WASHINGTON – Right around noon today, if all goes as planned, a spacecraft called Messenger will swoop past the planet Mercury and begin two days of unprecedented picture-taking and data-collecting.

The flyby, the first visit to Mercury in more than 33 years by an emissary from Earth, will mark a key moment in a NASA mission that will ultimately place the first satellite into orbit around the tiny planet that sits closest to the sun.

The planetary science community is eagerly awaiting images and information that should shed light on some of the enduring mysteries about the planet – such as where in the solar system it was formed and why its hard metal core is so large and its outer rock crust so scant, compared with those of Earth and the other rocky planets.

“Mercury is a difficult place to get to, and it’s taken a long time to get back,” said principal investigator Sean Solomon, who has worked on the mission for more than 11 years. “But now we’re in place to learn things about one of our few sister rocky planets, and we’re ready for some real surprises.”

The desk-size spacecraft was launched in 2004 and has taken a circuitous path to Mercury, swinging twice by Venus and once by Earth for gravity assists. Messenger will make two more passes by Mercury to let the planet’s gravity slow it down enough for it to swing into orbit in 2011.

Still, today’s whisker-close flyby will be, NASA officials say, a high point of the mission. It will be the closest pass by Messenger in the entire mission, and the nearest to the planet’s equator.

“The biggest mystery of Mercury is why it has so much heavy metal – a core very different in size from other planets,” Solomon said. “We think we can begin to unravel the mystery once we know the chemical makeup of the planet’s surface.”

There are several competing theories on how Mercury came to be what and where it is. One is that the searing heat of the sun stripped the crust off a once larger planet and left primarily the core. Another is that the planet collided with another celestial body during a time when the early solar system was cluttered with them. Under this theory, Mercury’s outer crust and mantle were smashed away and the planet was knocked into its close-in orbit.

Because Mercury is so close to the sun, designing a spacecraft that could stand the heat and calculating a trajectory that would place Messenger into orbit – rather than plunging into the sun – were daunting tasks. Temperatures on the ceramic-cloth sunscreen that protects its instruments will reach 600 degrees Fahrenheit.

The spacecraft’s instruments were designed never to face the sun because they would otherwise quickly overheat and be destroyed, and even pointing them at Mercury will be done for very limited periods. On the side facing the sun, the planet reaches 1,100 degrees at the equator, and on the dark side, it drops to as low as 300 degrees below zero.

The name of the probe, Messenger, is an the acronym for Mercury Surface, Space Environment, Geochemistry and Ranging. It is the first spacecraft to visit Mercury since Mariner 10 in 1974, at a cost of $446 million for the life of its mission. Reflecting the new technologies and logistical knowledge that made the mission and its unprecedented orbiting possible, the European and Japanese space agencies will also be sending spacecraft to Mercury soon.

The planet they will scrutinize has craters, volcanoes, and many unusual and unexplained features, including what might be frozen water in polar crevasses protected from the sun. Its mass is only 5 percent that of Earth’s, but its metal core accounts for 60 percent of that mass (compared with about 30 percent for Earth and Venus, and 20 percent for Mars). Mercury also has an active magnetic field in its thin atmosphere, the only rocky planet other than Earth with that feature. And it contains one of the largest impact craters in the solar system – the Caloris Basin, which is 800 miles in diameter, or about a quarter of the planet’s diameter.

Adding to the picture of a very different kind of planet, it takes Mercury 88 Earth days to circle the sun and six months to rotate around its axis to make a full “day.”

When Mariner 10 passed by Mercury in 1974 and 1975, it was able to capture images of only about half the planet’s surface. Messenger is expected to photograph the entire planet before it is done.

Monday’s flyby will include some anxious moments as the spacecraft passes behind Mercury, going without solar power for 14 minutes and out of radio contact for at least 48 minutes. After about 55 hours after the probe’s instruments were turned on Sunday, data will begin to stream back to Earth. The instruments will take more than 1,200 images during the flyby and will fill the onboard recorder with more than 700 megabytes of data, said systems engineer Eric Finnegan of the Johns Hopkins University Applied Physics Laboratory in Laurel.

“Fifty minutes prior to closest approach, signals from the spacecraft will go quiet as Messenger passes behind Mercury, out of Earth’s view,” he said. “Forty-eight minutes later, engineers and scientists on the ground will attempt to witness the gravitational pull of the planet firsthand by reacquiring the transmitted signal from the spacecraft within minutes of the closest approach point.”

As of Monday, Messenger is only slightly more than halfway through its journey, which will ultimately cover almost 5 billion miles. It will make its next close approach to Mercury in October.

Asked why anyone might care about the characteristics and dynamics of Mercury, Solomon, the principal investigator, gave two reasons: “natural human curiosity” and because the inner planets – the “siblings of Earth” – have special meaning to people.

“We are the shepherds of our planet, and part of that role is understanding what makes it work,” Solomon said. “Well, Mercury was formed at about the same time as the other siblings, under common processes, but it resulted in quite a different outcome. Quite directly, the depth of our understanding of Earth is taxed by our inability to explain other outcomes, and so learning about Mercury is an extension of understanding how Earth began and how it operates.”

AP-NY-01-13-08 1736EST

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