SALT - Southern African Large Telescope

Studying Asteroids with SALT

Close up images of four known asteroids taken with the Galileo and NEAR probes.

Asteroids are small, mostly rocky bodies in orbit around the Sun. They are principally found in the “main asteroid belt” between the orbits of Mars and Jupiter and are thought to be the remains of a planet that was unable to form. There are also many Trojan asteroids, those that stably orbit in Jupiter's L4 and L5 Lagrangian points (60 degrees ahead and behind of Jupiter in its orbit). Asteroids with orbits that reach into the inner solar system are termed Near-Earth Asteroids.

Asteroids vary in size considerably, with the smallest being a few metres across and the largest, Ceres (which is a dwarf planet), being 933 km across. Most asteroids are irregular in shape and are covered in craters from impacts with other asteroids. As there is no atmosphere on an asteroid, there is no erosion to wear the craters away over time. The majority of asteroids fall into three types: C-type, S-type and M-type. C-type asteroids are carbonaceous objects and are the most numerous (75% of all asteroids are this type). S-types are silicaeous (stony) objects and M-type asteroids are metallic objects.

Plot showing the diameter and rotation period of previously discovered VSAs. The red markers indicate some of the new measurements taken with SALT. The newly measured asteroids are all less than 100 m long!

SALT partner institution astronomers in Poland, led by Dr. Tomasz Kwiatkowski have used SALT to study a sample of near-Earth Very Small Asteroids (VSAs).  VSAs are faint objects with absolute magnitudes H > 21.5 mag, which translates to effective diameters smaller than 0.15 km. Due to their faintness, most of them are observed as near-Earth asteroids but there are also several VSAs from the Main Belt that have been observed. VSAs are thought to be the building blocks of larger asteroids and, as such, are interesting to study. Many of these monolithic or deeply fractured objects display rapid rotations with periods as short as several minutes!

SALT imaging observations have been conducted for a sample of VSAs, and the resulting lightcurves (plots showing the asteroid brightness versus time) were used to calculate how fast the asteroids rotate on their own axis and the elongations of these bodies. From these measurements astronomers were able to determine the tensile strength of their interiors.

In total, SALT observations have led to new spin period determinations for 26 very small asteroids in near-Earth orbits, which increases the number of known spins by about 50%. The average period of rotation was 15 minutes. One of the asteroids was found to be a possible non-principal axis (NPA) rotator, which adds to the 3 previously known NPA rotation asteroids. The analysis of the spin periods showed that the tensile strength of VSAs, after scaling them to the same size, is of the same order as the minimum tensile strength estimated for stony meteoroids undergoing fragmentation in the atmosphere.