The Mercury-bound MESSENGER spacecraft will be assaulted by temperatures as high as 700°F as it orbits the planet closest to the Sun, and the only thing that will stand between its room-temperature science instruments and the blistering heat is a handmade ceramic-cloth quilt just one-quarter of an inch thick. The man largely responsible for making sure that MESSENGER would be able to stand up to such harsh heat once imagined he’d make his living in a darker, much cooler environment: the coal mining industry.
Carl Jack Ercol, MESSENGER’s lead thermal engineer, grew up in Ebensburg, a small town outside of Pittsburgh, Pennsylvania. “The area was very blue-collar – pretty much steel mills and coal mines,” he said. “My dad worked in the coal mines, and so since I was good in math, my original plan was to major in mining engineering.”
But as he made his way through studies at the University of Pittsburgh at Johnstown, he gravitated toward mechanical engineering instead. And it’s a good thing: by the time he had earned a Bachelor of Science degree in mechanical engineering in 1982, both the coal mining and steel industries had taken a nose dive.
After earning another degree – a master’s in mechanical engineering with a concentration in energy conversion and heat transfer from the University of Maryland – Ercol landed a job at the Naval Research Laboratory working on top-secret military space programs. “I was doing thermal engineering for this very high-level program, and I liked it,” he said Ercol.
The Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Md., hired him in 1991 as the lead thermal engineer for the Electronics Section of the Midcourse Space Experiment (MSX), a Ballistic Missile Defense Organization satellite experiment to map bright infrared sources in space. Ercol developed detailed thermal and geometry models of MSX’s electronics section, which were used to predict heater power requirements and to size thermal radiators; and he developed thermal models used for temperature predictions during MSX pre-flight testing at Goddard Space Flight Center.
In the years since, Ercol has amassed significant experience in making sure that spacecraft operate as designed throughout a variety of environmental conditions. He was the lead thermal engineer for the solar powered Near-Earth Asteroid Rendezvous (NEAR) Shoemaker spacecraft, which was the first Discovery mission. “The NEAR Shoemaker thermal design was difficult in that the spacecraft had a maximum solar distance farther than the orbit of Mars but inside that of Jupiter,” Ercol explained. “The elliptical orbit created a wide range of thermal environments that constrained the power available for heaters. The spacecraft eventually orbited and then successfully landed on the asteroid Eros.”
Jack Ercol checks the condition of MESSENGER's ceramic-fabric sunshade after testing in the thermal-vacuum chamber at NASA's Goddard Space Flight Center in 2004. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington.
With these missions under his belt, Ercol was well-qualified to serve as the lead thermal engineer for a spacecraft bound for the closest planet to the Sun. He was among the original group of scientists and engineers who, in 1997, submitted a proposal to NASA for a mission to orbit Mercury. “Of course the big concern was how to affordably design a spacecraft that would be able to endure the hostile Mercury environment,” Ercol said. “The Sun shines up to 11 times brighter at Mercury than we see from our own planet.”
MESSENGER's first line of thermal defense is a heat-resistant and highly reflective sunshade, fixed on a titanium frame to the front of the spacecraft. Measuring about 2.5 meters (8 feet) tall and 2 meters (6 feet) across, the thin shade has front and back layers of Nextel ceramic cloth surrounding several inner layers of plastic insulation. While temperatures on the front of the shade could reach 370°C (700°F) when Mercury is closest to the Sun, behind it the spacecraft will operate at room temperature, around 20°C (68°F).
Radiators and one-way heat pipes are installed to carry heat away from the spacecraft body, and the science orbit is designed to limit MESSENGER's exposure to heat re-radiating from the surface of Mercury. MESSENGER will spend only about 25 minutes of each 12-hour orbit crossing Mercury's broiling surface at low altitude. The combination of the sunshade, thermal blanketing, and heat-radiation system allows the spacecraft to operate without special high-temperature electronics.
“So far the spacecraft has been thermally stable and very well behaved,” Ercol said. “During the two Mercury flybys last year, MESSENGER received only a fraction of the thermal emission from the planet that it will face during the orbital mission phase. The mission’s maximum solar heating, however, was experienced shortly after the second Mercury fly by, on October 15, 2008, when the spacecraft was subjected to 11 time the heat at Earth.” And the thermal subsystems, to Ercol’s satisfaction, performed as designed.
Up next is a third flyby in September 2009 and then Mercury orbit insertion in March 2011. “I’m enjoying seeing it work through the different sets of environments,” he added. “It’s been slow going from proposal to acceptance to launch – I’ve never been on a program this long – but it’s been very satisfying to grow with it.”
Shortly after MESSENGER’s launch in 2004, Ercol was assigned as lead engineer for the New Horizons mission to Pluto. “It doesn’t get any colder than that,” he said, referring to Pluto’s distance from the Sun of over 28 astronomical units (AU) at the nearest point in its orbit. The solar energy at Pluto is on the order of 1/1000 of the radiance received in Earth orbit.
The New Horizons spacecraft thermal design utilizes a “thermos-bottle” approach that minimizes heat leaks and uses the electronics waste heat to maintain the average structure temperature near room temperature. “It was very easy to understand the thermal challenges for New Horizons,” said Ercol, who compares the approach to that of NEAR Shoemaker. “NEAR Shoemaker and New Horizons followed a similar thermal philosophy because both were power constrained and both needed thermal designs that would minimize heat leakage and utilize spacecraft waste heat to maintain temperatures while not overheating when near the Earth,” Ercol explained.
While those two missions are en route, Ercol is involved in yet another mission to an extreme environment: the Solar Probe, which aims to study the streams of charged particles the Sun hurls into space from a vantage point within the Sun’s corona – its outer atmosphere – where the processes operate that heat the corona and produce solar wind. At closest approach the spacecraft will have to survive solar intensity more than 500 times what spacecraft experience while orbiting Earth; flying past the Sun at 125 miles per second, protected by a carbon-composite heat shield that must withstand up to 2,600°F and survive blasts of radiation and energized dust at levels not experienced by any other spacecraft.
“The Solar Probe job will be extremely difficult,” said Ercol, laughing. But his work on MESSENGER has informed the team’s approach to the challenge. Solar Probe’s power system will be fortified with heat-resistant technologies developed for MESSENGER. Preliminary designs include a 9-foot-diameter, 6-inch-thick, carbon-foam-filled solar shield atop the spacecraft body. Two sets of solar arrays would retract or extend as the spacecraft swings toward or away from the Sun during several loops around the inner solar system, making sure the panels stay at proper temperatures and power levels.
When he’s not in the hot seat, so to speak, Ercol spends a lot of time with his wife, Maureen, and two daughters: Marisa, age 15, and Carly, age 7. The physical fitness advocate starts each day with a workout. “I get up at 5:30 every morning and get at least an hour or so in,” he said. “There’s no better way to start your day.”