David Jewitt and Herve Aussel

Institute for Astronomy, University of Hawaii

and Aaron Evans

Dept. Physics and Astronomy, State University of New York at Stony Brook

NATURE, 2001, 411, 446-447 (May 24 issue)

Observations within the last decade have revealed the existence of a large number of bodies in orbit about the sun beyond Neptune. These bodies, commonly known as Kuiper Belt Objects (KBOs), are products of agglomeration in the rarefied outer regions of the protoplanetary disk of the sun. Scientific interest focuses on the primitive nature of the KBOs and on their role as the likely source of short-period comets. Unfortunately, the KBOs are difficult astronomical targets, so that even such basic physical properties as the sizes and albedos remain unknown. Here we report the first simultaneous thermal and optical measurements of a bright KBO and use them to solve separately for the albedo and size. (20000) Varuna has equivalent circular diameter D = 900(+125/-145) km and red geometric albedo p = 0.070(+0.030/-0.017). The surface is darker than Pluto, suggesting a composition largely devoid of fresh ice, but higher than the canonical albedo of 0.04 previously assumed for these bodies.

PS Figure PDF Figure TIFF Figure

Blink Pair showing the motion of (20000) Varuna. The interval is 1 day. R-band, 250-sec integrations from the UH - 2.2 meter telescope.

The individual pictures from Day 1 (2001 Apr 24) and Day 2 (2001 Apr 25). Images by Scott Sheppard.

The PS version of the paper and the PDF version, too..

The editorial by Tegler and Romanishin and the Trout piece.

Additional Figures from the paper:

PS Figure 1, PDF Figure 1 and PS Figure 2, PDF Figure 2.

After we measured the size, we went back to measure the rotation period and shape of Varuna. To our surprise, we were able to estimate the density of the object. The low value (1000 kg per m**3) is due to internal porosity. The resulting paper is here as a PDF.

Last updated May 2002

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