Today Mars made its closest approach to the Sun -- as seen from Earth, that is. Why is this important? Do a Google search on "Mars solar conjunction" and nearly all the results you'll get will be astrology-related, but Mars' solar conjunction is of great importance to real solar system exploration as well.
The reason it's important is because of what the Sun does to radio communications from Mars. We have four (hopefully five) active spacecraft up there that depend on radio waves being able to traverse the millions of kilometers separating Earth and Mars unhindered. But when a great big blazing radio-noisy star gets in between Mars and Earth, it can be very hard to separate the faint signals from our little explorers from the hissing and crackling of the Sun. What transmissions do get through are degraded in quality from all that noise.
If it's bad to try to get data back from spacecraft when a planet's near solar conjunction, it's worse to try to command spacecraft at this time. We do have more powerful radio transmitters on our dishes on Earth than we have sent on those small spacecraft in space. But that doesn't matter; any degradation in quality of the data we send to spacecraft carries great potential risk. Tiny errors in bad places in the commands we send can kill a spacecraft.
So there's a command moratorium for all of the Mars spacecraft right now, for a couple of weeks on each side of conjunction. All the Mars explorers were given a set of commands a couple of weeks ago and told to run them autonomously for several weeks. They're still transmitting signals back to Earth, so we know they're still up there and still healthy (that is, except for Spirit, about which we know little right now).
Here is a really cool video from the Mars Express VMC blog showing Mars -- and its invisibly tiny attendant space explorers -- approaching the Sun over the last month.
Mars and the Sun viewed by SOHO The video above shows the Sun from January 1 to February 4, 2011 as viewed by the Large Angle and Spectrometric Coronagraph (LASCO) Experiment aboard the ESA/NASA SOHO solar observatory mission. This instrument puts a disc in front of the Sun to block the direct light. Coming in from the left of the video is a bright speck - Mars! Mars reached solar conjunction on February 4. As a result of its apparent proximity to the Sun, radio communications with all the Mars spacecraft (Mars Express, Mars Odyssey, Mars Reconnaissance Orbiter, and the two Mars Exploration Rovers) were curtailed.Video: ESA/NASA
The video was produced using the JHelioViewer tool. It shows in blue and red the view from the LASCO instrument on the ESA / NASA SOHO solar observatory mission. This instrument puts a disc in front of the Sun to block the direct light, leaving the corona and background stars (and planets like Mars) in view. In this video the center has been filled with images from the NASA Solar Dynamics Observatory AIA instrument.