Mercury looms large as probe closes in for final flyby

Messenger snapped these images over a span of four days as it approached Mercury for its third and last flyby on Tuesday. The image on the far right was captured on Monday, when Messenger was about 542,000 kilometres away from the planet’s surface (Images: NASA/JHU/APL/CIW)

29 September 2009 by Rachel Courtland

NASA’s Messenger spacecraft is set to make its third and final flyby of Mercury on Tuesday. If all goes well, the manoeuvre will use Mercury’s gravity to slow the probe down enough to go into orbit around the planet in 2011.

Until Messenger’s first flyby of Mercury last year, the only spacecraft to view the diminutive planet up close was NASA’s Mariner 10. Over the course of three flybys in 1974 and 1975, the Mariner probe collected thousands of images of some 45 per cent of the planet’s surface.

Since then, Messenger has helped fill in the rest of the map. The spacecraft’s first two flybys revealed some 50 per cent of the planet that had not been imaged before by nearby spacecraft. This last flyby, which will take Messenger within 230 kilometres of the planet’s surface around 2200 GMT, is expected to photograph much of the remaining 5 per cent.

Although Messenger will collect most of its data after it reaches orbit on 18 March 2011, the spacecraft’s flybys have produced some tantalising results.

Messenger’s first pass above Mercury’s surface on 12 January 2008 revealed more evidence that the planet harbours a molten inner core and seems to have shrunk more than expected.

The spacecraft’s second flyby nine months later uncovered a bizarre spoke-like pattern in the planet’s second-largest impact basin and revealed yet more evidence of an unidentified dark, bluish material, patches of which now seem to be spread all around the planet.

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