We present new time-resolved photometric observations of the bright trans-Neptunian object ( 20000 ) Varuna and use them to study the rotation period , shape , and color .
In observations from 2001 February and April , we find a best-fit two-peaked lightcurve with period 6.3442 \pm 0.0002 hr .
The peak-to-peak photometric range in the R-band is 0.42 \pm 0.02 mag .
We find no rotational variation in colors over the 0.45 \leq \lambda \leq 0.85 \micron wavelength range .
From the short double-peaked period and large amplitude we suggest that Varuna is an elongated , prolate body perhaps close in shape to one of the Jacobi ellipsoids .
If so , the ratio of the axes projected into the plane of the sky is 1.5:1 and the density is near 1000 kg m ^ { -3 } .
( 20000 ) Varuna may be a rotationally distorted rubble pile , with a weak internal constitution due to fracturing by past impacts .
The high specific angular momentum implied by our observations and recent detections of binary Trans-Neptunian Objects both point to an early , intense collisional epoch in which large Trans-Neptunian Objects were \sim 100 times more abundant than now .
In order to maintain a cosmochemically plausible rock : ice mass ratio \sim 0.5 , Varuna must be internally porous .