Located between the Shuttle's
belly and the payload bay, OARE includes a single triaxial sensor head that
measures acceleration on all three axes (sample
at left).November 30, 1997
United States Microgravity Payload-4 - Flight
Day 11
A satellite slowly spiraling back towards Earth shouldn't have much in common with an earthquake. They don't, except for how you analyze the subtle ways that the two shimmy.
"I like to say we're measuring earthquakes in space," said Melissa Rogers, OARE project scientist for the Orbital Acceleration Research Experiment (OARE). "It's all vibrations."
Measurements taken by OARE, and made available with other data through the Principal Investigator Microgravity Services (PIMS) at NASA's Lewis Research Center, tell scientists how smooth a ride their experiments are getting on the fourth U.S. Microgravity Payload (USMP-4) mission now aboard Space Shuttle Columbia.
"OARE measures a different part of the acceleration environment than the Space Acceleration Measurement System (SAMS)" described in an earlier story (Shake, Rattle and Hum), said Rogers, who works for the Universities Space Research Association. "It's what we call the quasi-steady microgravity environment."
That stems from the fact that the Shuttle and all other satellites aren't really circling the world. They're spiraling inward, ever so slowly, as the last traces of Earth's atmosphere drag on them. The amount of drag depends on the size of the craft, its altitude, and how active the sun is (it heats and expands the outer atmosphere).
Located between the Shuttle's
belly and the payload bay, OARE includes a single triaxial sensor head that
measures acceleration on all three axes (sample
at left).
Materials experiments aboard USMP-4 and other space missions are sensitive to vibrations and movement. So NASA outfits the shuttle with accelerometers to measure their effects. This lets scientists correlate their results with what the Shuttle was doing at the time.
"The orbiter is experiencing drag and that's slowing it down," Rogers said. "But an object in the orbiter doesn't experience that same drag because the orbiter protects it."
So unless an object is strapped down, it moves relative to the orbiter - until it hits a wall. Rogers compared it to riding in your car and hitting the brakes: you press against the seat belt while anything loose slides forward until it hits the dashboard.
"The second part of the problem has to do with most things not sitting on the spacecraft's center of mass," she continued. At any distance from the satellite's center of mass, an object is at a slightly different orbital altitude and tries to pull ahead or fall behind.
"So just based on the location of an object in the orbiter, you get some acceleration," Rogers said.
For that reason, missions are planned so the orbiter holds a specific attitude relative to the Earth (called local vertical/local horizontal) and minimizes accelerations on the experiment samples. Like the Moon, it rotates once per orbit to hold the same face to the Earth.
"All these things are predictable if you know the altitude, atmospheric density for the solar cycle, and other factors," Rogers said. In addition, PIMS has built up a file of the effects of thruster firings, waster water dumps, and even cabin depressurizations so scientists can plan experiments around those, or request that they be deferred to a more convenient time.
This is where it matches seismology, the field in which Rogers earned her master's degree at Virginia Tech. When her husband took a job in Huntsville she knew that earthquakes and oil exploration were not in demand.
But her skills did come in handy in 1989 when the Center for Microgravity and Materials Research at the University of Alabama in Huntsville needed someone to figure out how to analyze the vibrations aboard the shuttle and determine how they affect experiments.
The work later took her to the National Center for Microgravity Research located at NASA/Lewis in Cleveland. The PIMS team at NASA/Lewis provides investigators with both data and lessons from SAMS and OARE as part of NASA/Lewis's Microgravity Measurements and Analysis Project.
"PIMS provides accelerometer data, analysis, and interpretation to investigator teams," Rogers explained. "We work with the principal investigators to figure out what is needed for their missions and how best to schedule their experiments."
Since PIMS was started in 1993, "we've built up a strong reputation for knowing what the data mean."
That includes finding things that many people would miss. When Rogers was in graduate school at Virginia Tech, she saw an old saying which was reversed to describe how they studied seismic data: "I wouldn't have seen it if I hadn't believed it." It applies here.
"We go in looking for something and see it because we believe it has to be there," Rogers said. Not that they invent things. It's just that accelerometer data are inherently noisy, and patterns can be as tough to see as ripples in a storm.
"We have some very experienced people on the team - including an electrical engineer and a materials scientist - who are very good at dissecting the data," she said. But, "Some of this stuff we never find." Right now, the team is trying to keep a 56 cycle/second (56 Hz) ripple from going into the unsolved category. It has been riding with USMP-4 since soon after the experiments were activated.
The PIMS team also is marketing itself and holding short courses for scientists (until now they worked mostly with scientists with experiments that will fly or have flown). Part of the push is because of International Space Station. With more than a dozen crew and lab modules, plus large trusses sprouting solar arrays, station will be larger and more flexible than the Space Shuttle. PIMS won't be able to collect and analyze all the data all the time.
Initially, station will be equipped with a modified version of SAMS to collect data during assembly. The gradual buildup will help them isolate new vibrations in newly arrived modules.
These data will be recorded on board and the hard disks returned to Earth. Eventually, the complete ISS will be equipped with a new SAMS II system sporting up to nine triaxial sensor heads. To ease the load on the station's data transmissions, scientists will be asked to select the vibration bands to which their experiments will be most sensitive. If they don't choose wisely, then there won't be an archive from which to retrieve the data.
"They're going to have to be really smart" to figure out ahead of time which data to request, Rogers said. "We've had principal investigators ignore the microgravity environment, but come back when the found out that something affected them."
On station, they won't have that luxury.
Check
out the daily
Mission Status Reports prepared by Marshall's Public Affairs Office.
Author: Dave
Dooling
Curator: Linda Porter
NASA Official: Gregory S.
Wilson