Seek out 1998 OR2, the brightest predicted near-Earth asteroid encounter of 2020

By Ade Ashford

12 April 2020 Ade Ashford
This graphic shows the orbit of 52768 (1998 OR2), a near-Earth object (NEO) and potentially hazardous asteroid (PHA) that passes just 16.4 lunar distances (6.3 million kilometres) from Earth at 09:56 UT (10:56am BST) on Wednesday, 29 April 2020. The asteroid is visible in 15-cm (6-inch) aperture telescopes and larger during moonless nights throughout April. Click the graphic to open an interactive orbit model in a new window. AN graphic by Ade Ashford/NASA JPL Small-Body Database Browser.

If you’re feeling a bit disappointed with the breakup of Comet ATLAS (C/2019 Y4), console yourself by searching for a large near-Earth asteroid with the catchy official name of 52768 (1998 OR2) that is already visible with instruments as small as 15-cm (6-inch) aperture and will brighten to eleventh-magnitude by the end of April.

We have produced a detailed printable PDF finder chart for download that shows the track of 1998 OR2 when moonlight will not be a problem for observers in the British Isles as darkness falls, which for the heart of the UK is about 10:40pm in mid-April, or 11pm a week later (all times BST).

Asteroid 52768 (1998 OR2) shone at magnitude +12.5 and left a trail 1¼ arcminutes long against the stars of Cancer in this ten-minute exposure concluding at 22:04 UT on 19 April 2020. Ade Ashford used a TS 65mm f/6.5 astrograph and an astromodified Canon 6D at ISO1600. Background stars to magnitude +16.5 are shown.
Buzzing the Beehive
Astrophotographers may care to note that as night falls in the UK on the nights of 17 and 18 April, magnitude +12.7 1998 OR2 lies in the constellation of Cancer within 3 degrees of the beautiful naked-eye open star cluster Messier 44, better known as the Beehive Cluster, or Praesepe. On these two nights the asteroid leaves trails about 6 arcseconds long per minute of exposure time.

By new Moon on 23 April, 1998 OR2 is predicted to be magnitude +12 or brighter and crossing the constellation border of Cancer into Leo at a rate of close to 4¼ degrees per day. By the time that the asteroid passes closest to Earth at a distance of just 16.4 lunar distances (6.3 million kilometres) at 09:56 UT (10:56am BST) on Wednesday, 29 April, it’s predicted to peak at magnitude +10.8, moving at a rate of 6.9 degrees/day against the stars of Hydra.

Last quarter on 14 April means that the Moon is currently confined to the morning sky, so observers in the British Isles should look for near-Earth asteroid 52768 (1998 OR2) as soon night falls for much of the remainder of the month. Around 16 April, 1998 OR2 is a magnitude +13 object moving against the background stars of Cancer at a rate of 2 degrees per day. By 28 April, the asteroid is two magnitudes brighter and zooming through the stars of Hydra at 6½ degrees day. Click this graphic for a printable A4 PDF finder chart, while a two-hourly ephemeris for the period that 1998 OR2 is visible from the UK is located at the bottom of this page. AN graphic by Ade Ashford.

Discovered on 24 July 1998 by the Near-Earth Asteroid Tracking (NEAT) program at Haleakalā Observatory on the Hawaiian island of Maui, 1998 OR2 has a diameter of 2 to 4 kilometres and rotates once on its axis every 4.1 hours. The asteroid has a 1345-day (3⅔-year) orbit that brings it to within 1.0179 astronomical units (AU) of the Sun at perihelion, out to 3.7509 AU at aphelion.

Classified as a near-Earth object (NEO) and potentially hazardous asteroid (PHA), most of the time 1998 OR2 satisfies the criteria for an Amor-group body. However, gravitational perturbations can bring it closer to the Sun than Earth’s aphelion (1.017 AU), at which time the asteroid is reclassified into the Apollo group.

Finding 1998 OR2 the easy way
Observers with computerised GoTo mounts or digital setting circles may prefer to locate the asteroid from the two-hourly topocentric ephemeris computed for the heart of the UK below. Note that the coordinates are for the J2000.0 epoch and the tabulated times are in UTC, so you need to add one hour to obtain British Summer Time.

This topocentric ephemeris of near-Earth asteroid 52768 (1998 OR2) is computed for the centre of the British Isles (54°N, 2.5°W) on the Dates and UTC (= BST -1 hour) times shown. The body’s right ascension (R.A.) and declination (Dec.) are for the J2000.0 epoch; Delta is its distance from Earth in astronomical units; V.Mag is the predicted visual magnitude; Rate is the asteroid’s daily motion in degrees; Con is the constellation in which it resides (Cnc = Cancer, Leo = Leo, Hya = Hydra, Sex = Sextans, Cen = Centaurus). AN computation and graphic by Ade Ashford/Minor Planet Ephemeris Service/JPL’s HORIZONS system.

The next occasions that 1998 OR2 gets closer to Earth than on 29 April this year are 16 April 2079 and 16 April 2127, when the asteroid passes safely by our planet at 4.6 and 6.5 lunar distances, respectively.

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