By Jonathan Amos
Science reporter, BBC News
The first probe to visit Mercury in more than 30 years has passed within just 200km (125 miles) of the planet.
The fly-by was the first of three to be made by the Messenger spacecraft as it prepares to enter into orbit around the Solar System's smallest planet in 2011.
The US probe was programmed to collect more than 1,300 images and make other observations during the encounter.
No mission has viewed Mercury up close since the Mariner 10's third and final fly-by in March 1975.
Marilyn Lindstrom, the US space agency (Nasa) mission's programme scientist, said: "[Messenger's] goal is to understand the surface, the interior, the magnetosphere and the atmosphere of this innermost planet; but in the process of doing that we hope to apply that [knowledge] to understand how all four of the terrestrial, Earth-like, planets formed."
Messenger is half-way through what will be a seven-year tour of the inner Solar System.
It is not due in orbit around Mercury until March 2011. To get there, it must perform a series of fly-bys and engine firings to put it on a correct course and, crucially, slow its final approach.
Mercury pictured during Messenger's approach
This week's pass, which took place some 53 million km (33 million miles) from the Sun, was intended to reduce the spacecraft's velocity by 8,000km/h (5,000mph). Even so, it still moved over the cratered surface at a relative speed of 25,000km/h (16,000mph).
"Messenger's orbital period around the Sun will be decreased by 11 days thus setting up a planetary car race with Mercury," explained Eric Finnegan, mission systems engineer at Johns Hopkins University Applied Physics Laboratory.
"Using its engine and future gravity assists, the spacecraft after being lapped by Mercury many times in its race around the Sun will eventually match the 88-day orbital period of the innermost planet."
Messenger began its fly-by observations on Sunday. The probe's instruments were expected to gather data over a period of 55 hours.
The moment of closest approach occurred at 1904 GMT on Monday. The data treasure from the pass was due to be transmitted to Earth on Tuesday.
THE PLANET MERCURY
Closest planet to the Sun
Mercurian year: 88 days
Has global magnetic field
Messenger is operating in an extremely harsh environment.
Its electronics and observational instruments are protected behind a shield that allows them to operate at "room temperature". The Sun-facing side of the shield, however, experiences temperatures in excess of 300C.
All the terrestrial planets are believed to have formed at the same time by common processes - but Mercury itself is a bit of an oddball.
It is so dense that more than two-thirds of it has to be of an iron-metal composition.
It is so close to the Sun that the temperature variation between day and night at the equator is more than 600 degrees; and yet there may be water-ice at the poles in craters that are in permanent shadow.
Europeans to follow
"Mariner 10 showed us a surface that was so heavily cratered that it looked like geological activity on Mercury ended very early in the history of the Solar System," said Sean Solomon, Messenger's principal investigator from the Carnegie Institution of Washington.
"And yet, Mercury is the only other inner planet that like Earth has a magnetic field which we believe means it must have a very dynamic molten iron core.
"So how to reconcile this ancient surface with this modern-day internal dynamic activity is one of the mysteries we hope to solve."
Messenger's first fly-by was designed to:
- obtain the first detailed view of the hemisphere of the planet missed by Mariner 10 (it only saw 45% of the planet's surface)
- make the first measurements of the elemental composition of Mercury's surface
- use a laser altimeter to study the shape and topography of the planet
- take gravity measurements to try to understand better Mercury's internal structure
On Friday this week, the European Space Agency (Esa) will sign an industrial contract with EADS Astrium to build BepiColombo.
This mission will be launched to Mercury in 2013. It consists of two spacecraft - an orbiter for planetary investigation, led by Esa, and one for magnetospheric studies, led by the Jaxa (Japan Aerospace Exploration Agency).
The satellite duo will reach Mercury in 2019 after a six-year journey towards the inner Solar System.