SAMPEX mission patch (Credits: NASA).

The Solar, Anomalous, and Magnetospheric Particle Explorer, or SAMPEX, is ending its long operational phase, begun in 1992. The orbit of the 20-year-old spacecraft has decreased enough that, some time around the end of 2012, it will be burned up completely while re-entering Earth’s atmosphere.

“SAMPEX was launched on a shoe string budget,” says Shri Kanekal, a space weather scientist at NASA’s Goddard Space Weather Center, who has been involved with SAMPEX research since its launch. “It was proposed as a minimum one-year mission with a goal of three years, but it lasted for an unexpectedly long time. It has provided 20 years of high quality data, used by nearly everyone who studies near-Earth space.”

In 1992, when SAMPEX was launched, the 11-year solar cycle’s peak was about to reach its maximum. NASA’s scientists wanted to observe the space weather in the 5 year period between this peak and the solar minimum. SAMPEX was supposed to be de-orbited around 2000 by the same solar output it was going to watch. Luckily for space weather scientists, the predictions were wrong and the small explorer survived the atmospheric drag created by the solar peak, continuing its orbit long enough to get close to the next maximum, predicted for 2013.

During its 20 years of observation, SAMPEX has collected data on the near Earth radiation environment during the solar cycle, observing the change over time due to incoming particles from the Sun and the galaxy. Data from SAMPEX has validated the theory that outer space cosmic rays are trapped by Earths’ magnetosphere and gathered in a belt around the planet. Moreover, it helped identify the composition of those energetic solar particles.

SAMPEX’s observation on the radiation belts hold practical value, since these two toroidal regions around Earth can occasionally damage satellites’ electronics due to the presence of high energy particles. When solar activity is particularly strong or localized due to the slight offset of the belts from Earth’s geometric center, the inner boundary of the inner radiation belt can drop to an altitude of just 200 km above the planet’s surface.

Scientists are eager to compare the last data from SAMPEX with the new data from Radiation Belt Storm Probes (RBSP), a NASA mission launched in August 2012, to study the microburst phenomenon. The combination of data from RBSP and the soon-to-be-deorbited SAMPEX will help to understand events in which electrons drop out of the radiation belts. SAMPEX will continue collecting data until its final burst as it burns up in the Earth’s atmosphere.

Here, a simulation of Earth’s radiation belts constructed from SAMPEX data around the time of the well studied 2003 “Halloween” solar storms: