# Can James Webb see Kuiper Belt Objects like 2014 MU69 (Ultima Thule)?

In this paper, we learned that Hubble is the only telescope in the vicinity of Earth that can detect Ultima Thule directly. Hubble will eventually fail, and will be somewhat replaced by James Webb. James Webb will operate on a completely different frequency, however. Will James Webb be able to detect Ultima Thule, or objects like it?

• It's an interesting problem! With an albedo of say 0.1, 10% of the sunlight that hits it reflected in the visible (roughly speaking) and 90% is re-radiated in thermal IR (though more widely) so you might think a dark asteroid is always brighter in thermal IR than reflected visible. The problem is that once you get past (very rough) 10 AU, the equilibrium temperature is so low that most of the "thermal" IR is much longer than 30 microns and so outside of the JWST range. So at very large distances, visible light wins. But for NEOs thermal IR wins like crazy. – uhoh Jan 7 '19 at 16:03
• No time to do the full calculation right now, might in a few days. See Planck distribution and approximate equilibrium temperature for the math. Cows are spheres and π = 1. – uhoh Jan 7 '19 at 16:06

This page gives us this equation, which gives a resulting thermal equilibrium as follows: $$280K [1-A]^{1/4}/a^{1/2}$$