To calibrate, researchers put space instrument in vacuum chamber and subject it to extremes
In movies like “Apollo 13” and “The Martian,” there are scenes where there’s a mechanical problem in space and engineers turn to a copy on the ground to fix it.
That copy is called an engineering model, and one has been calibrated for an Arizona State University-built instrument launching to an asteroid next month. “The reason we’re doing it is to improve the flight instrument,” said Dan Pelham, opto/mechanical engineer. “It gives us the opportunity to improve the one that’s in space.”
Called OTES, for OSIRIS-REx thermal emission spectrometer, the device is the first space instrument built entirely on campus at ASU. It will sniff out what types of minerals are on the asteroid, how big particle sizes are and what the temperature is. The information will be vital to mapping and studying the space object called Bennu, before decisions are made on where to pick up samples.
The OSIRIS-REx mission will travel to Bennu, study it for a year, reach out and grab 4 pounds of material from the surface, return to Earth, and drop the sample capsule in the Utah desert.
Scientists think asteroids may contain clues to the origins of life. The small, rocky bodies have never been explored in this level of detail before. No one knows how they form, how they behave or what’s on them.
“No one has ever seen an asteroid like this up close,” said Phil Christensen, project leader, OTES instrument scientist, director of the Mars Space Flight Facility in the School of Earth and Space Exploration, and Regents' Professor of geological sciences. “That’s fun. That’s exploration. That’s exciting.”
“These samples will be studied by scientists for decades,” project engineer Greg Mehall said. “People think (asteroids) are the building blocks of life.”
BEFORE: The back of the thermal vacuum test chamber has insulated pipes and valves that regulate gases and temperature.Photo by Charlie Leight/ASU Now
AFTER: Some of the components have iced over from the testing process.Photo by Charlie Leight/ASU Now
Senior optical/mechanical engineer Bill O'Donnell starts removing screws from a stand holding the engineering model of the OSIRIS-REx thermal emission spectrometer, so he can move it into the vacuum test chamber on Aug. 12. The engineering model is a replica of the OTES, the first space instrument built entirely at ASU.Photo by Charlie Leight/ASU Now
Engineers Ian Kubik (left) and Rob Woodward connect the OTES to the thermal vacuum test chamber platform as Bill O'Donnell (right) supports it on Aug. 12.Photo by Charlie Leight/ASU Now
Project engineer Greg Mehall and other engineers look at initial data as testing begins for calibrating the engineering model of the OSIRIS-REx Thermal Emission Spectrometer (OTES), in the thermal vacuum test chamber in ISTB4 on the Tempe campus.Photo by Charlie Leight/ASU Now
Principal investigator Phil Christensen (left), doctoral student John Hill and project engineer Greg Mehall look over initial data as testing begins for calibrating the engineering model of the OSIRIS-REx Thermal Emission Spectrometer.Photo by Charlie Leight/ASU Now
Bennu is about the size of one of Giza’s smaller pyramids, large enough to be “a region-killer,” said John Hill, a doctoral student working on the calibration. And one of the reasons it was selected for exploration is its relatively high likelihood of hitting Earth late next century — though reported NASA estimates put that chance at less than a tenth of 1 percent. Still, knowing the physical and chemical makeup of the asteroid will be critical to know in the event of what NASA calls an “impact mitigation mission.”
Osiris-REx is a long mission: seven years. It launches Sept. 8 and spends a year orbiting the sun, building up speed to pick up some of Earth’s orbital energy, then slingshots into deep space.
“It’s a two-year cruise,” Mehall said. “Once we get there, we don’t orbit, because there’s no gravity. We sort of maneuver around it ... The mission starts for us at the end of 2019. Then we study the asteroid for a year.”
Once the samples have been collected, Bennu and Earth might not be aligned. The spacecraft may have to wait to leave. NASA hopes it will be able to leave in 2021. It’s another two-year cruise to return to Earth orbit in 2023.
The mission “isn’t a first, but it’ll be the first one to bring something back,” Christensen said. “We’re going to bring back about 4 pounds of material.”
“Absolute calibration is required for the geology at the asteroid,” Mehall said. “When that instrument says it’s 105 Kelvin, it has to be 105 Kelvin.”
That is what the team worked to ensure last week on the Tempe campus. In one of the clean rooms, on the first floor of Interdisciplinary Science and Technology Building IV, they cranked up a large vacuum chamber.
The chamber is little bit smaller than a Volkswagen bus. It repetitively squeaks like a hamster wheel. Liquid nitrogen boils off the top like Hollywood vampire mist.
They put the spectrometer in the vacuum chamber, heat it up, cook off the gunk (the kind of residue that comes off the dashboard of a new car on a hot day and coats the windows), and then switch on the instrument. Aerospace engineers call this process "shake and bake" because it reproduces the vibrations of a rocket launch as well as the extremes of heat and cold that OTES must survive to do its job.
