In my previous posts on this topic, I've discussed probes that have been visiting Saturn, Mars, and one headed shortly for the moon. However, there's still many places left to explore in our solar system. Currently there are two more probes headed to opposite ends of the solar system. One is known as MErcury Surface, Space ENvironment, GEochemistry and Ranging (MESSENGER) and is headed for Mercury. Another is known as New Horizons and is headed for Pluto.
For now, I'll only be talking about Mercury and its probe. Although Mercury can be viewed by the naked eye, it's still a rather difficult object to find for a few reasons. Cheif among these is, that since it's orbit lies within our own, it never strays to far from the sun. This means two things:
First that it will only be visible shortly after sunet or before sunrise which makes it easy to lose in the twilight.
The second reason has to do with the amount of visible surface lit in relation to distance. Since brightness falls off as the inverse square (ie, doubling the distance would make the object 1/4th as bright) we would like to view Mercury as close as possible. However, for Mercury to be its closest, this would mean that it were directly between the Earth and Sun. This means we would not see any of the surface lit. The best balance turns out to be when Mercury is at what's known as "greatest elongation", which means its as far from the sun in the sky as possible. But even at this point, only half of Mercury's visible surface is lit.
Additionally, Mercury isn't the largest of planets. Nor does it have a terribly surface reflectivity (albedo). All of this combined makes Mercury a rather tricky target.
That being said, if you know where to look and when, Mercury is possible to spot with the naked eye. However, even though a decent sized telescope, it's not much more than a bright disk. Mercury has been known since ancient times, although it wasn't until relatively recently that it was known to be a single object. Since it can appear both in the morning and the evening, Mercury had two names: The Morning Star (Lucifer) and the Evening Star (Noctifer).
Yet dispite having known of its existance for a very long time, little is still known about Mercury since only one previous spacecraft (Mariner 10) has visited the diminutive planet. Mariner 10 never entered into an orbit around Mercury though. Instead it merely made three flybys. During this period, Mercury had the same side of the planet facing the sun, so only ~45% of the planet's surface was actually mapped.
Thus, MESSENGER will be the first probe to actually enter into an orbit around Mercury. But although it was launched in August of 2004, it will not enter orbit until 2011. For those of you quick with math, you will realize this is almost 7 years to traverse a relatively small distance. Why so long?
The reason has to do with another important body in our solar system: The Sun. The sun has a massive gravitational influence. The further one is from it, the greater one's gravitational potential energy. By moving closer, that potential energy will get transformed into kinetic energy. Thus, by the time MESSENGER got to Mercury, it would be going far to fast to even latch on.
Therefore, that extra energy must be bled off in some way. To do that without having to constantly have the breaks on, the mission designers put MESSENGER into a long looping orbit that would burn off some of the energy as it falls towards the sun, and adds energy so that MESSENGER can catch up to Mercury as it orbits.
Therefore, MESSENGER will make several passes of Venus, Earth, and Mercury itself. Recently, MESSENGER made its first flyby of Earth and took a series of images like this one:
The entire series of images has been made into a very nice animation that can be found here.
Eventually when MESSENGER reaches Mercury, it has several science goals as well. These include determining more about Mercury's chemical composition (specificly why its density is so high), as well as studying its extremely weak magnetic field and tenuous atmosphere. It's also thought that Mercury may have water near its poles.
But all of this will have to wait till MESSENGER arrives in 2011.