MESSENGER Becomes the First Spacecraft to Orbit Mercury

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[Image: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington]
When NASA’s Mariner 10 spacecraft left Mercury in 1974, the probe left scientists scratching their heads about the nature of the innermost planet. Why is Mercury so dense? Is its outer core really molten? How does it hold onto its thin, tenuous atmosphere made of helium, sodium, and other lightweight elements?

Last night, NASA’s first spacecraft to this fast-moving planet in almost 40 years arrived in orbit around Mercury to answer these questions and more. Called MESSENGER (for MEcury Surface, Space ENvironment, GEochemistry, and Ranging), this spacecraft has taken almost seven years, plus two gravitational assists from Venus and three swingbys of Mercury, to finally achieve orbit. Flying directly to Mercury requires too much fuel, so MESSENGER borrowed gravitational energy from Earth, Venus, and Mercury itself before winding up in its current orbit around Mercury.

Mercury moves around the Sun in a little less than one Earth year, or 88 Earth days. However, Mercury’s day is 58.65 Earth days, making its day two-thirds the length of its year. On Mercury, then, one day lasts two Mercurian years: if you’re standing on Mercury’s surface, you’ll be in sunlight for one orbit, then in darkness for the next, all in the course of one day.

Orbital mechanics aside, MESSENGER is set to produce some great science results and images in the next few weeks. While there’s no livestream of video due to science embargoes, expect great images of volcanoes, craters, and the highly reflective material at the poles (Is it ice? Or sulfur?).

Also stay tuned to find out more about Mercury’s interior---is its outer core molten, or solid? Is it pure iron, or does it contain some sulfur as well? Knowing this will help better determine how Mercury formed, as well as the rest of the solar system. In addition to the core properties, expect MESSENGER to put some constraints on the composition of Mercury’s surface and mantle. Mercury’s crust shows a history of violently explosive volcanism, which means that the mantle has to have volatiles, lighter elements that wouldn’t be expected to have formed as close to the sun. Did they condense further out and collide with Mercury after the solar system formed?

MESSENGER is set to be in orbit around Mercury for at least a year, maybe even three. At the end of its mission, it’ll crash into Mercury, leaving a crater that’ll perhaps be explored by the next spacecraft to visit the innermost planet.

You can follow MESSENGER on Twitter, as well as others’ tweets about the Messenger Orbital Insertion (MOI). Some of the scientists involved with MESSENGER are also tweeting, including Jean-Luc Margot.


Alessondra Springmann greatly appreciates tweeting spacecraft. Her favorites? LRO and New Horizons.

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