It uses long-wavelength infrared light to map the asteroid's minerals, which will help mission scientists select where to collect samples. ASU is one of only a handful of universities in the U.S. capable of building NASA-certified space instruments.
It was -190 Celsius in the chamber. Outer space is absolute zero, about -271 Celsius.
“That’s space,” Mehall said. “That’s the coldest we can get with liquid nitrogen.”
“When you open that door, you do not want to go in there,” he said. Nitrogen itself is not harmful — Earth’s atmosphere is 80 percent nitrogen — but nitrogen can displace all the oxygen in the room. “People have died at aerospace companies.”
When they calibrate instruments for weeks on end, there are always two people in the room for safety’s sake, around the clock. To dispose of the nitrogen, they simply let it dissipate slowly.
The goal is to engineer something that can’t fail. There’s no way to repair it remotely, and as Christensen said once watching one of his instruments being launched into space, “Man, that sucker’s gone. It’s out of here, and it’s not coming back.”
“In the early stage, you look at the requirements, which is what it needs to do, what sort of science performance it needs to have, and the environment,” Pelham explained. “You select components that have a high likelihood of working in those conditions. Then what we do is what we call screening and qualification testing of all these sub-components: detectors, motors, things like that. We test them rigorously in the environment they have to work in.
"Generally, we look at the lifetime of the instrument on its mission, and we test those elements to twice that. The idea is you can’t eliminate the risk entirely, that you’re going to have a failure on orbit or in space, but you minimize the risk at hand. It’s only one instrument, so if one of those critical components fails, if it stops working, that’s the best we can do. It costs money, it takes time, but that’s what you do for building a space flight instrument.”
OTES cost in the $12 million to $15 million range. It’s considered low cost for this type of mission. “We’re using technology that was designed and built on previous missions,” Pelham said. TES and Mini-TES both went to Mars, and the optical elements thrived in the extremely dusty environment. OTES isn’t a copy of those instruments, but it is a similar design, which means the team didn’t have to spend as much time designing and building from scratch.
“In NASA-speak they call it TRL: technology readiness level,” Pelham said. “When you have a component that’s a TRL-9, that’s considered the best you can do. It’s got time on orbit.”
It’s an exciting mission, because no one knows what they’ll find on Bennu. There’s only one first time.
“I have been to Mars a lot of times, at least with instruments, and it’s actually fun to do something different,” Christensen said. “Part of the excitement of going to Mars the first time was that we had no idea what we were going to find. After you’ve been there a bunch of times you kind of know what you’re looking for and can expect. The beauty of going to this asteroid is we have no idea what we’re going to find. So it’s fun. It’s discovery versus detailed science — just pure discovery.”
Being able to build a NASA flight instrument on campus is great from a convenience standpoint and from an educational standpoint. While the team works away at the calibration, dozens of students gather at the windows and snap photos.
“The first five times we built (instruments) in Santa Barbara, and I spent, on average, 100 days a year sleeping in a hotel room in Santa Barbara, California, away from my family, away from my house, away from my kids, pets, job,” Christensen said. “To not have to travel is goal No. 1. It’s really nice to just stay home. Secondly, being at a university, we always felt if we could do it on campus, it was an incredible opportunity to get students involved. ... I’ll teach the first lecture of my freshmen class with about 150 students, and I’ll probably leave here, take this off, and go in there and say, ‘Hey, you know, I just spent the last hour building an instrument that’s going to go to an asteroid and now I’m going to tell you about it.’”
The OTES team is led by Christensen, deputy instrument scientist Victoria Hamilton of Southwest Research Institute, and Mehall.
How do you choose an asteroid to study?
There are more than half a million known asteroids in the solar system. Why this one? Here’s how NASA explained it:
The closest asteroids to Earth are those that orbit with a certain distance, about 124 million miles. The most accessible asteroids to reach are within that range and have orbits that don’t veer wildly all over space. When the mission selected an asteroid in 2008, there were about 7,000 in orbit near Earth, but only 192 met the criteria.
Small asteroids rotate more quickly than large asteroids. On a small one, they spin so fast all the loose material on the surface is flung off into space. A big one (diameter larger than 200 yards) spins slowly enough that a spacecraft can come safely near it and collect a decent sample. This criteria winnowed 192 candidates down to 26.
Asteroids are organized according to their chemical makeup. Primitive asteroids are carbon-rich and haven’t changed very much since they formed about 4 billion years ago. They hold the chemicals that may have led to life on Earth. Of the 26 candidates left, only 12 had a known chemical makeup. Only five of those were primitive and carbon-rich.
Bennu wins the beauty pageant because it comes close to Earth, it’s big, it’s primitive, and it might hit Earth — even if NASA estimates have been reported at a 1 in 2,700 chance.
Top photo: Optical/mechanical engineers Bill O'Donnell (left) and Dan Pelham prepare the platform in the thermal vacuum test chamber for calibrating the engineering model of the OSIRIS-REx Thermal Emission Spectrometer (OTES) on Aug. 12. Photo by Charlie Leight/ASU Now