NASA plans to return to Mercury, after an absence of more than 30 years, when its Messenger spacecraft reaches the innermost planet in 2008.

The Messenger mission to map Mercury is one of two new projects selected for NASA's "quicker, cheaper" Discovery programme (Flight International, 21-27 July). The other is the Deep Impact mission to excavate the interior of a comet, planned for 2005.

The $286 million Messenger (Mercury Surface, Space Environment, Geochemistry and Ranging) is managed by Johns Hopkins University's Applied Physics Laboratory, which will design and build the spacecraft. GenCorp Aerojet will supply the propulsion system, and Composite Optics the integrated structure.

The bi-propellant main engine and hydrazine thrusters will be integrated into the spacecraft structure to reduce weight and increase propellant mass. Despite this, after launch in 2004, the Messenger will have to make two gravity-assisted flybys of Venus, and two Mercury flybys in 2008, before it can enter a year-long orbit around Mercury in September 2009.

During its two flybys, the Messenger will produce images of Mercury and study the planet's atmosphere and magnetosphere to provide data for use in planning the orbital phase of the mission.

The second mission is the $240 million Ball Aerospace Deep Impact, which will be launched in January 2004 for an encounter with comet P/Tempel 1 on 4 July, 2005. It will fire a 500kg (1,100lb) copper projectile into the comet and observe the resulting debris.

Source: Flight International