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Asteroid 2017 FN1 caught on camera at closest approach

On the evening of 2017 March 20/21, Peter Birtwhistle managed a remarkable feat achieving a follow-up of a new discovery by the Mt. Lemmon Survey of a near-Earth asteroid measuring around just 3 metres across. Peter was the only observer, professional or amateur, to detect the object and his observations made it possible to define its orbit as reported in M.P.E.C. 2017-F47. Here's Peter's image of 2017 FN1 taken from Great Shefford in the UK when the NEO was just 6 minutes before closest approach to the Earth, passing a little further than the orbital distance of geostationary satellites:

For the full story, here's Peter's own account of his remarkable observation:


"Near-Earth Asteroid 2017 FN1 was discovered by the Mt. Lemmon Survey from three images taken starting at 08:32UT on 20th March and they managed to get another set of three positions for it two hours later.  It was already moving relatively fast, accelerating from 24 to 35" per minute during those two hours and probably only about 2 Lunar Distances (LD) from Earth. The Minor Planet Center added it to their NEO Confirmation Page (NEOCP) by 09:15UT but by nautical twilight in the UK that evening no other observers had reported it and it was predicted by the MPC to be somewhere in  a thin strip of sky up to 25 degrees long. The core uncertainty area was about 8 degrees long, aligned in the direction of motion (south-east to north-west) with the most likely location predicted to be at the far north-west  end of the strip. The NEO was obviously expected  to be very close to Earth by this time, with motion of over 800"/minute or 13 degrees per hour and brightness of about magnitude +16. The approach to Earth was so close that it would move from the north east of Leo at 19:30UT, rise nearly to the zenith but then set in the north by 22:00UT!

Objects with this amount of uncertainty and speed are almost always too difficult to recover, especially considering the CCD on my Meade 16" Schmidt-Cassegrain provides a field of view of only 0.3 x 0.3 degrees so a search for a very fast moving object in an area that size is highly unlikely to succeed.

However, I decided to use the the six positions Mt. Lemmon had reported to calculate an ephemeris using the orbit determination software FindOrb to see how it compared to the Minor Planet Center's prediction. FindOrb indicated a place 5 degrees south-east from the NEOCP nominal prediction and I started searching from the FindOrb prediction, working my way north-west towards the NEOCP's nominal, taking 9 one second exposures in each field. Although I didn't realise when I was taking the images, much to my amazement 2017 FN1 was caught in the first seven exposures of the very first field I exposed. However, by the time I had found it in those images I had already exposed a further four search fields extending a degree along the uncertainty area. It took some effort to detect as it was too faint to see on individual images and required stacking of various combinations of the images to be able to realise it was there at all. Sometimes luck is on your side.

2017 FN1 was moving at 790"/minute and was 0.33 Lunar Distances (LD) from Earth when I first detected it at 19:12 UT and it was followed for 40 minutes during which time it covered nearly 13 degrees of sky, had accelerated to a speed of 26 degrees an hour and was by then only 0.24 LD distant. I then went after some other new discoveries on the NEOCP, returning to try and find 2017 FN1 again about an hour later. It was within minutes of its closest approach at 0.16 LD or just 3.5 Earth diameters from the Earth's surface and moving so fast, 66 degrees per hour,  that I only managed to get the telescope positioned in the correct place once, recording it at 20:55:37UT as a 1.7 arc minute long streak coming into view from the eastern side of the field for just the last half of a 3 second exposure. At that speed it was passing through the entire field of view in 16 seconds.

No other observatories reported it and it was announced as 2017 FN1 in MPEC 2017-F47 on 21st March. Subsequently the JPL SENTRY system flagged it as a "virtual impactor" with a 1 in 3,450 chance of colliding with Earth in 16 years time. However, with an estimated diameter of only about 3 meters, nearly 10 times smaller than the Chelyabinsk meteor, it is too small to cause any concern."
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