Two small asteroids will pass the Earth this week, within the distance of the Moon.

The two objects were discovered on 5 September by Andrea Boattini working with the 1.5 metre reflector at Mount Lemmon in Arizona as part of the Mount Lemmon Survey, which also includes astronomers J. D. Ahern, E. C. Beshore, G. J. Garradd, A. R. Gibbs, A. D. Grauer, R. E. Hill, R. A. Kowalski, S. M. Larson and R. H. McNaught.

Asteroids and Remote Planets section director for the British Astronomical Association Richard Miles told Astronomy Now that the probable sizes of the two bodies are eight and 12 metres across, with possible ranges of 5-15 and 7-25 metres respectively.

The smaller of the two objects, 2010 RF12, passes the Earth at just 21 percent the Earth-Moon separation (that is, 80,000 kilometres). Currently moving at an apparent speed of up to 20 arcseconds per minute, it will brighten to 16th magnitude this evening (7 September). By the time it makes its closest approach to Earth at around 9pm on 8 September it will likely have accelerated to 70-160 arcseconds per minute and brightened to 14th magnitude. Its orbit takes it from 0.82 to 1.17 AU and it orbits the Sun once every year, only catching up with the Earth once every one hundred years.

The larger object, 2010 RX30, passes at 0.66 the Earth-Moon separation, but will be more favourably placed for UK observers to track than the smaller asteroid. The 12 metre wide asteroid will make its closest pass of Earth at 10am on 8 September, travelling at an estimate speed of between 110 and 400 arcseconds per minute and brightening to 14th magnitude. Its orbit stretches from 0.5 to 1.15 AU and is inclined 4.5 degrees from the plane of the ecliptic.

Early photometric observations using the two-metre Faulkes Telescopes South yesterday (September 6) of 2010 RF12 have now shown that this object is not a fast-rotator as initially thought. "It looks to be spinning quite slowly, certainly taking longer than 6 hours to turn on its axis," says Miles. "Over an interval of 2.5 hours, its brightnessed increased by a little over 10 precent (0.11 mag)."

Details of the positions of the objects can be found at the Minor Planet and Comet Ephemeris Service and instructions for using the webpage are detailed by the BAA here. The Faulkes Telescope Project has listed observations of the asteroids as urgent; details of how to image them using the Faulkes Telescopes can be found here.

Neither asteroid poses a serious threat to the Earth; if they were inbound they would disintegrate as they fall through our thick atmosphere. Most of the mass of the asteroid would burn up, with smaller surviving fragments landing as meteorites.

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