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Observing the Sky Solar System Satellites Navy Research Resources

Solar System: Inner Planets - Mercury

The Fastest Planet

Mercury, a terrestrial planet and the innermost planet in the solar system, has temperatures that are the most extreme in the solar system. On the day side of Mercury, the thermometer reaches 700 K (800°F/427°C). When night falls, temperatures plunge to just 90 K (-298°F/-183°C).

Most of what we know about Mercury we learned from three Mariner 10 spacecraft flybys in 1974 and 1975. During those flybys, Mariner 10 mapped nearly 45 percent of Mercury’s surface and gathered data about the planet’s atmosphere, temperature, geology, and more. Images from the probe revealed a crater-pocked planet that looks a lot like our Moon. The data from Mariner 10 are still being analyzed by scientists and have actually raised many questions, such as: Is there ice at Mercury’s poles? Why is Mercury so dense? What’s inside the planet’s core? Scientists have many different theories that attempt to answer these questions. Perhaps some of them will be answered during the next mission to Mercury.

Phot of Mercury
Courtesy of NASA JPL

In 2004, a spacecraft called MESSENGER (for MErcury Surface, Space ENvironment, GEochemistry, and Ranging) will be launched on a five-year journey across the inner solar system to Mercury. During its mission, MESSENGER will fly past Mercury twice (in 2007 and 2008), taking pictures and gathering data. In 2009, MESSENGER will begin a one-year orbit of Mercury, collecting data and pictures of the entire planet.

Because it appears to us that Mercury is never more than 28 degrees away from the Sun, it’s hard for us to see the planet except occasionally at either sunrise or sunset.

In 2004, Mercury is best seen in the early evening in late March through April, July, and November. In July and August, you can see Mercury and Mars together in the evening sky.