At 23:28 on Tuesday evening (8 November), the asteroid 2005 YU55 will pass closer to Earth than any other object is expected to for the next 17 years. But there is no need for alarm. The 400m-diameter chunk of rock will still be 324,604km (201,700 miles) away and, contrary to some reports in the tabloid press, will not pass between the Earth and the Moon.
The Solar System is littered with small asteroids in orbit around the Sun. Most of them reside in a region between the orbits of Mars and Jupiter, but not all of them stick to this region, known as the asteroid belt. Those, like 2005 YU55, that venture close to the Earth are known as near-Earth asteroids, or NEAs for short.
As technology has improved, so has our ability track such objects, and the Torino scale, which categorises asteroids and other near-Earth objects according to the threat posed to Earth, has been in use since 1995. YU55’s approach will be the closest by an object of this size yet to have been predicted in advance – the asteroid will pass just 0.84 lunar distances from Earth – but the asteroid is still rated as ‘zero risk’ on a scale of 0-10. The next ‘near miss’ will come in 2028, when asteroid 2001 WN5 will pass within 0.6 lunar distances.
The asteroid will not be visible to the naked eye, but observers with a 6-inch or larger telescope should be able to catch a glimpse of it for the next couple of nights, though its high velocity and the light of the Moon will make it a tricky one to catch.
But while amateur astronomers may struggle to spot YU55, professional observatories will be making a wide range of observations. Asteroids are some of the most ancient objects in the Solar System, and the close passage of this one will give scientists an opportunity to learn a lot more about their structure and composition. NASA’s Deep Space Network antenna and Arecibo Planetary Radar Facility (in Goldstone, California and Puerto Rico, respectively) will be key to such observations.
Observing the asteroid
These charts show the path of the asteroid starting at 00:00 UT on the 9th (midnight on the 8th). Its position in the sky will vary slightly depending where you are in the UK – if you want really accurate positions customised for your own location, these can be generated here.
At the start of the chart track, 2005 YU55 will be around mag. +11.5, until it’s lost in the dawn twilight on the 9th. Before this happens, the asteroid will pass quite close to the globular cluster M15 in Pegasus, an encounter best seen between 04:00 UT and 05:00 UT on the 9th. Over the course of the next night (18:00 UT on the 9th to 06:00 UT on the 10th), 2005 YU55 will dim from mag. +12 to around +13, with the virtue that it will be moving slower at this time. This is the best time to attempt so spot the asteroid, though you will need at least a 6-inch scope.
As the asteroid’s distance from Earth increases, so its apparent speed against the stars slows, along with a dimming of brightness. By the early morning of the 11th, it’ll be around magnitude +14.0.