Figuring out how Mercury ended up so different from the other three terrestrial planets -- Venus, Earth and Mars -- will provide valuable insights into how the solar system formed 4.5 billion years ago.

It also will help astronomers establish benchmarks for use when studying other solar systems, where many planets have been found orbiting as close or closer than Mercury to their parent suns.

"Mariner 10 was a mission that was designed as a reconnaissance of Mercury in order to characterize it to plan a Mercury orbiter," Robert Strom, a member of the Mariner 10 team and a co-investigator with the MESSENGER project, said before launch. "That orbiter was supposed to be planned and launched by about 1980. Well, it's been 30 years and until now, nothing has happened.

"So now we've got not only a mission to Mercury, an orbiter to Mercury, but we have a world-class orbiter of Mercury. This is a super mission, never in my wildest imagination did I think that we would get a spacecraft like this for a Mercury orbiter. It has the instruments on board to answer the questions that were raised by Mariner 10 and its going to do that in spades."

Equipped with seven miniaturized instruments, including two cameras, four spectrometers, a magnetometer and a laser altimeter, MESSENGER will photograph the entire planet in color and stereo, map its magnetic field, the mineralogy of its crust and probe the nature of its hidden core.

"The family of the four inner planets -- Mars, Venus, Earth and little Mercury -- shared a common origin," Solomon said before launch. "They all formed in the disk of gas and dust, the solar nebula, that surrounded our young sun. They formed by the same processes, they formed at the same time, their outcomes were extremely different. And Mercury is the most extreme of those four planets. Besides being closest to the sun, it's the most dense, the highest variation of temperature over its surface, it's of course the smallest of the four.

"What we know about mercury mostly came from Mariner 10, which flew by Mercury three times in 1974 and 1975. Mariner 10 imaged less than half the surface and so there's an entire hemisphere we've never seen. Mariner 10 discovered that little Mercury had a magnetic field and made a number of other discoveries that raised profound questions that we're trying to address.

"One of those questions is how Mercury ended up so dense, so massive for its size," Solomon continued. "On the basis of that density, Mercury must be at least two thirds iron metal and that's a much higher fraction of any of the other inner planets. Why did Mercury end up that way?

One possibility is that the solar nebula had a chemical gradient for some reason and that at Mercury's distance from the sun, more metals were present than in regions farther out.

"But competing ideas say Mercury started out more like the Earth, with a larger volume of rocky materials surrounding that metal core, but that silicate shell was largely removed, either because it was vaporized by the extremely hot temperatures in the inner solar nebula or it was blasted away by the impact of an object almost Mercury's size," Solomon said.

All three theories make predictions that MESSENGER will test.

"If Mercury formed out of a solar nebula that had a chemical gradient only in the ratio of iron metal to silicates, then the composition of the silicate material at Mercury's surface should have the major elements in approximately solar proportions," Solomon said.

"If Mercury was once more earth-like in composition but lost most of its silicates as a result of high temperatures because it was bathed in the solar nebula that was very, very hot, the consequences of that would have been vaporization of the upper part of the planet.

"But different elements vaporize at different temperatures. One would expect under that kind of scenario to see primarily a deficiency of the elements that would tend to vaporize easily and a concentration of what are known as the more refractory elements that tend to stay behind the longest. Those consequences are very different from the chemical gradient scenario."

And if Mercury suffered a catastrophic impact with a body nearly its own size, the crust that remained would have lower silicate concentrations.

"We expect to be able to easily distinguish whether any of these competing ideas are correct," Solomon said. "We may have to discard them all and start over. But it is a nice scientific experiment in this case where we have multiple hypotheses which make different predictions of what we will see."

Closely related to the question of how Mercury ended up with such an oversize core is the issue of its magnetic field.

"It is the most Earth-like of the magnetospheres in our solar system," Solomon said. "The mystery is, why tiny Mercury has retained a magnetic field when larger planets in the inner solar system -- Mars and Venus -- do not have a global magnetic field today. Is Mercury's field the result of an Earth-like dynamo mechanism in a fluid outer core? Or is the magnetic field that Mariner 10 measured a fossil?"

MESSENGER will answer that question by mapping subtle changes in the planet's rotation rate as the world swings through its highly elliptical orbit and experiences strong solar tides. The magnitude of that variation is dependent on the nature of the core and by measuring slight changes in a point of longitude on the surface, MESSENGER should be able to answer the question.

