Measuring a 31-hour spin

How observers in Mallorca and elsewhere timed one of Iphigenia’s slow rotations.

Published 2026-10-23

Measuring how fast a distant asteroid rotates sounds impossible. In practice, it is done with a camera and a lot of patience.^[4]

The method

As an irregular asteroid turns, the area facing us changes and its brightness rises and falls. Plotting brightness against time gives a light curve; its repeating pattern gives the rotation period.^[4]

The challenge of a slow rotator

A 5-hour rotation can be covered in one night. Iphigenia takes more than 31 hours, so no single night shows a full cycle. Observers must stitch together several nights and match the pieces.^[2]

The results

• Photometry in 2007 at the Observatorio Astronómico de Mallorca, published in 2010, gave a synodic period of 31.385 ± 0.006 hours and a brightness variation of 0.30 ± 0.02 magnitude.^[2]
• Independent results reported in 2008 gave 31.466 ± 0.001 hours.^[2]

The two values are close but not identical; the source describes them as agreeing.^[2] JPL adopts 31.466 hours.^[1]

An amateur-friendly science

Much asteroid light-curve work is done with modest telescopes and CCD or CMOS cameras.^[4]

Numbered citations link to the sources list on the Iphigenia overview page.