See space rock 2018 RC whizz by Earth closer than the Moon this weekend

By Ade Ashford

This graphic shows the orbit of Near-Earth Object 2018 RC and the terrestrial planets to scale with their positions at 12:08 UT (1:08pm BST) on 9 September – the instant that the asteroid passes just 218,300 kilometres from the surface of our planet, or less than 60 percent of the Moon’s average distance. Click the graphic to open an interactive orbit model in a new window. AN graphic by Ade Ashford/NASA JPL Small-Body Database Browser.
It would seem that near-Earth asteroids are rather like buses: you can wait ages for one, then two come along in quick succession. Shortly after I told you about 2015 FP118, news broke of the first observations of 2018 RC at ATLAS-MLO Hawaii on 3 September 2018.

Believed to be up to 70 metres in size – the same as the wingspan of a Boeing 747 Jumbo Jet – Aten-type asteroid 2018 RC has a 289¾-day orbit that brings it to within 0.653 astronomical unit (AU) of the Sun at perihelion, out to 1.061 AU at aphelion.

Earth has a very close encounter with 2018 RC at 12:09 TDB (1:08pm BST) on Sunday, 9 September 2018, the asteroid passing just 218,300 kilometres from the surface of our planet, or slightly less than 60 percent of the Moon’s average distance.

Observing 2018 RC from the UK on the night of 8–9 September
Despite the asteroid’s relatively small size, its proximity to Earth this weekend means that it could attain magnitude +12.1, making it a viable target for 6-inch (15-cm) aperture and larger backyard telescopes. Saturday night to Sunday morning in the UK finds the asteroid in southern Aquarius, some 9 degrees north of first-magnitude star Fomalhaut in the constellation of Piscis Austrinus, the Southern Fish.

The UK observing window for viewing 2018 RC at its brightest lasts from approximately 10pm BST on Saturday, 8 September to 3am BST the following morning. The asteroid is best seen highest in the southern sky of the British Isles around 1am BST on 9 September when it’s moving against the background stars of Aquarius at a rate equivalent to the width of the full Moon every 12 minutes.

Near-Earth Object 2018 RC is observable at its brightest from the British Isles during an observing window that lasts from approximately 10pm BST on Saturday, 8 September to 3am BST the following morning. The asteroid moves 12⅔° against the stars of southern Aquarius within this five-hour interval, attaining a predicted visual magnitude of +12.1. For an observer in the heart of the UK, 2018 RC reaches a peak altitude of 15° in the southern sky some 9° north of first-magnitude star Fomalhaut (aka α Piscis Austrini) close to 1am BST on Sunday, 9 September. For scale, this star chart is about twice the span of an outstretched hand at arm’s length wide. Stars to magnitude +6 are shown. AN graphic by Ade Ashford.
Finding 2018 RC the easy way
Observers with computerised GoTo mounts or digital setting circles may prefer to locate the asteroid from the half-hourly topocentric ephemeris computed for the centre of the British Isles found at the bottom of the page.

See asteroid 2018 RC live online
If you are clouded out or don’t have a telescope large enough to see or image it, then why not follow this fascinating NEO online? The Virtual Telescope Project 2.0 is hosting a live event starting at 22:00 UTC (11pm BST) on Saturday, 8 September.

The table above shows the position of asteroid 2018 RC every 30 minutes for the interval that it’s brightest and visible from the UK. British Summer Time (BST) is used, so subtract one hour for Universal Time. Topocentric central UK equatorial coordinates of right ascension (R.A.) and declination (Dec.) are for the current epoch (J2018.7) for direct entry into digital setting circles or GoTo mount hand controllers. The asteroid’s predicted visual magnitude (Mag.) and distance in astronomical units (Delta) is also shown. Data credit: Minor Planet Center / NASA HORIZONS / Ade Ashford.