Mercury: The Swift Planet

Despite the fact that astronomers have been observing Mercury with telescopes for centuries, how much do we really know about this strange world? The planet Mercury lies closest to the Sun, orbiting every 88 days. This tiny, crater-ridden world has intrigued planetary scientists for decades, housing water ice at its poles and zipping around on an orbit with enough ellipticity that the apparent size of our star changes dramatically between its closest and farthest points.

Although secure today in its place as the innermost planet, astronomers once thought that Mercury might actually hold second place. Its unique orbit precesses, or shifts in orientation, around the Sun, which was believed due to an unseen planet orbiting closer than Mercury. Now, though, we know that this precession is actually due to the effects of general relativity, as the Sun's mass deforms space-time around it.

Mercury rotates on its axis with nearly no tilt whatsoever, meaning the planet experiences no seasons and there are places at the poles where sunlight never touches, blocked by steep crater walls. Solar radiation and micrometeorite impacts are two of the major forces that have shaped — and continue to influence — this world, and we are still digging for clues on its history and evolution. Only two spacecraft have ever visited, and the entire surface wasn't even imaged until earlier this century.

Below, we've compiled a few resources on Mercury, to help you learn more about our solar system's smallest, swiftest planet before you cast your eyes skyward tonight.

Mercury at its evening best
Our March issue's Sky this month section highlights Mercury's best apparition of the year; you'll also find a sky chart to help you spot the planet.

Tour the solar system: Mercury
Our Tour the solar system video series takes a closer look at the planet Mercury.

Web Extra: Mercury in the spotlight
Our February 2016 story, "Torrid Mercury," highlights one of MESSENGER's biggest discoveries — water ice at the planet's poles.

Weird Object: Mercury
Get the facts on Mercury and its place in the solar system in this overview by Astronomy contributor Bob Berman.

Astronomy magazine podcast: MESSENGER at Mercury
Step back in time to just before MESSENGER passed Mercury in early 2008 to hear Senior Editor Rich Talcott explain the spacecraft's journey.

Astronomy magazine podcast: Mercury revealed
Continue your time travel journey with this July 2008 podcast, which takes you through some of MESSENGER's earliest discoveries.

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