Bagnall Beach Observatory

MPC Observatory Code 433

To contact us:

Phone: 555-555-5555

Fax: 555-555-5555

E-mail: someone@example.com

Photometry of Asteroids

Photometry of asteroids helps to build up a series of “light curves“ for each asteroid. These are graphs of the amount of light reflected by the asteroid over time. Since many asteroids are not spherical, the amount of light they reflect varies as they tumble. It is amazing what things can be discovered from this information. To begin with, the rotation period can be discovered from the light curve, and from this, more things inferred. From the rotation period, the centrifugal force of the tumbling asteroid can be calculated. In turn, this gives an indication whether the asteroid could be a conglomerate of materials bound loosely by the weak gravitation of the asteroid itself, or a monolithic structure able to withstand rapid rotation. Such knowledge about particular asteroids may prove useful if we ever need to divert one from a collision course with the Earth. Brian Warner outlines further information gleaned from lightcurves in his book, A Practical Guide to Lightcurve Photometry and Analysis:

· Find the correlation between taxonomic class and rotation period.

· Find the correlation between rotation rate and size.

· Resolve the mystery of the slow rotators.

· Determine the size and shape of asteroids.

· Determine the H and G values of asteroids

· Infer the shape of an asteroid by correlating with occultation observations.

· Assist radar observations of asteroids.

· Remove observational biases

Lightcurve analysis is a complex process and the above book provides an excellent introduction for the amateur.

I use MPO Canopus for asteroid light curves. This software assists in the calculation of transformation coefficients as well as determining the correct light curve period. I highly recommend it.

The animation below shows the tumbling of the asteroid Eros. Some asteroids tumble at a relatively fast rate, doing a complete rotation in as little as four hours. Others may take many days to complete a rotation. Asteroids probably rotate because they have been involved in collisions with other asteroids and the energy of the impact has been converted into angular momentum.

Why do photometry of asteroids?

The asteroid, Eros, shows a distinctly irregular shape. Light curves from such asteroids help to establish both spin rates and shape.

 

Animation courtesy JPL/NASA