It would seem that All Hallows’ Eve is a good time to spot near-Earth objects (NEOs). In 2015 we had the close passage of an appropriately skull-shaped dead comet, designated 2015 TB145, that missed our planet by just 1.3 lunar distances, or about 302,000 miles (486,000 kilometres). This year, it’s the turn of asteroid 164121 (2003 YT1) to pass Earth by a somewhat safer margin — 3.2 million miles (5.2 million kilometres) at 9:24am GMT on 31 October 2016.
NEO 164121 (2003 YT1) is a 1.7-kilometre-(1.1-mile)-wide Apollo asteroid discovered by the Catalina Sky Survey on 18 December 2003. It completes an orbit every 427 days in a somewhat eccentric ellipse that carries it to within 73 million miles (118 million kilometres) of the Sun. The asteroid also has an orbit with a relatively high inclination of 44 degrees, which has interesting consequences for the object’s visibility over the next few days.As the last day of October 2016 dawns, 2003 YT1 is already a circumpolar object for observers in the British Isles where it will remain above the horizon 24/7 until the third week of November. The asteroid’s motion is northerly, almost entirely in declination at the present time.
Photographing 2003 YT1 near Polaris: no expensive tracking mounts required!
As seen from the heart of the British Isles, 2003 YT1 passes just 4.6 arcminutes — less than one-tenth of a degree — from Polaris at 02:30 UT (2:30am GMT) on 2 November. Since this near-Earth asteroid passes so close to the pole star, an interesting astrophotographic opportunity presents itself: almost anyone with a tripod-mounted DSLR camera who can get to a dark sky site can have a go at capturing 2003 YT1.Any object passing this close to the north celestial pole will be almost stationary in the sky for the duration of an exposure that will be long enough to record it. An 85mm lens, for example, used with a typical APS-C sensor DSLR will deliver a field of view almost 15 x 10 degrees in extent. (Our online DSLR Calc web app will enable you to work out the field of view of other lens/camera combinations.) Recall that the asteroid passes less than one-tenth of a degree from Polaris at closest approach, so the use of a longer focal length lens means that the NEO’s faint trail is less likely to be lost in the glare of the pole star.
Simply focus your chosen DSLR lens on infinity at full aperture (you can use a distant landmark during the daytime for this, being careful not to disturb the focus of the lens until nightfall), set the ISO rating to 800 or 1600, point at Polaris and shoot some 30 second, 1 or 2-minute exposures with a cable release. If you have the appropriate software on your computer, stacking several of these carefully registered images (each of the same exposure time) will improve signal-to-noise ratio and 2003 YT1 will be identified as the dot that moves between images.
Viewing 2003 YT1 near Polaris
If you are fortunate to have clear skies around 02:00 UT (2am GMT) on 2 November, note that this is a rare opportunity for users of undriven telescopes such as Dobsonians to observe an object at length without the need for tracking. In fact, owners of telescopes on alt-azimuth mounts are at a distinct advantage here since equatorial mounts are more difficult to use so close to the celestial pole.
If you have a telescope with an aperture of 6-inches (150-millimetres) or larger, get Polaris within the field of view of your medium- to high-power eyepiece close to the appointed time and look for the twelfth-magnitude speck of light that is 2003 YT1 moving against the background stars of Ursa Minor at a rate of 12 degrees per day, or half a lunar diameter per hour.
Ephemeris of 2003 YT1 for the UK
Inside the magazine
For a comprehensive guide to observing all that is happening in the current month’s sky, tailored to Western Europe, North America and Australasia, obtain a copy of the November 2016 edition of Astronomy Now.
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