Ultima Thule (Minor planet designation: (486958) 2014 MU69) is a trans-Neptunian object visited by the New Horizons space probe on 1 Jan. 2019, the farthest object in the Solar System to visited up to that date. It is a contact binary with a long axis 31 km in length. Ultima Thule is classified as a cubewano based on its location and orbit.

As of 9 Jan. 2019, the mass of Ultima Thule has not been announced. The following size and mass information is for other cubewano objects for comparison. (Mass is known for few cubewano objects.)

BodyRadius (km)Mass (kg)Spherical vol. (km^3)Density (g/cm^3)
Makemake (136472)7154.4x10^2117000000002.3
Quaoar (50000)5551.4x10^217160000002.2
Salacia (120347)4250.438x10^213210000001.16
Varda (174567)3582.65x10^201920000001.25
2004 UX25 (55637)3351.25x10^201570000000.82
Varuna (20000)3343.7x10^201560000000.99
Sila (79630)12511x10^1881800001.3*
Logos (58534)40270x10^152680001.0*
Ultima Thule (486958)16?1.4 (avg. of above)

* Calculated from body size and mass.

The shape of Ultima Thule is approximately an ellipsoid with a circular cross-section perpendicular to the major axis. Semi-minor axis of approximately 10 km gives a volume of approximately 6700 km^3.

If Ultima Thule is of average density for known cubewano objects, it would have a mass of approximately 9.4 x 10^15 kg.

(1 g/cm^3 = 1x10^12 kg/km^3)

(10^15 kg = 1 Eg (exagram))

Source: List of Solar System objects by size. Wikipedia. Accessed 9 Jan. 2019.