A solar probe on a mission to take the closest ever images of the sun will pass incredibly close to Earth today, but debris from the recent Russian anti-satellite missile test will make its visit more risky and unpredictable.
The European Space Agency's (ESA) Solar Orbiter spacecraft will zoom just 285 miles (460 kilometers) above Earth's surface on Friday at 11:30 pm EST (Saturday, 4:30 GMT). This close encounter, a maneuver called flyby, will help nudge the satellite closer to the sun so that it can commence its scientific exploration of the star.
But the Nov. 15 anti-satellite missile test by Russia, which shattered the nearly 2-ton defunct Kosmos 1408 satellite into pieces, adds worry for the ground control teams who have been carefully plotting Solar Orbiter's path to make it efficient and safe.
"Solar Orbiter will fly through the most polluted areas around Earth," Andrea Accomazzo, ESA's head of solar system and exploration, who oversees the flyby, told Space.com. "We run calculations in which we compare the path of Solar Orbiter with the trajectories of all known space debris objects. The problem is that the [Russian ASAT] test happened so recently that there is only partial information about the debris it created."
While the $1.5 billion Solar Orbiter nears Earth, the operators are finetuning those calculations. So far, Accomazzo said, all looks good. But in case a piece of known space junk would appear to be on a collision course with the precious sun explorer, the operators would perform a last-minute maneuver to guide the spacecraft through a safer zone. This maneuver, Accomazzo said, would change the altitude of Solar Orbiter's closest approach to Earth by about 12 miles (20 km). While it might seem insignificant, such an adjustment would make the flyby less efficient for its purpose—to tighten Solar Orbiter's path around the sun with the help of Earth's gravity.
"It would slightly change the parameters of the gravity assist maneuver," Accomazzo said. "Later on, we would have to perform a correction using the propellant on board of Solar Orbiter."
Debris from the ASAT test is only a small part of the problem facing the sun probe. ESA estimates that there are currently some 36,500 pieces of space debris larger than 4 inches (10 centimeters) hurtling around Earth at incredible speeds (the Russian test created only about 1,500 of these). These, at least, can be tracked by ground-based radars and therefore avoided. In addition, there are some 1 million fragments between 0.4 inches and 4 inches (1 to 10 cm) across, and a staggering 330 million that are smaller than 0.4 inches (1 cm) but bigger than 0.04 inches (1 millimeter). These fragments are mostly invisible, yet, each of them can destroy or significantly damage a satellite.
In August 2016, a fragment of space junk about 0.04 inches (1 mm) in size smashed through a solar panel of the European Earth-observing Copernicus Sentinel-1A satellite, creating a 16-inch (40-cm) wide hole. Ground control teams managed to make up for the loss of power, and the mission successfully continues to this day, Accomazzo said. But the outcome could have been much more severe.
"If this particle had hit the main body of the spacecraft, it could have destroyed its onboard computer," said Accomazzo. "Without the onboard computer, the spacecraft is basically dead. Or it could have hit the propellant tank. And if it's a mission that relies on propulsion, if you start leaking, that's it."
Unlike Sentinel 1A and thousands of other spacecraft that live their whole lives in the heavily polluted low Earth orbit, the region of space up to the altitude of 600 miles (1,000 km), Solar Orbiter will find itself in the danger zone for only about two hours. One hour before it's closest approach to Earth, the satellite will zip through the geostationary ring, the orbit at the altitude of 22,000 miles (36,000 km) where satellites appear suspended above a fixed spot on Earth. This region is populated by broadcasting, telecommunication and weather forecasting satellites and also harbors a lot of clutter.
Solar Orbiter will then dive down right through the most cluttered region of space, the area of low Earth orbit between 250 and 500 miles (400 and 800 km) above the planet's surface, Accomazzo said. The spacecraft will pass just above the orbit of the International Space Station and head out of Earth's vicinity, crossing the geostationary ring again one hour after the closest approach.
Accomazzo previously oversaw three Earth flybys of perhaps the most famous ESA spacecraft—the comet chaser Rosetta. He says that although the risk for these briefly visiting spacecraft is relatively low, maneuvers relying on the gravity of our planet have become more challenging over the past decade.
"The three Rosetta Earth flybys took place in 2005, 2007 and 2009," said Accomazzo. "They all were at a higher altitude where there is less stuff. But it is certainly true that there is much more debris now than it was back then."
Hopefully, Solar Orbiter will wave farewell to Earth unscathed. Although it will not capture any images of the planet, scientists working with its sensitive instruments hope that it will make measurements of Earth's magnetic field and the solar wind in the planet's vicinity, ESA said in a statement.
Most importantly, this flyby will give Solar Orbiter the final kick it needs to get to its target distance from the sun. The big moment will come in about four months, in March 2022, when the sun observer will pass at a distance of only 34 million miles (50 million km) from the sun's surface (about a third of the Sun- Earth distance). This will be the closest any spacecraft carrying a camera will have approached the sun. NASA's Parker Solar Probe flies much closer but the environment it finds itself in is so hot that no existing imaging technology could take pictures of the sun from this close.
Scientists have high expectations from Solar Orbiter's next big moment. Already during its first close approach, which took the probe some 48 million miles (77 million km) from the sun, Solar Orbiter discovered new phenomena on the star's surface, miniature solar flares dubbed campfires. These campfires might be behind one of the biggest mysteries of the sun's behavior, the extreme heating of its corona.
But even then, Solar Orbiter's journey will only have just begun. Accomazzo said the spacecraft will keep periodically visiting Venus to use its gravity to tilt its orbit out of the ecliptic plane, the plane in which planets orbit. This will enable the spacecraft's sensitive imagers to take the first ever up-close images of the star's poles. And there, the scientists expect many more new discoveries.
Solar Orbiter, launched in February 2020, just before the onset of the COVID 19 pandemic. The mission went through the most sensitive commissioning phase during the first COVID lockdown in Europe with control teams operating in restricted socially distanced conditions.