Western Slope Skies - North Pole of an Asteroid

Mar 19, 2021

Credit NASA/Goddard/University of Arizona

After a recent presentation on the OSIRIS-Rex mission to asteroid Bennu, someone asked "How do you define the north pole of an asteroid?" According to the International Astronomical Union, the correct answer is that asteroids do not have north poles!



Everyone understands the concept of the North and South Poles on Earth. They define the axis around which the Earth rotates. This rotation, of course, defines the length of one day. Furthermore, the ORBIT of the Earth around the Sun defines one year.

Definition of North and South Poles for the planets is relatively straightforward. All the planets in our solar system orbit the Sun in roughly the same plane. The North Pole of the Earth is above that plane. Hence, by convention, the North Pole of any other planet, as well as the Sun, is defined as the pole that is ABOVE this plane.

If you were to look down on the Sun from above the Sun’s north pole, the Sun rotates counterclockwise. All 8 planets orbit the Sun in the same direction as the Sun rotates. This is called ‘prograde’ orbital motion. The opposite of prograde is ‘retrograde.’ Similarly, 6 of the planets rotate in the same direction as they orbit, that is, they have prograde rotation. Venus and Uranus, however, have retrograde rotation, even though they have prograde orbital motion.

Bennu has a prograde orbit, but it has retrograde rotation.

In 2009, the International Astronomical Union (IAU) developed a new definition for polar alignment in solar system bodies smaller than the 8 planets. The "right-hand rule" may be familiar, if you work with carpentry screws or bolts. If you know the rotational direction of an object, then imagine curling the fingers of your right hand in that direction. Your thumb, by IAU definition, will point in the ‘positive’ direction. The negative pole is the opposite of the positive pole. The two poles define the axis of rotation for the object.

Many people still tend to think of positive as north. However, the IAU wanted to avoid the confusion between north and south, because orientation of rotation axes of small bodies can change radically and rapidly.

Here is a video of asteroid Bennu’s rotation. Using the right-hand rule, see if you can tell where Bennu’s positive pole is.

Western Slope Skies is produced by members of the Black Canyon Astronomical Society. This episode was written and recorded by Bryan Cashion.