Watch asteroid 2015 FP118 fly by Earth in early September

By Ade Ashford

This graphic shows the orbit of Near-Earth Object 2015 FP118 and the terrestrial planets to scale with their positions at 10:09 UT (11:09am BST) on 3 September – the instant that the asteroid passes just 4.7 million kilometres from our planet, or 12¼ times the average distance of the Moon. Click the graphic to open an interactive orbit model in a new window. AN graphic by Ade Ashford/NASA JPL Small-Body Database Browser.
An Apollo asteroid estimated to be up to 800 metres in size will safely fly by our planet on 3 September 2018. Now known as 2015 FP118, the Burj Khalifa-sized body was discovered by Pan-STARRS 1 on 21 March 2015. The asteroid has a 3.09-year-long orbit that carries it to within 0.954 astronomical units (AU) of the Sun at perihelion, out to 3.288 AU at aphelion.

At 10:09 UT (11:09am BST) on 3 September 2018, 2015 FP118 passes just 4.7 million kilometres from Earth, or 12¼ times the average distance of the Moon. FP118 features on the Minor Planet Center list of Potentially Hazardous Asteroids (PHAs), but no approach as close as the September 2018 flyby occurs within the next 173 years.

This extract of a high-resolution PDF finder chart suitable for printing obtained by clicking on the graphic shows the track of 2015 FP118 shortly after its closest approach to Earth on 3 September. The asteroid is brightest on the UK night of 6–7 September, visible in telescopes of 10-inch (25-cm) aperture and larger as a magnitude +14.5 dot moving against the stars of Cassiopeia at a rate equivalent to the diameter of the full Moon every 100 minutes. Photographically, FP118 can be imaged with much smaller apertures. AN graphic by Ade Ashford.
Finding FP118 on the night of 4–5 Sep 2018
Although closest to Earth on 3 September, 2015 FP118’s phase angle is changing and its brightness is still increasing as the asteroid gets farther away. As darkness falls on the UK night of 4 September you can find FP118 amid the stars of northern Cepheus close to magnitude +4.4 star pi (π) Cephei. The asteroid is predicted to glow at magnitude +14.7 this night, which means it’s a viable target for 10-inch (25-cm) aperture backyard ‘scopes and larger under moonless skies (note: the 24-day-old waning crescent Moon rises in the UK around 12:30am BST on 5 September). FP118’s motion against the background stars presently exceeds 9 degrees/day.

Observers with computerised GoTo mounts or digital setting circles may prefer to locate 2015 FP118 from the hourly topocentric ephemeris computed for the centre of the British Isles found at the bottom of the page.

The night of 5–6 Sep 2018
As darkness falls in Western Europe, the NEO is 13.3 lunar distances (5.1 million kilometres) from Earth and predicted to be magnitude +14.6 amid the stars of Cepheus. FP118 crosses the constellation border into Cassiopeia shortly before 1am BST on 6 September when it’s travelling at a rate close to one-third of a degree per hour against the stars.

The night of 6–7 Sep 2018
As darkness blankets the British Isles on 6 September, 2018 FP118 lies in an easily located position amid the familiar W-shaped constellation of Cassiopeia high in the northeastern sky. Throughout the hours of darkness the asteroid lies within the triangle bounded by gamma (γ), beta (β) and alpha (α) Cassiopeiae, stars more commonly known as Navi, Caph and Schedar, respectively. The asteroid is brightest this night, a magnitude +14.5 dot some 14½ lunar distances away moving towards Schedar at a rate equivalent to the diameter of the full Moon every 100 minutes.

The night of 7–8 Sep 2018
The asteroid is now approaching 16 lunar distances or 6 million kilometres from Earth and its motion against the stars of Cassiopeia has slowed to about the diameter of the full Moon every two hours. FP118 is predicted to be magnitude +14.6 this night.

The night of 8–9 Sep 2018
As dusk fades to dark in the UK on 8 September the asteroid lies in southern Cassiopeia between magnitude +4.5 star omicron (ο) Cassiopeiae and magnitude +4.3 phi (φ) Andromedae. The space rock is still around magnitude +14.6 as it crosses the constellation border into Andromeda close to 3am BST on 9 September.

The night of 9–10 Sep 2018
This is the night many astrophotographers may be waiting for since 2015 FP118 lies within 4 degrees east of the Andromeda Galaxy, Messier 31. Throughout the hours of UK darkness the magnitude +14.7 asteroid moves around 1½ degrees against the background stars, crossing a line drawn between magnitude +4.3 phi (φ) Andromedae and magnitude +3.8 mu (μ) Andromedae.

See asteroid 2015 FP118 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 21:30 UT (10:30pm BST) on 3 September.

The table above shows the position of asteroid 2015 FP118 every hour for the interval that it is brightest and visible in darkness from the UK. Universal Time (UT) is used, so add one hour for British Summer Time. Topocentric central UK equatorial coordinates 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.), distance in astronomical units (Delta) and interference from moonlight (Mn) is also shown. Data credit: Minor Planet Center / NASA HORIZONS / Ade Ashford.