On May 10 and 11, 2024, a solar storm treated residents in the Northern Hemisphere as far south as southern California to a heavenly sight: the cascading sheets of multi-colored energized particles known as the aurora borealis, or northern lights.
Meanwhile, satellite teams at NASA's Goddard Space Flight Center in Greenbelt, MD, were in a race against time to prevent the storm from damaging Earth observation satellites.
The May event that created intense auroras and problems for satellite teams was the result of one of the strongest geomagnetic storms to hit Earth in more than two decades. One satellite, NASA's Ice, Cloud, and Land Elevation Satellite-2 (ICESat-2), transitioned into safe mode to protect its systems as the storm caused the spacecraft's attitude control to become questionable; two other satellites (NASA's Aqua and Aura) came within minutes of having to go into safe mode, according to Russell DeHart, Mission Operation Assurance Lead Engineer at NASA Goddard. It was a close call, but one that mission teams are prepared to address to protect satellites, instruments, and NASA Earth science data.
While the Sun powers our planet and makes life possible, it can create headaches for teams managing NASA's constellation of Earth observation satellites. Solar flares, coronal mass ejections (CMEs), and the constant bombardment of solar radiation—all of which fall under the umbrella of space weather—contribute to the mix of conditions that make space an unforgiving environment and one that constantly is changing. Three ways our closest star can make life difficult for mission teams are in the areas of satellite orbits, long-term instrument health, and data transmission from satellite to ground.
Low Earth Orbit is a Crowded Place
Low Earth orbit (which refers to orbits 1,200 miles or less above the planet) is home to more than 5,000 satellites, including the International Space Station and many NASA Earth observation missions. But this is only part of the story. It's also full of space debris, which can be pieces of satellites, expended launch vehicles, and even flecks of paint. NASA's Orbital Debris Program Office (ODPO) puts the number of particles larger than 1 mm in low Earth orbit at more than 100 million.
Now throw a solar storm into the mix. The Sun is constantly emitting bursts of highly charged particles and goes through cycles called maximums (periods with more solar activity) and minimums (periods of lower solar activity) roughly every 11 years. A solar storm refers to a more powerful than normal emission of energy. These can occur any time but are more prevalent during a solar maximum. Two of the most common types of solar storms are solar flares and CMEs. Solar flares are eruptions of radiation that hit Earth about eight minutes after leaving the Sun and can interfere with satellite radio signals. CMEs are slower moving eruptions of huge clouds of charged particles that hit Earth one to two days after leaving the Sun and can cause massive damage to power grids, satellites, and communications.
When a CME hits Earth's atmosphere, it causes the atmosphere to warm and expand. This denser atmosphere creates a thicker medium through which a satellite must travel, and this additional drag can slow a satellite's momentum enough to lower its orbit. Satellite orbits are constantly being monitored, and many spacecraft carry fuel for emergency maneuvers if they come too close to other satellites or space debris (called conjunction events).
"At Goddard there are two entities [monitoring satellite orbits]," says Dr. Yihua (Eva) Zheng, research astrophysicist with NASA's Heliophysics Science Division. "NASA's Earth Science Mission Operations Project [ESMO] monitors six low Earth orbit missions. Then there's NASA's CARA—Conjunction Assessment Risk Analysis—team that works with the U.S. Space Force."
The CARA team performs risk analysis of close approach prediction data and assists with maneuver planning to mitigate identified high-interest events. The May solar storm significantly impacted satellite orbits.
"For some of the [low Earth orbit] science missions, we saw a lowering of the [satellite's] orbit of anywhere from dozens of meters to hundreds of meters," says DeHart. "It does remind you of how powerful the Sun is that just one event can lower spacecraft orbits by, as we saw, as much as 400 to 600 meters for some of our spacecraft."
These orbit changes can, in turn, impact instrument data. When a satellite's orbit changes, finely tuned instruments aboard the spacecraft may not be able to collect required imagery.
"Most satellites are designed to operate at a specific distance from Earth, and instruments are designed and calibrated based on operating at a particular orbit," says Eric Moyer, NASA ESMO deputy project manager technical, in an email. "When missions go outside that mission orbit, the science data may become degraded or unusable (out of focus). [ESMO] corrects for this by performing more frequent and/or larger maneuvers to maintain the desired mission orbit."
But using maneuvering fuel can shorten a satellite mission. For missions that carry propellant (not all satellite missions do), using fuel to correct an orbit can result in reduced mission life since spacecraft often need to reserve a minimum amount of fuel to maintain science operations and conduct a controlled re-entry. Extra maneuvers cut into how long the spacecraft can operate.
"You can see years shaved off the estimate for how many years [of fuel] would be left if a solar cycle was more active than originally anticipated," says DeHart.
Protecting Instruments
Instruments aboard Earth observation satellites and the computers and software that control them are designed to withstand the constant bombardment of solar and cosmic radiation. However, fully protecting instruments and computers from radiation would lead to significant increases in cost and weight. Just like a home computer, a sudden burst of energy can cause issues. And, like a home computer, mission teams have similar fixes.
"When we have these highly energetic particles coming to Earth, they can end up hitting a computer chip aboard a spacecraft and cause a [computer programming] bit to flip, which can then cause either the [spacecraft] computer software or the hardware to have an anomaly where we’ll have what’s called a single-event upset," says DeHart.
