Launched in 2012, NASA’s twin Van Allen radiation belt probes have been swinging through the hazardously charged-particle region around Earth while returning valuable scientific data that has not only helped scientists better understand this protective region around Earth but also plan for future missions of exploration that will travel beyond the Van Allen belts.
More than six years after launch, controllers have begun the process of ending the Van Allen probes mission by lowering their orbital perigees from 385 miles (619 km) to 190 miles (305 km) – a change that will set up an end to the probes’ mission in early 2020 and a controlled re-entry roughly 15 years from now.
The coming end:
It might seem odd that the Van Allen Probes mission has begun taking steps for a destructive reentry to Earth’s atmosphere 15 years from now.
But in the world of close-to-Earth space travel, it all comes down to ensuring that the probes do not contribute to the large population of space junk and potentially hazardous debris in Earth orbit.
And this, for the Van Allen probes, is something that requires action now for an event that won’t occur until 2034.
“In order for the Van Allen Probes to have a controlled reentry within a reasonable amount of time, we need to lower the perigee,” said Nelli Mosavi, project manager for the Van Allen Probes at the Johns Hopkins Applied Physics Laboratory. “At the new [perigee], aerodynamic drag will bring down the satellites and eventually burn them up in the upper atmosphere.”
To lower their orbital perigees (point of closest approach to Earth), controllers will command the probes through a 10-day series of engine burns one at a time that will lower their perigees from 385 miles (619 km) to 190 miles (305 km).
This process began for Van Allen probe-B two days ago on 12 February 2019 and will culminate on 22 February. The second spacecraft, Van Allen probe-A, will perform the same set of maneuvers from 11-22 March 2019.
The perigee lowering operations themselves are a series of five, two-hour burns of each probes’ engine. But lowering the Van Allen probes’ perigees was not simply a matter of aligning each probe’s engine for a retrograde burn at orbital apogee (farthest point in orbit of Earth). [NOTE: Burning retrograde at apogee will lower an orbit’s perigee.]
Because the probes spin as part of their operation, mission controllers had to calculate when each spinning probe’s engine would align in the needed direction at just the right point in the orbit so that burning the engine would lower the spacecraft’s perigee.
When calculated, NASA noted that the needed geometry for the burns only happens once or twice every year.
February and March 2019 turned out to be the optimal months to perform the perigee lowering operations as doing so now leaves both Van Allen probes with enough onboard fuel to keep their solar arrays pointed at the Sun for another year.
“We’ll continue to operate and obtain new science in our new orbit until we are out of fuel, at which point we won’t be able to point our solar panels at the Sun to power the spacecraft systems,” said Mosavi.
Once the probes are unable to keep their solar panels pointed at the Sun, an event expected to occur in early 2020, their mission will officially come to an end.
But the coming final year of operation will not only see the probes continue to return information about Earth’s radiation belts, but also provide unique and new insights into how oxygen in the upper atmosphere degrades satellite instrumentation.
It is hoped this new information will eventually contribute to more robust spacecraft instrumentation builds.
Once the mission formally comes to an end in 2020, the Van Allen probes will remain in Earth orbit for another 14 years as their orbits are gradually lowered and circularized by atmospheric drag from their low-altitude perigees.
By 2034, their orbits will be nearly circular and will quickly degrade from that point due to atmospheric drag, with the two probes burning up in Earth’s atmosphere and safely disintegrating – thus preventing them from becoming potentially dangerous space debris.
“The spacecraft and instruments have given us incredible insight into spacecraft operations in a high-radiation environment,” said Mosavi. “Everyone on the mission feels a real sense of pride and accomplishment in the work we’ve done and the science we’ve provided to the world – even as we begin the de-orbiting maneuvers.”
But like all spacecraft, the Van Allen probes have much yet to teach us in their final year, and their exploration of this high-radiation region around Earth will continue to influence spacecraft design long after they cease to function.
This is especially true for planned human missions beyond Low Earth Orbit – including missions to Earth-Moon Lagrange points, the Moon, and outward to Mars.
These regions are all located beyond the protective Van Allen radiation belts and will – by necessity – require spacecraft to function in high-radiation environments while simultaneously doing what they can to protect the crews they carry.
Likewise, on Mars (which does not have a protective radiation band like the Van Allen belts), engineers will have to design structures that can shield future Martian explorers from as much harmful radiation thrown outward by the Sun as possible.
To this end, the Van Allen probes have been immensely valuable not just in the information they have returned about how the radiation belts function in and react to different types of space weather during the Sun’s solar cycle, but also in the effectiveness of their radiation-hardened systems and engineering.
Unlike other spacecraft which can afford the option of being turned off during passage through the radiation belts or shut down during intense solar storms, the Van Allen probes had to remain functional in the high-radiation environment and throughout anything the Sun could throw at them.
Thus the probes were a critical step in designing our future human-rated exploration craft, which cannot simply have their life-critical systems turned off in a potential high radiation exposure event.