Asteroidal photometry

The brightness of asteroids vary according to how close they are to the Sun and to the Earth. Asteroids also rotate and so show varying amounts of their surface to each of those bodies. Accordingly, their brightness varies as they rotate. By measuring the variation in brightness over time it is possible to determine the rotational period of an asteroid.

The British Astronomical Association's Asteroids and Remote Planets Section, selects suitable asteroids for which the rotational period is unknown or poorly known and coordinates observers around the world to make observations. "Around the world" is important because the sky is dark at some longitudes when it is daylight at others so longer periods of time are available for taking data as the asteroid rotates.

When a satisfactory rotational period has been determined, a paper is submitted to the Minor Planets Bulletin with all observers who have contributed observations being co-authors.

The observational technique is quite simple once one has learned how to perform photometry, which I did by measuring variable stars. Because only the time-varying brightness is of interest, asteroids can be imaged unfiltered to maximise the signal to noise ratio, which should be in the range 100-200 to give a precision of 0.01 magnitude or better. With the 0.4m Dilworth and SX814 camera this corresponds to an exposure between one and three minutes for targets in the range 14.5 to 17 magnitude. I have yet to get to grips with standard analysis software named Tycho Tracker so my images are uploaded to a site where they can be analysed by a colleague in the BAA-VSS. With luck it should soon be possible to perform all the analysis in-house.

Asteroids which I have observed include