"If only the silicate shell is solid you get one answer," Solomon said. "If the entire planet is solid, you get a different answer."

By the end of the mission, he said, "we will be able to know how big the core is, we'll be able to know ... whether there's a fluid outer core or not and of course, that's very closely tied to the question of how we account for the magnetic field today."

Another major mystery is the thermal state of the planet. At the equator, sun-side temperatures exceed 800 degrees Fahrenheit while on the darkside, they plunge to more than 300 degrees below zero. That 1,100-degree variation is the most extreme in the solar system.

But in permanently shadowed craters near Mercury's poles, scientists have discovered deposits of radar-reflective materials that could be ice. If ice is, in fact, present, MESSENGER should be able to see it from its orbit around the planet's poles.

Designed and built at the Applied Physics Laboratory at Johns Hopkins University, the solar-powered MESSENGER measures 4.7 feet tall, 6.1 feet wide and 4.2 feet deep. Its two square side-mounted solar panels extend 20 feet tip to tip. The spacecraft is equipped with seven miniaturized scientific instruments, a computer system, maneuvering thrusters, a main engine, communications equipment, navigation sensors and a large sunshade, all crammed into a half-ton dry-weight package.

The sun shines 11 times brighter at Mercury than at Earth and MESSENGER will experience temperatures as high as 700 degrees when the planet is closest to the sun. While getting to Mercury posed one major challenge, engineers also had to come up with a way to protect the compact spacecraft from the extreme heating it will experience in Mercury orbit.

The solution was an 8-by-6 foot sunshade, made of Nextel ceramic cloth surrounding multiple layers of Kapton insulation, mounted on a titanium frame attached to the side of the spacecraft that will always face the sun.

"The front of the sunshade will get up to about as hot as a pizza oven while the rest of the spacecraft will be at nearly room temperature," said James Leary, mission systems engineer at APL.

Radiators and heat pipes also are in place to carry heat away from the body of the spacecraft, which is protected by multilayer insulation. As an added precaution, MESSENGER's orbit was designed to minimize the effects of heat radiating away from Mercury's surface.

As a result of all that thermal protection, MESSENGER's subsystems and science instruments did not require expensive high-temperature electronics. But the spacecraft still requires an unusual level of complexity.

MESSENGER's twin solar panels measure 5-by-5.5 feet each and generate a combined 640 watts in Mercury orbit. They could produce up to 2,000 watts of power, but output is limited to what the probe's electrical system actually needs to minimize stress.

Two-thirds of each sun-facing panel is made up of rows of mirrors, with two rows between each row of solar cells. With the arrays tilted to reduce heating, only 28 percent of the sunlight hitting the panels will be converted into electricity.

MESSENGER is equipped with a main engine capable of producing 150 pounds of push, along with four 5-pound maneuvering thrusters and a dozen 1-pound jets for small maneuvers. To beam back its scientific data, the spacecraft will use two phased array high gain antennas, one on each side of the craft. Rather than using more common steerable antennas that must be mechanically aimed at Earth, MESSENGER's stationary antennas are electronically steered, the first such antennas ever used on a NASA deep space mission.

The craft's electronic brain is made up of redundant Integrated Electronics Modules, each one built around a radiation-hardened RAD-6000 PowerPC processor. Two solid state data recorders, each capable of storing a gigabyte of information, are on board to store images and other science data until they can be transmitted to Earth.

Once in orbit around Mercury, MESSENGER will beam back eight hours of data per day as it passes through the high point of its orbit. But the distance between Earth and Mercury varies from 54 million to 132 million miles. When the planets are at their closest, up to 104,000 bits of data can be transmitted to Earth each second. When they're at their farthest, the data rate drops to 9.9 bits per second. As a result, data will be prioritized and only the most significant imagery and engineering data will be transmitted when Mercury is on the far side of the sun.

"It's a wonderful mission, it's one that's addressing a range of scientific questions at the scale of an entire planet," Solomon said. "One must ask why it hasn't been done for 30 years. And that has to do with the engineering challenges of first building a spacecraft that can withstand the environment at Mercury and then designing a very clever mission that makes use of advances in orbital mechanics."