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Planetary News: Mars (2004)

All Systems 'Go' for MESSENGER Launch to Mercury

MESSENGER -- the first mission to Mercury in more than 30 years and the second voyage ever to the planet closest to the Sun -- is being readied for its pre-dawn launch from Cape Canaveral Air Force Station at 2:16 a.m. Monday, August 1 local time, which is 11:16 p.m., tomorrow night Pacific Daylight Time. A Delta II launch vehicle will send the historic $426 million mission on its way.

Plans currently call for MESSENGER to be "powered up" starting at 4:30 tomorrow afternoon local time at the Cape, Launch Director Chuck Duvall told reporters at a press conference held earlier today at Kennedy Space Center. The 1.2-ton spacecraft will separate from the third stage of the Delta rocket "at an Earth escape velocity at 56 minutes, 43.7 seconds after liftoff," added Chris Welch of Boeing, manufactures of the Delta launch vehicle.

Following a 4.9 billion mile [7.9 billion-kilometer] flightpath designed by Chen-Wan L.Yen, of the Jet Propulsion Laboratory (JPL), MESSENGER -- short for MErcury Surface, Space ENvironment, GEochemistry and Ranging -- will loop around the Sun 15 times, and fly around Earth once, Venus twice, and Mercury three times, before slipping into that planet's orbit in March 2011.

While Mariner 10 was the first spacecraft to flyby and study Mercury, MESSENGER will be the first to actually orbit the innermost planet.

On arrival in March 2011, it will begin a year-long study of our moonless neighbor, the second smallest planet in the solar system. If all goes as planned, the spacecraft's suite of seven scientific instruments, which are locked onto its compact and durable composite frame, will send home the first color images of the entire planet, as well as other data about the composition of Mercury, and the characteristics of its magnetic field.

While all fingers are crossed for MESSENGER to launch tomorrow, everyone acknowledges that the very quick 12-second window is a challenge and weather can always be an issue during the summer at the Cape, especially with hurricane season getting underway. There is a "good chance" that late afternoon showers and thunderstorms will drench the Cape tomorrow afternoon, and there are two potential hurricane systems brewing in the Atlantic Ocean that the meteorological team is keeping an eye on, Joel Tumbiolo, launch weather officer and chief weather forecaster noted. But all in all, he said, things are looking good for a 'go.' "As soon as the weather clears we'll be able to roll the tower and go from there."

Launching in the middle of the night may seem less than desirable, but it is actually "the best news", Tumbiolo said, because in the summertime in Florida "the best opportunity" to fly is in the middle of the night. He cited a "70 percent chance of good weather" tomorrow night.

If MESSENGER misses this window, though, it won't be the end of the world. Since a 13-day launch period is just beginning, it will have a dozen more 12-second chances to get off the ground if it doesn't fly tomorrow, according to Mission Manager Robert Farquhar.

Planetary scientists are interested in knowing more about Mercury because it is an 'extreme' planet that scientists are hoping will shed new light on fundamental questions about the formation of our solar system. "The family of the four inner planets -- Mars, Venus, Earth and little Mercury -- shared a common origin -- they all formed from a disc of gas and dust, the solar nebula that surrounded our young Sun," Sean C. Solomon, principal investigator, Carnegie Institution of Washington, told reporters. "They formed by the same processes and formed at the same time and yet their outcomes are extremely different, and Mercury is the most extreme of those four planets."

But Mercury is the least explored of the inner planets. "What we know about Mercury mostly came from Mariner 10, which flew by Mercury in 1974 and 1975," Solomon noted. Because of its proximity to the Sun, Mercury has been off-limits even to the Hubble Space Telescope because of the potential for the intense radiation to damage its sensitive instruments.

While Mariner 10 gathered enough information to map about 45% of the surface at what would be considered "fairly course resolution" by today's standards, "there's an entire hemisphere unknown to us by high resolution imaging, and we have no idea whether the hemisphere we haven't seen looks similar or different than the hemisphere that we have," Solomon pointed out.

