Astronomy Down Under

Watching Mercury

9 Jan at 16:16 by Wilson

Now is a good time to look at that small rock sitting very close to our Sun… Mercury is reaching it maximum eastern elongation, which means that it’s about as far away from the Sun (when seen from Earth) as it is going to get this time around. If you have a nice, flat western horizon, look for it soon after the sunset, slightly north of the place where the Sun disappeared. The sky will still be quite bright by the time Mercury sets, but the best day to see it will be on the 21st. For observers in Melbourne, Mercury will set at 21:41 today and 21:42 tomorrow (the Sun sets at 20:45 and 20:44, respectively). For other locations, check this page from the US Naval Observatory.

But, of course, you may prefer to watch it from a better vantage point. If so, now is a very good time: MESSENGER, the first probe to visit that planet since the late 70s, is less than one week away from its first fly-by. The closest approach, at just 200km from the surface, will happen on the 14th, but the cameras will start to take pictures of the planet tomorrow, and some images should be released soon afterwards.

MESSENGER was launched in August 2004 and is in a very complex and long trajectory that will end up with the probe in orbit of the planet… on 18 March 2011. Two more fly-bys happen before then, one in October 2008 and other in September 2009. The reason for this is that it is very hard to send a probe that close to the Sun without having it go into the Sun; it will use its several planetary fly-bys (two of the Earth, two of Venus and three of Mercury) to match its speed to that of Mercury, enabling it to enter the planet’s orbit with relative ease.