An asteroid classed as potentially hazardous, because its orbit can cross the Earth’s, is making its closest flyby since 1890. It will be visible in binoculars and small telescopes.
The 5 km (3 mile) wide chunk of space rock, named after Crimean War nurse Florence Nightingale, will reach magnitude 8.5. But though this encounter is unusually close, there is no danger of a collision. The asteroid will miss us by 7,066.600 km (4,391,000 miles), which is nearly 18 times further away than the Moon.
Discovered in March, 1981, by U.S. astronomer Schelte Bus, from Siding Spring Observatory, New South Wales, Australia, the asteroid is formally classified as 3122 Florence. It orbits the Sun once every 2 years and 4 months (859 days) but will not come as close to Earth again until after the year 2500.
The current close approach means that although the asteroid is tiny compared to the planets, it will become bright enough for amateur astronomers to see without too much difficulty. And once you have found it, its motion against the starry background will become obvious from hour to hour.
On the date this story is being published, 31 August, 2017, the asteroid is rapidly heading north through the constellation of Aquarius. Around midday U.T. (Universal Time) it nips into the small neighbouring star grouping of Equuleus, before entering another compact constellation, Delphinus.
Delphinus’s brighter stars form a diamond pattern, and the asteroid will be especially easy to locate when it enters this diamond and passes close to the double-star Gamma (γ) at around 0h U.T. on 3 September.
How to find asteroid 3122 Florence
Asteroid 3122 Florence is well-placed for viewing through out the night from the northern and southern hemispheres. Our charts are with north at the top, and readers at southern latitudes will see the sky the other way up!
Our coloured map gives a general, wide angle view of the area of sky though which the asteroid passes over the next few nights. This region can be seen towards the south east as soon as the sky gets dark after sunset. It will then swing round towards the west during the night, as the Earth rotates.
The detailed black and white charts, produced using Cartes du Ciel software, will help locate the asteroid with your binoculars or small telescopes. The asteroid will appear as a star-like point of light. Don’t expect to see anything more. However, what will be obvious, if you are patient, is the asteroid’s rapid motion across the starry background, compared to the speed of planets.
If you are using binoculars, it will help to make sure you are comfortable. Sit in a deckchair, or lean against a wall, to help support you as you hold the binoculars steady.
You could also attempt to photograph the asteroid’s flyby if you have a camera capable of taking time exposures. Light from the waxing Moon nearby will limit your exposure times, but if you set the “film speed” to ISO 200 or 400, an exposure of 10 to 15 seconds should show the asteroid.
Longer exposures, or faster film speeds, risk being washed out by moonlight. You will need to focus your camera lens manually, by setting it at infinity, as stars are usually too faint for autofocus to work. It is difficult to focus on the camera’s viewscreen unless you can find a bright star in the area, such as Altair in Aquila, and focus on that.
Take a number of shots to experiment and see what you get. The beauty of digital photography is that you get the chance to check your settings immediately! If it stays clear and you are able to photograph the patch of sky containing the asteroid every few minutes, you should be able to see how quickly it has moved by comparing your photos. Do let us know, in the comments below, how you get on!
Update: Radar images of asteroid Florence, obtained with the 70-metre antenna at NASA’s Goldstone Deep Space Communications Complex between 29 August and 1 September have revealed that the asteroid has two small moons.
This makes it only the third triple near-Earth asteroid known. The tiny moons are probably between 100 – 300 meters (300-1000 ft) across. The inner moon takes approximately 8 hours to orbit Florence, and the outer moon 22 to 27 hours. Read more about this discovery on the NASA JPL blog.
Update 2: Here is an image showing asteroid Florence, taken by the author, on the evening of 2 September. Several short exposures were stacked on the asteroid (at center), so that the stars appear as strings of dots.
Related: The night sky this month
★ Keep up with space news and observing tips. Click here to sign up for alerts to our latest reports. No spam ever - we promise!