  The formation of the largest objects in the Kuiper belt, with measured
densities of ~1.5 g cm-3 and higher, from the coagulation of small bodies, with
measured densities below 1 g cm-3 is difficult to explain without invoking
significant porosity in the smallest objects. If such porosity does occur,
measured densities should begin to increase at the size at which significant
porosity is no longer supported. Among the asteroids, this transition occurs
for diameters larger than ~350 km. In the Kuiper belt, no density measurements
have been made between ~350 km and ~850 km, the diameter range where porosities
might first begin to drop. Objects in this range could provide key tests of the
rock fraction of small Kuiper belt objects. Here we report the orbital
characterization, mass, and density determination of the 2002 UX25 system in
the Kuiper belt. For this object, with a diameter of ~650 km, we find a density
of 0.82+/-0.11 g cm-3, making it the largest solid known object in the solar
system with a measured density below that of pure water ice. We argue that the
porosity of this object is unlikely to be above ~20%, suggesting a low rock
fraction. If the currently measured densities of Kuiper belt objects are a fair
representation of the sample as a whole, creating ~1000 km and larger Kuiper
belt objects with rock mass fractions of 70% and higher from coagulation of
small objects with rock fractions as low as those inferred from 2002 UX25 is
difficult.
