with Yepun, the fourth Unit Telescope of the VLT, equipped with the adaptive optics instrument NACO, as well as with the 0.5m "Campo Catino Austral Telescope" at San Pedro de Atacama (Chile), and with the 2.15m "Jorge Sahade" telescope at Cerro El Leoncito (Argentina).

An accurate timing of the occultation seen at the three sites provides the most accurate measurement of Charon’s size: its radius is found to be 603.6 km, with an error of the order of 5 km.

This accuracy now allows astronomers to pin Charon’s density down to 1.71 that of water, indicative of an icy body with about slightly more than half of rocks. Quite remarkably, Charon’s density is now measured with much more precision than Pluto’s.

Thanks to these observations, Sicardy and his collaborators could determine that if an tenuous atmosphere exists on Charon, linking it to the freezing ­‑220­ degrees centigrade or so surface, its pressure has to be less than one tenth of a millionth that at the surface of the Earth, or 0.1 microbar, assuming that it is constituted entirely of nitrogen.

A similar upper limit is derived for a gas like carbon monoxide. This is more than a factor one hundred smaller than Pluto’s surface pressure, which is estimated to be in the range 10-15 microbars.

"Comparing Pluto and Charon, we seem to cross a borderline between bodies which may have bound atmospheres - like Pluto - and airless bodies like Charon", said Olivier Hainaut, from ESO and member of the team.

The observations also indicate that methane ice, if present, should be restricted to very cold regions of the surface. Similarly, nitrogen ice would be confined at best to high northern latitudes or permanently shadowed regions of Charon.

As Pluto and its satellite sweep across the Milky Way, observations of more occultations will be tempted from the ground, while the NASA’s Pluto-Kuiper Belt Mission, to be launched in January 2006, will be travelling towards the planet, that it should reach in July 2015.

A report of these results is to be published in the January 5, 2006 issue of Nature ("Charon’s size and upper limit on its atmosphere from a stellar occultation", by B. Sicardy, A. Bellucci, E. Gendron, F. Lacombe, S. Lacour, J. Lecacheux, E. Lellouch, S. Renner, S. Pau, F. Roques, T. Widemann, F. Colas, F. Vachier, N. Ageorges, O. Hainaut, O. Marco, W. Beisker, E. Hummel, C. Feinstein, H. Levato, A. Maury, E. Frappa, B. Gaillard, M. Lavayssière, M. Di Sora, F. Mallia, G. Masi, R. Behrend, F. Carrier, O. Mousis, P. Rousselot, A. Alvarez-Candal, D. Lazzaro, C. Veiga, A.H. Andrei, M. Assafin, D.N. da Silva Neto, R. Vieira Martins, C. Jacques, E. Pimentel, D. Weaver, J.-F Lecampion, F. Doncel, T. Momiyama, and G. Tancredi).

Contacts
Bruno Sicardy
Paris Observatory, France
Mobile: +33-(0)6-19 41 26 15
Email: Bruno.Sicardy@obspm.fr

Olivier Hainaut
ESO, Chile
Phone: +56 55 43 5336 or +56 2 463 3118
Email: ohainaut@eso.org

Henri Boffin | EurekAlert!
Further information:
http://www.eso.org/outreach/press-rel/pr-2006/pr-02-06.html
http://www.eso.org

More articles from Physics and Astronomy:

nachricht CCNY physicists master unexplored electron property
26.07.2017 | City College of New York

nachricht Large, distant comets more common than previously thought
26.07.2017 | University of Maryland

All articles from Physics and Astronomy >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: Carbon Nanotubes Turn Electrical Current into Light-emitting Quasi-particles

Strong light-matter coupling in these semiconducting tubes may hold the key to electrically pumped lasers

Light-matter quasi-particles can be generated electrically in semiconducting carbon nanotubes. Material scientists and physicists from Heidelberg University...

Im Focus: Flexible proximity sensor creates smart surfaces

Fraunhofer IPA has developed a proximity sensor made from silicone and carbon nanotubes (CNT) which detects objects and determines their position. The materials and printing process used mean that the sensor is extremely flexible, economical and can be used for large surfaces. Industry and research partners can use and further develop this innovation straight away.

At first glance, the proximity sensor appears to be nothing special: a thin, elastic layer of silicone onto which black square surfaces are printed, but these...

Im Focus: 3-D scanning with water

3-D shape acquisition using water displacement as the shape sensor for the reconstruction of complex objects

A global team of computer scientists and engineers have developed an innovative technique that more completely reconstructs challenging 3D objects. An ancient...

Im Focus: Manipulating Electron Spins Without Loss of Information

Physicists have developed a new technique that uses electrical voltages to control the electron spin on a chip. The newly-developed method provides protection from spin decay, meaning that the contained information can be maintained and transmitted over comparatively large distances, as has been demonstrated by a team from the University of Basel’s Department of Physics and the Swiss Nanoscience Institute. The results have been published in Physical Review X.

For several years, researchers have been trying to use the spin of an electron to store and transmit information. The spin of each electron is always coupled...

Im Focus: The proton precisely weighted

What is the mass of a proton? Scientists from Germany and Japan successfully did an important step towards the most exact knowledge of this fundamental constant. By means of precision measurements on a single proton, they could improve the precision by a factor of three and also correct the existing value.

To determine the mass of a single proton still more accurate – a group of physicists led by Klaus Blaum and Sven Sturm of the Max Planck Institute for Nuclear...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

Event News

Clash of Realities 2017: Registration now open. International Conference at TH Köln

26.07.2017 | Event News

Closing the Sustainability Circle: Protection of Food with Biobased Materials

21.07.2017 | Event News

»We are bringing Additive Manufacturing to SMEs«

19.07.2017 | Event News

 
Latest News

CCNY physicists master unexplored electron property

26.07.2017 | Physics and Astronomy

Molecular microscopy illuminates molecular motor motion

26.07.2017 | Life Sciences

Large-Mouthed Fish Was Top Predator After Mass Extinction

26.07.2017 | Earth Sciences

VideoLinks
B2B-VideoLinks
More VideoLinks >>>