In search of moons of VenusPosted: July 16, 2009
Venus still has no moons, not the most interesting start to an article, but I thought I may as well start this post by giving the ending away. Why? Partly because I get a strange kick out of spoiling your enjoyment of my own blog post (by the way, Murder on the Orient Express, turns out Poirot did it) and partly because I don’t want to leave you under the misapprehension that this article is about the greatest discovery in modern Solar System astronomy, it’s not. What it is about is this interesting paper which set out to find moons around Venus and (you may know this already) found none.
And so to Venus, second planet in of the Solar System, about the same size as the Earth, bit on the hot side on its surface, probably what you saw that time you thought you spotted a UFO. The idea of Venus having a moon first surfaced shortly after the discovery of satellites around other planets such as Jupiter (Galileo, 1610), Saturn (Huygens, 1655) and Earth (Homo erectus, 1,500,000BC). So it didn’t take a huge leap of the imagination to believe the object discovered by Cassini in 1672 was a moon of our Venerean neighbour. Named Neith, this supposed moon was described by Cassini as being up to a quarter of the diameter of Venus. However despite many various claims of subsequent observations of the satellite later searches failed to confirm its existence and it is likely that the sightings were bright stars which happened to be close to Venus at the time of the observations that astronomers confused with a moon.
So alas Neith as we knew it does not exist and after that short historical detour it’s time to return to our feature presentation, A survey for satellites of Venus. The idea for the study was simple, look for the first time with modern astronomical equipment (CCD cameras) for satellites of Venus. But where to look? Where could a moon of Venus exist? Well there is a good rough maximum limit beyond which a satellite couldn’t orbit its parent body. This is known as the Hill radius and is the limit within which a planet’s gravity dominates over that of the Sun. Around Venus this is approximately a million kilometres. Calculating the maximum distance a moon can be from its parent planet is actually a bit more complicated than just working out the Hill radius (it can depend on which way the satellite orbits for instance) and it is actually a little smaller than it, but the Hill radius serves as a good approximation to how close to a planet you have to look to have a chance of seeing a moon.
So the team of astronomers based in the US used one of the Magellan telescopes in Chile to look for a new Neith. The area around Venus was divided into 5 fields and each of these were observed three time with 2-4 minutes between observations. Incredibly, Venus’ motion across the sky compared to background stars is so great that a four minute time baseline is enough to distinguish objects which are satellites of Venus (and hence move across the sky with it) from coincidence objects in the same field. Unfortunately the study found only 5 known, unrelated background asteroids.
The authors calculate that they cover 90% of the Hill sphere (a sphere with a radius of the Hill radius) and taking into account the fact that the maximum stable orbit separation is slightly smaller than the Hill radius they reckon they cover 99% of the volume in which a satellite might exist (even taking into account the areas where scattered light from Venus would drown out the signal from any moon). Of course there still could be moons smaller than the smallest moon they could detect (which the authors calculate to be 300m in radius).
So in summary, Venus still has no known moons.
Sheppard, S., & Trujillo, C. (2009). A survey for satellites of Venus Icarus, 202 (1), 12-16 DOI: 10.1016/j.icarus.2009.02.008