Photo: NASA

Mercury is getting its first man-made visitor in more than 30 years. NASA’s Messenger space probe is heading for a rendezvous with the planet, where sunlight is 11 times as bright as here on Earth and temperatures can swing from a metal-­melting 450 ºC in the sunlight to lows of –180 ºC in the shade.

In the first of several encounters, Messenger (an acronym for MErcury Surface, Space ENvironment, GEochemistry, and Ranging) is scheduled to fly by Mercury on 14 January 2008 at a little more than 25 000 kilometers per hour, ­coming within 200 km of the planet’s surface. Because it must perform scientific observations and relay them to Earth while in the scorching glare of Mercury’s tight solar orbit, the craft boasts a multi­layer sunshade and the most advanced communications systems ever deployed in an interplanetary mission.

Although Mercury is relatively close to Earth, Messenger is just the second craft to visit it. Only about 45 percent of the planet’s surface has been mapped. “We anticipate a flood of data that will provide new insights on the origins and evolution of the [inner] planets—including Earth,” says Ralph L. McNutt Jr., the project scientist for Messenger, which was designed and built by the Johns Hopkins University Applied Physics Laboratory, in Laurel, Md.

The January flyby is only a tease for what is to come. Messenger is on a 7.9-billion-km trek that is scheduled to take it around the sun 15 times and past Earth once, Venus twice, and Mercury three times, before it finally settles into an orbit around the sun-blasted innermost planet on 18 March 2011.

To protect Messenger’s wiring, electronics, and scientific instruments from the heat of being within 46 million km of the sun, it has a highly reflective and heat-resistant 5-square-meter ­micrometeorite-proof sunshade. The shade is made from alternating ­layers of Nextel ceramic cloth (to protect against any micrometeorite damage) and Kapton plastic insulation (to guard against direct sunlight and radiation). It’s the same combination that protects the space shuttle’s main engines during reentry into Earth’s atmosphere. Messenger also has a series of radiators and pipes to divert heat from the spacecraft body. The result is that while the outer layers of the sunshade reach temperatures of 370 ºC, the instruments behind it stay a cool room temperature (20 ºC).

But not every Messenger ­component can be cooled to that extent. The ­orbiter’s two main communications antennas, situated on each side of the sunshade, will have to withstand ­temperatures that range from –150 ºC to almost 300 ºC. Engineers planning for the mission knew that such temperature swings would endanger internal ­components and the steering mechanism on a conventional gimballed dish antenna such as the one on NASA’s Mariner 10—the first craft to reach Mercury, flying by the planet twice in 1974 and once in 1975.

To function in such extreme conditions, Messenger carries the first phased-array antenna ever flown in deep space. Although it has no moving parts, the antenna can be electronically steered through a full 90 degrees. Through variations in the phase of signals on ­different parts of the array, the ­antenna’s radiation is enhanced in one direction and suppressed in all the others. The array is expected to return around 100 gigabits of data per year.

Mariner 10 gathered data and images from less than half of the planet’s surface. It left behind many questions for Messenger to answer about Mercury’s density and geologic history, the nature of its core and magnetic field, and how the solar wind interacts with the planet. Unlike Messenger, Mariner 10 was not equipped to achieve orbit around Mercury. It was the first spacecraft to use the gravitational pull of one planet (Venus) to reach another (Mercury), and engineers of the day didn’t feel confident enough in such a maneuver to use it to put the spacecraft into orbit. Orbital mechanics experts have much more experience now with such slingshot maneuvers. Messenger will use a gravity assist from Venus plus 16 ­thrusters to finally drop into a slow, one-­revolution-per-year orbit around Mercury. More than half the craft’s 1100-kilogram mass at launch was thruster fuel. In order to achieve orbit it will burn nearly 30 ­percent of this precious resource.

The data the craft gathers could be put to work solving problems here on Earth. Messenger will not observe the sun directly, but by examining the effects of the solar wind on Mercury’s magnetic field, scientists can make inferences about how solar storms cause radio interference, electricity grid disruptions, and other problems on Earth, McNutt says.

Illustration: Bryan Christie Design