That's no Moon: It's Mercury, as seen by the MESSENGER spacecraft. Photo credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

Dec 30, 2014 Now's your chance to see Mercury

Sen—If I had to guess, I’d bet you’ve never seen Mercury in the sky with your own eyes.

That’s OK. Most people haven’t; even a lot of astronomers haven’t. But now’s your chance; for the next couple of weeks the tiny planet will be visible in the western sky just after sunset. It can be tough to spot, but this time around there’s a nice beacon marking its location: Venus.

Over the next few days, Mercury will sidle up quite close to Venus in the sky, and on January 10, 2015 they’ll be only about half a degree apart: That’s about the size of the full Moon on the sky (in fact, on the 21st the thin crescent Moon itself will slide by the pair, making a lovely triangle low to the west).

Venus is easy enough to find; it’s the third brightest natural object in the sky, and it’s low to the southwest after sunset. You can even spot it when the sky is still bright. For northern hemisphere observers Mercury will be below and to the right of its brighter sister (for austral folks it’s below and to the left of Venus after sunset). Binoculars should help a lot here.

If you have access to a telescope, I strongly urge you to use it to look at Mercury, too. The planet undergoes Moon-like phases as it orbits the Sun, and later in January it’ll get between us and the Sun, becoming an ever-thinner crescent which you can track over the course of a few nights. It’s really lovely. As it does so it’ll get closer to the Sun in the sky, lower to the horizon, and be harder to see, but it’s very much worth your time to try.

Mercury is hard to observe from Earth because it never gets far from the Sun in the sky, so it always hangs near the horizon. Our first really good view of the planet was in the 1970s when Mariner 10 passed by, but things got a lot better when the MESSENGER probe arrived there. It flew by Mercury three times before finally settling into orbit in 2011.

It’s been taking magnificent pictures ever since (over a quarter of a million so far). It’s revealed a world saturated by impacts, covered nearly pole to pole with craters. Some have long rays; bright radial features that extend over much of the world. It’s confirmed ice deep in the eternally shadowed craters at Mercury’s north pole, and revealed that the planet has an incredibly tenuous atmosphere composed of material blasted off the surface by impacts and particles trapped from the solar wind. It even found evidence that Mercury sees meteor showers just like Earth does.

The original nominal mission of MESSENGER was to orbit Mercury for a year on a highly elliptical path. After an extension was granted, the orbit was shifted to bring it down lower to the surface so that higher-resolution images could be taken (as well as other observations that could be done closer in).

But all that is drawing to a close now. Its fuel is nearly exhausted, and that’s critical. Every time it flies close to the surface the orbit is affected by the changing gravity due to density differences under Mercury’s surface. Without fuel, these cannot be corrected, and eventually MESSENGER will impact the surface of the planet it has studied for many years.

This was scheduled to happen in March of 2015, but then a lovely thing happened. The liquid propellant is almost gone, but the fuel tanks were kept pressurized using helium. This element is lightweight and doesn’t make a great propellant, but it’s there, and useable. Engineers figured out a way to utilize it as a propellant to change the craft’s trajectory. This will extend the mission lifetime by as much as another month; a huge gift to planetary scientists.

I’m always amazed to see the images from MESSENGER; such highly detailed pictures of a planet so hard to observe from Earth. That’s why I always take the chance to see it with my own eyes when I can; watching its position shift in the sky, seeing its phase change as it moves. I know that what I’m seeing is an entire world, and one whose nature was hidden to humanity until my own lifetime. It’s a connection across space and time, and it’s one you can have too.