See asteroid Florence’s close dolphin encounter on 2—3 September

By Ade Ashford

The motion of near-Earth asteroid 3122 Florence against the background stars of Aquarius is clear to see in this two-minute exposure concluding at 22:44 UT (11:44pm BST) on 31 August. The 4.4 kilometre-wide Amor group asteroid was magnitude +8.7 and 0.047 astronomical units, or 7 million kilometres, from Earth at the time. Image credit: Ade Ashford, using a HyperStar-equipped Celestron C11 with a Starlight Xpress Ultrastar C camera.
During the first week of September, observers have an opportunity to see a bright near-Earth asteroid known as 3122 Florence (aka 1981 ET3) as it sails by our planet. Some 4.4 kilometres (2.7 miles) in diameter and orbiting the Sun every 2⅓ years, this Amor-group asteroid was discovered in 1981 and is the fourth-largest NEO known.

Around midday UK time on 1 September, 3122 Florence passed little more than 7 million kilometres (4.39 million miles), or 18 lunar distances, from Earth. This was its closest approach since 1890 and the closest it will get for 500 years. Given its size and current proximity, the asteroid will be brighter than the tenth magnitude for the next few days as it tracks across the Milky Way, hence it’s easy object for small telescopes despite the light of a waxing gibbous Moon.

On the UK night of 2—3 September (Saturday night, Sunday morning) 3122 Florence lies 0.049 astronomical units (7.33 million kilometres) from our planet, travelling at a rate of 9 degrees/day against the stars of Delphinus, a small constellation better known as the dolphin. The following wide-angle star chart shows you how to locate Delphinus.

The principal stars of the diminutive summer constellation of Delphinus the dolphin are encompassed within the field of view of wide-angle binoculars, easily located from its proximity to the summer triangle. Gamma (γ) Delphini represents the snout of this celestial aquatic mammal, the upper-left vertex of a four-star asterism sometimes known as “Job’s Coffin”. On the UK night of 2—3 September, ninth-magnitude near-Earth asteroid 3122 Florence lies within this asterism, seen in greater detail in the finder chart below. The five-degree marks on the scale bar equate to the field of view of a 10×50 binocular, while 20 degrees is the span of an outstretched hand at arm’s length. AN illustration by Ade Ashford.
Once you have located Delphinus with the naked eye in relation to the Summer Triangle of stars Deneb, Vega and Altair (avoiding streetlights and masking the 11-day-old waxing gibbous Moon with a tree or wall will help you locate the principal stars outlining the dolphin), turn your telescope towards the diamond-shaped four-star “Job’s Coffin” asterism defining the head of Delphinus, starting with your lowest power eyepiece.

On the night in question, 3122 Florence is predicted to shine at magnitude +9.1, considerably brighter than the faintest field stars shown in the detailed finder chart below. For scale, this chart is about five degrees wide — roughly the field of view of a 10×50 binocular. The asteroid is moving at a rate of about 9 degrees/day this night, or slightly more than 22 arcseconds/minute. Put simply, it’s travelling at a rate equal to the apparent angular size of planet Jupiter every two minutes. In 10cm aperture telescopes and larger at magnifications of 100x or more, the realtime motion of 3122 Florence should be readily observable.

This detailed finder chart shows a five-degree-wide section of Delphinus centred on the four-star asterism known as “Job’s Coffin” traversed by magnitude +9.1 asteroid 3122 Florence on the night of 2—3 September. Stars to magnitude +10.5 are shown with the asteroid’s position shown at hourly intervals. Note that the times are Universal Time (UT), so add one hour for British Summer Time. Click the chart for a full-size PDF suitable for printing and use outside at the telescope. AN graphic by Ade Ashford.