June 18, 2019

2009 UN3:A (Semi-) Bright Asteroid flyby.

This week, a brief cosmic interloper graces our skies. Tonight, 2009 UN3 will glide silently past Earth, at a distance of 0.03667 Astronomical Units, or about 3,400,000 miles. That equates to roughly 13 times the Earth-Moon distance. Not especially close, as Near Earth Asteroids go; 2009 UN3 isn’t considered a hazard on this pass, but has been classified as a Potentially Hazardous Asteroid, (PHA) or one that warrants watching. What is interesting about this particular asteroid is the fact that it is nearly a kilometer in size, and thus should appear moderately bright. At maximum approach, 2009 UN3 will be approximately +12 in apparent magnitude, bright enough for moderate (8” aperture or larger) scopes. Closest approach occurs at 4:48 Universal Time (UT) on the 9th, at which time the asteroid will be moving in a south to north direction through the constellation  Corvus into Virgo around Right Ascension 12h 23’ 26.0” and southern declination -08° 55’ 30”. Keep in mind, the coordinates mentioned are topocentric; with Near Earth Objects (NEOs), parallax as viewed from along the Earth’s surface comes into play.

For a more precise local ephemeris, I refer you to the above link. At maximum apparent speed, 2009 UN3 should exhibit a speed of 50” arc seconds a minute, discernible to the eye at moderate powers. Also, an interesting photo-op occurs the evening of the 10th, when 2009 UN3 glides just south of the bright galaxy M60…simply center the galaxy, sketch the field, and watch for the “star” that moves! Goldstone radar is planning on pinging 2009 UN3 this week…there is also a 1 in 6 chance that it may be binary.

The astro-term for this week is Apollo asteroid. These are a class of Earth-crossing asteroids that have a semi-major axis that extends beyond the orbit of our Earth but have a perihelion interior to 1.017 A.U., or mostly inside our orbit. They get their name from 1862 Apollo, the first of its class discovered.  Such famed alumni of the Apollo group include 2008 TC3, which impacted in the Nubian desert, 4179 Toutatis, which has made several memorable passes, and 2010 AL30 which whizzed by just last month. The largest Apollo asteroid is 1866 Sisyphus, at 10 kilometers in size and discovered in 1972. The current number of Apollo asteroids stands at 3,348, although this number can and does change daily!


  1. John says:

    What is a semi bright asteroid? By semi bright does that mean how bright it burns? Im doing a school project on asteroids and meteorites, so if you could give me as much information as possible, that would be extremely helpful in my research. Thank you,


  2. David Dickinson says:

    Asteroids, like planets, shine only by reflected sunlight. One of the brighest asteroids is Vesta, which can reach naked eye visibility from a dark sky site. An asteroid such as 2009 UN3 may reach +12 magniutde, within range of a moderate sized telescope… of course, the asteroid will still appear as a star like point.

  3. John says:

    Thank you david for answering my question. Glad to see someone has an answer to this. But i have one more question that i cant seem to find an answer to. Its also out of y personal curiosity. As the ices of a comet nucleus evaporate, they expand rapidly into a large cloud around the central part of the comet. If the clouds are made up of the evaporated ice, wouldnt the medium in space also make ther clouds evaporate into nothing in a small amount of time? This is also for the school project im doing so it would help greatly. Thank you,


  4. David Dickinson says:

    You are correct; the rapidly expands and cools in the near vacuum. Keep in mind, this gas is also sublimating directly to a solid and skipping the gaseous phase; the tail of a comet is mostly nothing or a most a near vacuum.

  5. John says:

    Thak you for your time. Ill be sure to ask you for any futrue questions that i may have. Thanks again,



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