Van Allen Belts
How many scientific spacecraft does NASA have up there specifically for the purpose of surveying the Van Allen Belts?
The answer stunned me – nine satellites!
In addition, there will be another satellite from JAXA going up later this year.
A challenge to understanding the Van Allen Belts is that it’s a vast region of nearby space that varies in composition and energy spectrum as a function of both space and time. One satellite won’t do as it has no way to distinguish between spatial variation and time-fluctuation. There are time-dependent variations on long time-scales due to the 11 year sunspot cycle as well as short-term variations due to individual bursts of energy from the Sun. The composition of the Van Allen Belts consists of electrons, protons and ions, each with its own energy spectrum, as well as electric and magnetic fields and plasma waves.
The first launched by NASA was a set of five satellites nine years ago called “Time History of Events and Macroscale Interactions” (THEMIS). Two of these have since been moved closer to the Moon, the remaining three are still surveying the Earth’s magnetosphere.
Three and a half years ago NASA launched a pair of satellites originally called “Radiation Belt Storm Probes” “RBSP” and now called “Van Allen” one and two. The original mission for these was two years, but that’s now been extended for another two years as a method has been found to reduce fuel usage required for varying the orbits. The mission will probably be terminated when fuel for manoeuvring runs out. The two spacecraft are in similar orbits but lap one another every now and then giving the full range of distances between craft.
Then just half a year ago NASA launched a group of four satellites called “Magnetospheric Multiscale” (MMS).
3 from Themis + 2 from Van Allen + 4 from MMS makes 9 NASA spacecraft in all.
At the same time, NASA also has a series of balloon flights operating together with the Van Allen spacecraft. That project is called BARREL, short for Balloon Array for RBSP Relativistic Electron Losses.
The new Japanese JAXA satellite studying the Van Allen Belts is “Exploration of energisation and Radiation in Geospace” (ERG), due for launch later this year.