Spacecraft Carrying Livermore Lab Instrument Poised to Orbit Mercury

By Janna Brancolini -- Bay City News Service

Space exploration often involves bringing the seemingly impossible to fruition, but when leaders of a NASA mission to Mercury approached a group of physicists at in 2002 for help, the numbers were particularly daunting.

Could the team find a way to get measurements near the surface of Mercury, which reaches about 800 degrees Fahrenheit, from an instrument that operates at about -330 degrees?

The answer will be revealed next week, when the team's gamma-ray spectrometer is scheduled to begin sending back information after nearly seven years aboard the Mercury Messenger spacecraft.

The craft, which launched from Cape Canaveral, Fla., between a hurricane and a tropical storm in August 2004, is expected to finally reach planetary orbit on Thursday.

The Lawrence Livermore crew's spectrometer is one of several instruments aboard the craft sending back data that scientists will use to learn more about Mercury's magnetic field and planetary formation. The information will likely lead to insight about our own planet, the researchers said.

"We've done every test possible, and (the spectrometer) has passed every test. But until it performs in orbit, you're nervous," said Morgan Burks, a Lawrence Livermore physicist who worked on the instrument's cooling system.

The spectrometer resembles an elaborate gold coffee can with a hunk of silver metal -- the element germanium -- inside.

The germanium measures gamma rays emitted by Mercury's surface so scientists can determine the elemental composition, but germanium comes with both benefits and complications.

The substance produces clearer, more precise results but has to operate at cryogenic, or ultra-low, temperatures -- no small feat near the surface of Mercury, which is hot enough in some places to melt lead.

Scientists will turn on the spectrometer a week after the Messenger -- short for Mercury Surface Space Environment, Geochemistry and Ranging -- begins orbiting the solar system's smallest planet, and data should start to come in a day or two later, Burks said.

Burks and the Lawrence Livermore team are counting down the days.

"I was really excited delivering the instrument and really excited when it launched," Burks said.

Then came the years-long wait, and now the moment when the team will see their work come to fruition is just around the corner.

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The spectrometer is one of seven instruments aboard Messenger, a $446 million mission funded by NASA and managed by the Applied Physics Laboratory at Johns Hopkins University. The mission is the first to Mercury, the closest planet to the sun, since 1975, and Messenger will be the first craft to orbit the planet.

Scientists say understanding Mercury is key to understanding the formation of the solar system, particularly the processes that produced the Earth, Venus and Mars -- the other terrestrial planets.

Of those, Mercury is the smallest, densest and least explored. It has the greatest variations in daily temperature and the oldest surface, said Ed Rhodes, a Johns Hopkins instrument scientist for the gamma-ray spectrometer.

Mercury has a highly radioactive surface that gives off gamma rays -- waves of energy that act like fingerprints of the elements that emit them -- when cosmic rays hit the planet.

The spectrometer will measure gamma rays emitted from Mercury, and the information will be used to evaluate theories about how the planet's surface formed, Rhodes said.

"Some elements are much more abundant depending on the model you choose," said Rhodes, who does everything from sending commands to the instrument to analyzing the spectral data it produces. "The very important elements are iron and titanium," he said.

Iron is more volatile than titanium, so detecting more iron would provide evidence for a different model of planetary formation than detecting more titanium, Rhodes said.

He said the mission command brought the Lawrence Livermore physicists into the picture because of their expertise with the more-precise germanium detectors.

The Livermore team had already created a germanium spectrometer for a spacecraft orbiting Earth, so they were asked to develop one for Messenger as well.
"At the outset, it was not clear that this would be possible due to the harsh thermal environment found at Mercury," Burks wrote in a 2004 technical paper.

A feasibility testing program was undertaken, and a tiny Stirling cycle mechanical cooler was developed. Several shields surrounding the germanium also reflect infrared heat.

The researchers built a prototype and worked with Johns Hopkins scientists who had done thermal modeling of the mission.

About a year later, the Livermore team was confident its spectrometer could withstand the extreme conditions of a Mercury orbit, Burks said.

Messenger will complete an orbit of Mercury once every 12 hours for the next year, flying within 124 miles of the surface on each circumnavigation.

"It gets a big heat pulse every 12 hours," Burks said. "We had to prove the instrument could handle that."

Between the cooler and various electronics, the spectrometer package is about the size of a soccer ball and weighs just over 20 pounds.

Messenger also needed to be protected from the sun, which from Mercury appears three times larger and 11 times brighter than on Earth, so the craft is surrounded by a sunshade.

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The team will be on call this week during the initial orbits in case anything goes wrong. Scientists can manage the instrument's operating temperature and power from Earth, and can change the software if needed, Burks said.

Each communication signal with the craft takes about 15 to 20 minutes to travel roundtrip.

The craft has done three fly-bys of Mercury, which provided opportunities to test the spectrometer, but the fly-bys were too brief to collect much valuable data, Burks said.

Additionally, the Johns Hopkins team has been remotely mending thespectrometer during its trip, he said.

Radiation damage in space has degraded the spectrometer's resolution, but the instrument includes a device that lets scientists heat the germanium crystal to repair it -- a process called "annealing."

"The detector has been bombarded by cosmic rays for seven years," Burks said. "That's the biggest issue."

Burks has also spent the years since the launch adapting the technology developed for the Messenger craft for the Department of Homeland Security.

Gamma-ray spectrometers can be used to detect bomb-making materials such as uranium and plutonium, he said, and Lawrence Livermore's team had previously developed a handheld device for use at shipping ports and border-patrol checkpoints.

The low-resolution radiation detectors currently don't use germanium, which is difficult to cool in a handheld device because the entire system is powered by battery.

Burks is using the cooling developments from the NASA mission to create smaller, lighter handheld devices that use less power and perform better.

"The technology for space really helped with that," he said. The new handheld spectrometers will use germanium to produce more accurate readings and will weigh about nine pounds, compared to the older 30-pound model.

The new technology has been licensed to a company and is undergoing field tests, and the hope is to commercialize it within a year, Burks said.