A bit flip is a change in a computer's magnetic memory data caused by high energy particles interacting with the computer’s memory hardware. Computers store data in the form of bits, which are sequences of 0s and 1s. High energy particles during a solar radiation storm can alter the properties of a data bit and cause it to change, or flip, from a 0 to a 1 or from a 1 to a 0.
DeHart explains that if a single-event upset disrupts a critical system during these events, the satellite is programmed to automatically stop doing anything not necessary for basic operations and point its solar panels at the Sun to make sure its battery stays charged while the ground team figures out what went wrong. This is referred to as putting the satellite into safe mode. Safe mode is designed to protect the spacecraft bus and the instruments on board. Safe mode can be caused by a single-event upset or if a piece of hardware is approaching its operational limit, as was the case with ICESat-2 and was nearly the case for Aqua and Aura during the May 10 solar storm.
"When you enter safe mode, science [data collection] stops and you investigate what’s going on with the spacecraft," says DeHart. "And of course you don't want to rush these things; you have to do a very methodical investigation on the ground."
Sometimes getting a satellite and its computers back in operation is as simple as rebooting the system. "One way to address issues is to design components that can be power cycled numerous times, which often clears out issues encountered," says Moyer. "Also, when referring to [computer] processors, bit error check and auto correction are used to identify if a bit gets flipped and to correct [this issue]. Multi-bit errors are not as easy to correct and may require [putting the satellite or instrument in safe mode] to allow code to be reloaded."
As DeHart observes, though, "it's certainly not where you can just go to your friend down the street who happens to write software. Thankfully, we maintain that expertise."
Astrophysicist Zheng also notes that timing is everything. "The most direct damage [to a satellite] can be from the [solar] flare. The flare can arrive at a satellite in about eight minutes," she says. "The CME comes to Earth, usually, in one to two days."
While one to two days is still a short amount of time, it does enable mission teams to be aware of an impending CME and take precautions to protect a satellite and instruments. Alerts and warnings come from many sources, but the primary alerts are from NOAA's Space Weather Prediction Center (SWPC) and NASA's Community Coordinated Modeling Center (CCMC) and Moon to Mars (M2M) Space Weather Analysis Office, both of which are located at NASA Goddard.
The CCMC is a multi-agency partnership that performs research and development for next-generation space science and space weather models. The M2M was established to support NASA’s Space Radiation Analysis Group (SRAG) with human space exploration activities, and its space weather analysis activities originated with the CCMC. The CCMC, M2M, and SRAG partner together on model validation. After a space weather event, M2M, in conjunction with SRAG and CCMC, runs an evaluation of the model output to identify any forecast delays and the reasons for these delays to help improve their models.
"One to two days is the most time you’ll have for a concrete warning. We received an alert on May 9 that there may be a very strong geomagnetic storm on May 10," says DeHart. "I sent a warning to the Earth science mission directors that we may have a strong geomagnetic storm and we might have a single-event upset occur with one or more spacecraft and to keep this in mind in case we see an anomaly pop up."
Satellite Communications
Periods of strong solar activity can make it difficult to downlink data from a satellite instrument to ground stations. Satellites transmit data to Earth using different frequencies of the electromagnetic spectrum. Solar flares are often accompanied by solar radio bursts (SRBs), which have the potential to disturb satellite data transmission. Zheng points to a powerful solar storm in 2006 that compromised numerous global positioning system (GPS) receivers due to the frequency of the SRBs being similar to those used by the GPS satellites to transmit data.
Another problem occurs when a satellite's orbit lowers due to increased atmospheric drag from a CME. As the satellite orbits closer to Earth, it circles the planet more rapidly. This makes the spacecraft arrive into view of ground station antennas earlier than expected, which can cause difficulty in acquiring good communications to downlink data. Moyer notes that two ways to deal with this are to send computer files to ground stations to correct the expected overpass timing of the satellite or to use maneuvering fuel to perform a drag make up maneuver, boosting the satellite back to its mission orbit.
Here Comes the Sun
"Space weather is not nearly to the point of Earth weather, where you can say, for example, five days from now we're going to have rain or a storm," says DeHart. "The physics of the Sun is very complicated; this is still an evolving science."
Missions such as NASA's Parker Solar Probe and Solar TErrestrial RElations Observatory (STEREO), the joint NASA/ESA (European Space Agency) Solar and Heliospheric Observatory (SOHO), and the joint NASA/NOAA/U.S. Air Force Deep Space Climate Observatory (DSCOVR) are continually collecting data that are helping to improve forecasts of solar events. Zheng notes that with the chronographs aboard SOHO and STEREO, it's possible to measure CMEs from two points of view, providing a sense of spread and direction. This, in turn, improves forecasting of these events. "Even though it's not perfect yet, the space weather services and [their] advanced warnings have been very helpful in terms of giving mission teams early warning so they can safeguard their instruments," she says.
DeHart notes that data from ongoing missions like STEREO, SOHO, and DSCOVR are contributing to better models being provided to NASA's CCMC, M2M, and SRAG to further improve forecasts of potential solar events. In addition, new missions such as the joint NASA/NOAA Geostationary Operational Environmental Satellite (GOES)-U (launched on June 25, 2024) will further help refine space weather forecasts. Still, he stresses that when it comes to the Sun, mission operations teams can only do so much.
"There are certainly actions that are taken during spacecraft design and assembly and during operations [to keep satellites, instruments, and data safe]," says DeHart. "But ultimately, the Sun is the boss, and we have to work around that."
