From Richard Miles
It seems to me that the solar system holds more great mysteries beyond the six you highlighted (23 January, p 28). There appears to be a diverse menagerie of objects, notably asteroids and comets, the true nature of which we have only just begun to explore.
Take, for example, one of the largest periodic comets known: 29P/Schwassmann-Wachmann. This strange body occupies a near-circular orbit beyond Jupiter. Since its discovery in photos from 1927, it has displayed energetic outbursts or eruptions between four and eight times a year, powered by some mysterious source yet to be identified.
Stourton Caundle, Dorset, UK