"I'll steal a quote from Paul Spudas who wrote an excellent review of the geology of Mercury," added Mark Robinson, a co-investigator from Northwestern University. "In his introduction, he lies out that we know about as much about Mercury as we knew about the Moon before the space age . . . we could only look at one-half the Moon through a telescope at about 1 kilometer per pixel. We had some idea there were compositional variations, because we could see differences in brightness between the smooth maria and the highlands. We have a similar view of Mercury from Mariner 10, because when it flew by the planet it could only see one side of the planet because of the geometry of the orbit and where the Sun was over the planet."

Of the four inner planets, also known as terrestrial or rocky planets, Mercury is the smallest, oldest, and densest. It is the planet with the largest daily variations in surface temperature -- "at the equator, the temperature of Mercury between day and night varies by 1100 degrees Fahrenheit," said Solomon. Tiny Mercury is the only other planet, besides Earth, to have a global magnetic field, and however bizarrely, considering just how close to the Sun it is, it features permanently shadowed regions around the poles that are extraordinarily cold and may harbor water ice.

"Mariner 10 discovered a lot of things about Mercury, but it raised many, many more questions than it answered," pointed out Robert G. Strom, a co-investigator from the University of Arizona, and a member of the Mariner 10 team back in the 1970s. The MESSENGER science team designed its investigation around the mysteries that have haunted planetary scientists for three decades: Why is Mercury -- the densest planet in the solar system -- mostly made of iron? Why is it the only inner planet besides Earth with a global magnetic field? How can the planet closest to the Sun, with daytime temperatures near 840 degrees Fahrenheit, have what appears to be ice in its polar craters?

The planet's density is so high that scientists hypothesize that two-thirds of the planet must be iron metal. Why did Mercury end up that way? "One suggestion in the literature is that it happened because there was one a gradient in chemistry of the solar nebula itself, such that at Mercury's distance there was more metal than rocky material," Solomon offered. "Competing ideas say that Mercury stated out more like the Earth, with a larger volume of rocky materials surrounding that metal core you see there, but that silicate shell was largely removed, either because it was vaporized by the extremely hot temperatures in the early solar nebula or was blasted away by the impact of an object almost Mercury's size." By looking at the composition of Mercury's surface, which would be diagnostic of how Mercury got put together, the team envisions that MESSENGER will be able to easily answer that question.

Mariner 10 found that Mercury has, somehow, retained a magnetic field. This innermost planet, as it turns out, is the only other terrestrial planet besides Earth that has a global magnetic field -- Mars and Venus do not -- although it is estimated to be about 100 times weaker than ours. Whether Mercury's magnetic field arises from motions in a fluid outer core as the Earth's magnetic field or by some other mechanism is still a mystery. "It is most Earth-like of the magnetospheres in our solar system," Solomon said. "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 as the fossil fields of Mars and the Moon, but one with a greater global coherence that gives rise to the patterns that we saw?" By mapping that magnetic field, MESSENGER should be able to distinguish among the dueling theories.

As for the permanently shadowed craters at the poles, which, Solomon said are colder than -300 degrees Fahrenheit, radar images taken at the Arecibo Observatory in Puerto Rico indicate "radar-bright materials in the floors of craters near the north pole" that return properties "well-matched" by water ice. "So we have the intriguing hypothesis that the planet with the most extreme variation in temperature at the equator might have water ice or other volatiles in those pole craters."

A geologic mystery, Robinson, noted, is that while the smooth plains on Mercury look morphologically the same as the expansive dark maria on the Moon, "the big mystery is why aren't they dark?" Looking closely at Mariner 10 images, he added, scientists see what they believe to be large scarfs, which, some hypothesize may be due to the planet shrinking. "That may seem a little crazy at first. When you think about it, as the core cools, and goes from a liquid to a solid, there's going to be a volume changes and if the crust was pretty much already formed when this happened as the planet shrank, the tension has to be taken up somewhere and that's buckling in the crust to make these large scarps which run for many hundreds of kilometers. But we don't know for sure if that's really what happened."

