Asteroid 2018 XG4 passed by the Earth at a distance of about 1 046 000
km (2.72 times the average distance between the Earth and the Moon, or
0.70% of the distance between the Earth and the Sun), slightly after 1.25 am
GMT on Saturday 15 December 2018. There was no danger of
the asteroid hitting us, though were it to do so it would not have
presented a significant threat. 2018 XG4 has an estimated
equivalent
diameter of 5-18 m (i.e. it is estimated that a spherical object
with
the same volume would be 5-18 m in diameter), and an object of this
size
would be expected to explode in
an airburst (an explosion caused by superheating from friction with the
Earth's atmosphere, which is greater than that caused by simply
falling, due to the orbital momentum of the asteroid) in the atmosphere
between 40 and 24 km above the ground, with only fragmentary material
reaching the Earth's surface.
The calculated orbit of 2018 XG4. Minor Planet Center.
2018 XG4 was discovered on 14 December 2018 (the day before its closest approach to the Earth) by the
University of Arizona's Mt. Lemmon Survey at the Steward Observatory
on Mount
Lemmon in the Catalina Mountains north of Tucson. The designation 2018
XG4 implies that the asteroid was the 107th object (object G4) discovered
in the first half of December 2018 (period 2018 X).
2018 XG4
has an 1000 day orbital period and an eccentric orbit
tilted at an angle of 6.55° to the plane of the Solar System, which
takes it from 0.98 AU from the Sun (i.e. 98% of he average distance at
which the Earth orbits the Sun) to 2.93 AU from the Sun (i.e. 2.93% of
the
average distance at which the Earth orbits the Sun, ,almost twice as far from the Sun as the Planet Mars). It is therefore
classed as an
Apollo Group Asteroid (an asteroid that is on average further from the
Sun than the Earth, but which does get closer).
