Of all the terrestrial planets, Mercury is the one we know the least about. The one and only spacecraft ever sent to the smallest and densest of the terrestrial worlds was Mariner 10, launched back in November 1973.

Mariner 10 was a two-for-one deal. It flew by Venus first for a gravity assist and then on to Mercury. But Mariner 10 was only able to capture data from half of Mercury on its three flybys in 1974 and 1975.

Approved by NASA in 1999, the $300 million Messenger spacecraft will seek to answer basic questions about Mercury's high-density core, magnetic field, geologic history, unusual materials at the poles, and the types of volatiles that exist on the surface.

"To get to Mercury, the 2004 opportunity is the best one in many years," says Dr. Robert Gold, Messenger Project Scientist at the Applied Physics Laboratory (APL). "We have a prime mission and a backup mission both in 2004. The next chance we would have to get there with this launch vehicle and this payload would be a 2012 launch. So we must go in 2004."

On its way to Mercury, Messenger will use the gravity of both inner planets to slow itself down for the 2009 Mercury orbit insertion. Messenger will fly by Venus in 2004 and 2006, and Mercury in 2007 and 2008.

The two flybys will give Messenger a chance to study Mercury's surface, atmosphere, and magnetosphere. Before it goes into orbit in 2009, the spacecraft will have mapped almost the entire surface of Mercury in color, including the parts unseen by Mariner 10.

"Mariner 10 is the only view we have ever had of Mercury beyond a dot of light in a telescope," says Gold. "I've been interested in Mercury ever since I knew that it had a magnetic field." But, he never thought he'd be lucky enough to send a spacecraft to study Mercury. When Mariner 10 flew by Mercury it had just enough time to spot a glimmer of the planet's magnetic field. "It seemed too strong to be just the interaction of the solar wind and the planet," Gold says. "I think the chances are good that there is a fluid layer, but we'll find out."

Gold says that Mercury's magnetic field probably froze into place as the material of the planet solidified billions of years ago. This process may be similar to what caused the magnetic fluctuations on Mars that were detected by the Mars Global Surveyor.

It's also possible that Mercury may have a fluid core or layer moving to form a dynamo. "The base theories say that if you just have an iron core, it should have solidified in under a billion years," says Gold. "But, if you have enough sulfur or another element that was mixed with the iron, you might form a eutectic that would be able to remain liquid to a much lower temperature. And that might be a thin shell around a solid iron core."

Messenger will study Mercury with several instruments, the Mercury Dual Imaging System (MDIS), Gamma-Ray and Neutron Spectrometer (GRNS), Magnetometer (MAG), Mercury Laser Altimeter (MLA), Radio Science (RS), Atmospheric and Surface Composition Spectrometer (ASCS), Energetic Particle and Plasma Spectrometer (EPPS) and an X-Ray Spectrometer (XRS). Production began in July of this year and the instruments are scheduled to be delivered by February 5th 2003.

The main issue is heat. "350 degrees, that's pizza oven temperature," says Gold. "As long as you can hide behind your thermal shade, you manage to survive. And using that ceramic cloth was one of the key ideas that made this possible."

"By using the combination of the thermal shade, the diode heat pipes to special radiators on the side, and other things, we're going to keep the body of the spacecraft inside the region, normally room temperature to plus 40," says Gold. "Which means we can do one of the things that has been stopping a mercury mission before this. We can do without special high temperature electronics."

The mission requires a large fuel fraction of 65%. Since Discovery missions are not allowed to launch on anything larger than a Delta II, this really limits the payload. "We've had to apply miniaturization principles wherever we could," says Gold. "It's a little hard when you have to purchase components. We only wish we had more money to pay people to miniaturize things, but you can't do that on a Discovery budget."

So, why choose to send an orbiter to Mercury? Gold explains, "You get a lot more science from an orbiter [along with] the problems of an orbiter. Getting to that part where you are in front of the planet [with sunlight from both sides] is so daunting that people are very afraid of it for good reason. It's not easy."

There are several layers of ceramic cloth in the sunshade on top of 20 layers of kapton thermal blanketing, which allow the spacecraft to withstand temperatures of 360 degrees centigrade. "By the time the heat gets all the way through the radiation stopping, it's about 100 degrees centigrade at the back of the thermal shade," says Gold. There is a gap between that and the spacecraft, which sits at 25 degrees centigrade, about room temperature."

Including fuel tanks, the Messenger spacecraft measures about 4 feet tall by 4 feet wide, and about 3 feet deep. Add on the thermal shade and the size increases to 6 1/2 feet tall.

Working on a Discovery mission has its advantages and disadvantages. "Although it's very intense, although you end up working lots of hours, it's in many ways very fulfilling because you don't waste months and years waiting for decisions to be made. You know you have to make decisions, you make them, and you move on. It's some of the most exhausting time, but some of the most rewarding time."

Science Writer Laura S. Woodmansee spoke with Dr. Gold after his presentation of the Messenger mission to the American Astronautical Society meeting in Pasadena.

Related Links
Messenger at APL
Mercury at PlanetScapes
Mercury at NinePlanets
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