To understand Mercury and definitively answer those and other fundamental questions, MESSENGER will image the entire planet in high resolution - "about 5 to 10 times higher resolution than Mariner 10," Robinson noted -- and collect detailed information on the composition and structure of Mercury's crust, its geologic history, the nature of its thin atmosphere and active magnetosphere, and the makeup of its core and polar materials.

Unlike many planetary missions, MESSENGER has been specifically tailored so that the seven scientific instruments contribute complementary data to answer these key science questions. "There are a total of seven scientific instruments on the payload addressing composition of the surface, the geometry of magnetic field, the gravity field and something of the magnetosphere and atmosphere - a host of questions all of which will illuminate how different Mercury is from the rest of the inner planets and by virtue of those differences will tell us a lot more about how Earth-like planets were assembled in general and how our family came to be," Solomon summarized.

The seven instruments on board include color cameras, a laser altimeter to map the planet's topography, a magnetometer to characterize the magnetic field and spectrometers to measure gamma rays, X-rays, ultraviolet and infrared light to map the make-up and abundance of Mercury's crustal materials, and an energetic particle and plasma spectrometer to measure the makeup and characteristics of charged particles within and around Mercury's magnetosphere.

"The key thing to take away from this is that once MESSENGER returns its data from the science instruments, we'll know the surface chemistry, the surface mineralogy, and with the morphologic pictures that we get back from the surface we'll be able to combine all this information to understand how these materials were in place on the surface and where they came from," Robinson said.

MESSENGER's Earth flyby, to occur one year after launch, and the Venus flybys, in October 2006 and June 2007, will use the pull of the planets' gravity to guide the spacecraft toward Mercury's orbit. The Mercury flybys in January 2008, October 2008, and September 2009 will fine-tune and slow the spacecraft's speed by more than 5000 miles per hour while allowing the spacecraft to gather data critical to planning the mission's orbit phase. The spacecraft will then ease into an elliptical orbit that will take it as close to the surface as 124 miles and as distant as 9400 miles.

Mercury's close proximity to the Sun presented engineers with an unprecedented spacecraft design challenge. The radiation can be up to 11 times brighter than what we see on Earth, which will essentially put the spacecraft in something of a "solar furnace" described Bruce C. Murray, the head of the Mariner 10 imaging team and co-founder of The Planetary Society.

"It's going to be flying in a very difficult environment," noted Strom. "Remember, we'll be going in towards the Sun. This is not like Cassini where it's going out where it's nice and cool. Cassini and the other orbiters of Mars and Jupiter are kind of orbiting paradise in a sense compared to MESSENGER which is going to orbit hell. But it's a very interesting hell."

In order to protect the spacecraft and her instruments, MESSENGER was ingenuously designed with a huge sunshield featuring layers and layers of heat-resistant ceramic fabric, similar to the shuttle tiles, stretched over a titanium frame, that actually allow the instruments to operate at room temperature. Each solar panel features two rows of mirrors to reflect sunlight as well as collect what it needs for power. In addition, the spacecraft will follow an orbit that requires it to pass only briefly over Mercury's hottest regions, limiting exposure to the intense heat bouncing back from the broiling surface.

"Mariner 10 was designed as reconnaissance of Mercury in order to characterize it and plan a Mercury orbiter," informed Strom. "That was supposed to be planned and launched by 1980. Well, it's been 30 years. Now we've got not only an orbiter to Mercury but a world class orbiter to Mercury. This is a super mission. Never in my wildest imagination did I think that we would get a spacecraft like this in Mercury orbit. It's got the instruments on board to answer the questions raised by Mariner 10 and it's going to do that in spades. It's basically a world class mission for discovery."