Asteroid 2020 SO passed by the Earth at a distance of about 225 100
km (0.58 times the average distance between the Earth and the Moon, or
0.15% of the distance between the Earth and the Sun), slightlybefore 9.40 pm
GMT on Tuesday 2 February 2021. There was no danger of
the asteroid hitting us, though were it to do so it would not have
presented a significant threat. 2020 SO has an estimated
equivalent
diameter of 5-15 m (i.e. it is estimated that a spherical object
with
the same volume would be 5-15 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) between 40 and 27 km
above the ground, with only fragmentary material
reaching the Earth's surface.
120 second image of 2020 SO taken with the Elena Planetwave 17" Telescope
at Ceccano
in Italy on 1 December 2020. The asteroid is the small point at the
centre of the image, indicated by the white arrow, the longer lines are
stars, their elongation being
caused by the telescope tracking the asteroid over the length of the
exposure. Inset a 60 second exposure showing the movement of the
asteroid, showing a pattern of brightening and darkening with a ten
second period and a double bright peak; this sugests the object has a
ten second rotational period. Gianluca Masi/Virtual Telescope. 2020 SO was first detected on 17 September 2020, by the University of Hawaii's PANSTARRS telescope. The
designation 2020 SO implies that it was the 20th asteroid (asteroid O -
in numbering asteroids the letters A-Y, excluding I, are assigned
numbers from 1 to 24, with a number added to the end each time the
alphabet is ended, so that A = 1, A1 = 25, A2 = 49, etc., which means that O = 20)
discovered in the second half of September 2020 (period 2020 S).
At the time of its discovery 2020 SO was found to have a
386 day (1.06 year) orbital period, with an elliptical orbit tilted at
an angle of 0.14° to the plain of the Solar System which takes in to
1.00 AU from the Sun (the distance at which the Earth orbits the
Sun) and out to
1.07 AU (7%
further away from the Sun than the Earth). As such it was classified as an Apollo Group Asteroid (an asteroid that is on average further from
the Sun
than the Earth, but which does get closer).
Animation of the orbit of 2020 SO (pink) and the Earth (blue) around Sun. Wikimedia Commons.
However, in November last year 2020 SO entererd the Earth's Hill sphere (an astronomical body's Hill radius is the radius of the surrounding
spherical region, the Hill sphere, within which smaller body's would
tend to orbit the body) and became captured as a 'second moon' of the Earth. Since this time the body has had close encounters with the Earth on 1 December 2020 and 2 February 2021, as well as repeated close encounters with the Moon, on 1 and 28 November, 22 December, and 23 January, and 3 April. 2020 SO will have one further close encounter with the Moon on 12 March 2021, before moving onto a new, Sun-orbiting path.
An animation of the nominal trajectory of asteroid 2020 SO. Wikimedia Commons.
The new orbit of 2020 SO will be tilted at an angle of 0.14° to the plain of the Solar System and take in to
0.98 AU from the Sun (98% of the distance at which the Earth orbits the
Sun and out to 0.99 AU (99% of the distance at which the Earth orbits
the Sun). As a Near
Earth Object that remains strictly inside the orbit of
the Earth 2020 SO will be classed as an Atira Family Asteroid. This means that 2020 SO will have further close encounters with the Earth, with the next predicted in January 2036.
A a press release issued by the European Space Agency on 12 January 2021 suggested 2020 SO is an old rocket part from a failed 1966 robotic mission to the Moon. The object was being pushed by radiation from the Sun.
Such solar pressure is so weak that astronomers knew the object in
question must be very light, and this suggested it was in fact a
human-made, artificial object (which tend to be far lighter and less
dense than natural bodies formed in space).
Astronomers traced the orbit of this mysterious object back in time and
now believe it is in fact the upper stage of a Centaur Rocket that left
Earth on 20 September 1966 carrying the Surveyor 2 lander bound for the
Moon. The mission failed, and the upper stage of the rocket, 12-metres
long by 3-metres wide, shot past the Moon and drifted off into orbit
around the Sun.
See also...
