posted April 25, 2013
In August of last year, NASA successfully launched two Radiation Belt Storm Probes— later dubbed the “Van Allen Probes”— with the central mission of mapping and examining the volatile Van Allen radiation belts: two doughnut-shaped radiation belts that extend from 1,000 km to 60,000 km above the Earth’s surface.
Aerospace engineers and scientists participated intensively in the mission planning and continue to be involved in data analysis seven months after the initial launch. The main mission is scheduled to take place over the course of two years, but NASA has put aside resources to potentially extend the mission for an additional two or three years depending on the success of the current research and data analysis.
Scientists are now putting together an impressive set of data from the probes that hints at some wonderful discoveries to come.
For generations, the Van Allen belts— named for James Van Allen, whose cosmic ray experiment aboard the Explorer 1 satellite led to the discovery of the belts in 1958 — have persistently caused problems for spacecraft. During the initial Explorer 1 mission, Van Allen and his colleagues observed a recurrent phenomenon in the satellite’s radiation detector. Explorer 1’s Geiger-Müller Tube consistently registered a null value while passing through what would later be recognized as the inner belt; a seemingly impossible occurrence since scientists expected at least some detectable level of radiation in the area. Van Allen and his team eventually realized that the Geiger 2 had actually been saturated with radiation to the point that it ceased to function, thus producing the null value. This realization led to the discovery of the inner belt, which is often cited as the first scientific discovery of the space age.
In a physical sense, the Van Allen belts consist of an inner belt, dominated by very energetic proton radiation, and an outer belt, comprised largely of energetic electrons. These charged particles can easily penetrate spacecraft shielding and cause any number of technical catastrophes in electrical devices.
Between the two belts is a dynamic slot region that, while typically void of radiation, will fill in with ionizing radiation from time to time. The slot and outer zones are the most volatile parts of the region and the wild environmental shifts that they exhibit are the primary catalyst for NASA’s Van Allen Probe mission. Understanding, and perhaps one day, predicting the behavior of the region is incredibly valuable information for both scientists and satellite builders alike.
Much of the current mission is focused on mapping and documenting the Van Allen belts in a comprehensive manner. The two satellite probes are being used to gather information and examine the dynamism of the radiation over time in order to enhance current climatology models. Aerospace scientist Dr. Paul O’Brien has been involved in the Van Allen probe mission from the very beginning and sees great synergy between the Van Allen research and the needs of Aerospace’s customers. “The data from the Van Allen probes will feed into the climatology models called AE9 and AP9, which will continue to be used to set environmental design and test specifications for future satellite systems,” said O’Brien. “This will allow satellite designers to reduce margins where possible and to have higher confidence that the specifications and tests that they are conducting will lead to on orbit success.”
Aerospace engineers have built sensors for the probes that are being used to observe geomagnetic storms in the belts. In addition, Aerospace has also collaborated with a national security agency to build a relativistic proton spectrometer that is measuring part of the proton population in the inner belt that has never been directly measured before.
The Van Allen Probes mission is the first of its kind to utilize two probes in order to study the Van Allen belts. In essence, the probes check each other’s work, crossing over previously observed orbital locations in order to observe temporal and spatial variations. “This is definitely the most comprehensive approach,” said O’Brien. “We have two probes now and we’ve learned a lot about how to design instruments so that they will function throughout the entire orbit. It’s a more comprehensive data environment for this mission than we’ve ever had before.”
The orbit of the Van Allen probes lasts for nine hours with each probe passing through each belt twice per orbit. This amounts to a total of about twelve passes through each belt in a little more than a day’s time. Over the past seven months, the probes have held up beautifully in a region that is outwardly hostile towards electrical devices.
The satellites are poised to deliver fascinating data sets for years to come and early results seem to confirm that there is a wealth of knowledge to be gleaned by the probes. Four days after the initial launch of the probes, scientists observed a third radiation belt that appeared just beyond the outer belt. It remained present for four weeks until it was destroyed by a shock wave from the sun. This discovery ignited the scientific community and has led to a lot of excitement among the general public as well.
Dr. James Clemmons, principal director, Space Science Applications Laboratory, is intrigued by a number of preliminary data sets, but acknowledges that it will take a lot of time and analysis to make sense of all the collected information. “Most of the results are going to take a lot of thinking,” said Clemmons. “They are going to take a lot of people to put together. There are five instrument suites onboard the probes and each one has its own science team. People will work on their own data, but we won’t get the whole picture until we put it all together.”
Though most of the key scientific breakthroughs will present themselves further on in the mission, there are a few unique belt behaviors that have been observed recently. “We have some preliminary results that are pretty interesting” says Clemmons. “One thing we’ve seen is that after the space weather activity increases, which makes the belts stronger a lot of times, we’ve seen some very coherent modulations in the fluxes. They go up and down in a regular sort of period. We saw a little of this back in the ’90s, but our instruments are superior now and we’ve seen that they [modulations] are a lot more common and they’re much better defined than what we’ve seen in our measurements.” By comparing this data with the electric and magnetic fields data from the same mission, scientists will be able to better understand what directly causes the unique behavioral shifts.
As the mission progresses, there will certainly be new and intriguing revelations about a pair of radiation belts that have beguiled and confounded the satellite and scientific communities for generations.
“The goal of science is understanding to the point of predictability,” said Clemmons. “And we have some of that understanding, but not all that we need. And that is why the Van Allen Probes are being flown.”