x rays
X-rayXenonXeroxSince the question asked for "three-letter" words beginning with "X", how about "xis"?
· weightlessness
Hubble telescope
Jupiter
x-ray
Xmas
A space word that begins with T is trajectory.
xunnig
n.a.s.a, Neptune, Nebula
· Zero gravity · Zodiac
X-Rays X-Rays from the sun
zoolonigeer