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Messenger will map Mercury's surface. Credit: NASA

A return to the innermost planet Mercury and a mission to excavate material from a comet's nucleus were selected by the US space agency NASA last week as the next in its Discovery line of planetary missions.

The $286 million Mercury Surface, Space Environment, Geochemistry and Ranging (Messenger) mission and the $240 million comet impactor, called Deep Impact, beat three other finalists in the funding competition. Those were a sample return from the Martian moons Phobos and Deimos, a Jupiter orbiter, and a spacecraft to study the atmosphere of Venus.

A return visit to Mercury has been a perennial on the wish list for Solar System missions. Messenger will launch in 2004, 30 years after Mariner 10 made two quick fly-bys, the only reconnaissance of the planet to date. The spacecraft will arrive at Mercury in 2008, carrying seven instruments for imaging, spectrometry, laser altimetry, and measurements of the planet's magnetic field.

During its year in orbit, it will map the surface, including the hemisphere that Mariner missed. Messenger's principal investigator is Sean Solomon of the Carnegie Institution of Washington, and the spacecraft will be built and managed by the Johns Hopkins University Applied Physics Laboratory.

NASA's Jet Propulsion Laboratory will manage Deep Impact. This will create its own fireworks on 4 July 2005, Independence Day in the United States, when it smashes a 500-kilogram copper projectile into Comet P/Tempel 1 at a velocity of ten kilometres per second.

The impact will gouge out a crater at least 25 metres deep and 120 metres in diameter — deep enough to expose pristine material from inside the comet, hopes principal investigator Michael A'Hearn of the University of Maryland. Cameras on the main spacecraft will image the evolution of the crater and the resulting plume in infrared and visible wavelengths.

A'Hearn was instrumental in organizing a ground-based campaign to observe the 1994 collision of comet Shoemaker-Levy 9 with Jupiter. He envisages a similar marshalling of professional and amateur telescopes to watch the artificial collision.

Deep Impact features another first for the Discovery programme — the offer of a ‘piggyback’ ride to other investigators. A'Hearn's team have an extra 10 kilograms of mass and 10 watts of power on the spacecraft, which it will make available for a technology experiment to be selected at a later date.