Physical and Mutual Orbit Characteristics of Binary PHA 1998 QE2

We present the physical and mutual orbit characteristics of potentially hazardous binary asteroid (285263) 1998 QE2. We use 9 days of Arecibo radar observations from 2013 and lightcurve observations spanning two months from 2013. Previously, preliminary shape models of 1998 QE2 indicated two possible spin-axis orientation solutions, both in the northern celestial hemisphere, with differences mainly attributed to the location of bumpy terrain. We now present a single likely solution based on the addition of lightcurves and fine-grain grid tests of orientation. 1998 QE2 has a volume-equivalent diameter of ~3 km, a rotation period of 4.74 h, and a spin-axis orientation of (119°, 35°) in ecliptic coordinates. The secondary component has an equatorial orbit about the primary with an orbital period of 31.74 h, semi-major axis of 6.3 km, and volume-equivalent diameter of 0.8 km. 1998 QE2 has a density of 0.7 g cm^-3 obtained via the primary’s size and the secondary’s mutual orbit parameters. Based on the density, 1998 QE2 is likely a porous, rubble pile type asteroid. The updated shape model provides key information about the bumpy surface terrain that is necessary for accurate thermophysical models.