A bundle of depleted batteries from the International Space Station careened around Earth for almost three years before falling out of orbit and plunging back into the atmosphere Friday. Most of the trash likely burned up during reentry, but it's possible some fragments may have reached Earth's surface intact.
Larger pieces of space junk regularly fall to Earth on unguided trajectories, but they're usually derelict satellites or spent rocket stages. This involved a pallet of batteries from the space station with a mass of more than 2.6 metric tons (5,800 pounds). NASA intentionally sent the space junk on a path toward an unguided reentry.
Naturally self-cleaning
Sandra Jones, a NASA spokesperson, said the agency "conducted a thorough debris analysis assessment on the pallet and has determined it will harmlessly reenter the Earth’s atmosphere." This was, by far, the most massive object ever tossed overboard from the International Space Station.
The batteries reentered the atmosphere at 2:29 pm EST (1929 UTC), according to US Space Command. At that time, the pallet would have been flying between Mexico and Cuba. "We do not expect any portion to have survived reentry," Jones told Ars.
The European Space Agency (ESA) also monitored the trajectory of the battery pallet. In a statement this week, the ESA said the risk of a person being hit by a piece of the pallet was "very low" but said "some parts may reach the ground." Jonathan McDowell, an astrophysicist who closely tracks spaceflight activity, estimated about 500 kilograms (1,100 pounds) of debris would hit the Earth's surface.
"The general rule of thumb is that 20 to 40 percent of the mass of a large object will reach the ground, though it depends on the design of the object," the Aerospace Corporation says.
A dead ESA satellite reentered the atmosphere in a similar uncontrolled manner February 21. At 2.3 metric tons, this satellite was similar in mass to the discarded battery pallet. ESA, which has positioned itself as a global leader in space sustainability, set up a website that provided daily tracking updates on the satellite's deteriorating orbit.
As NASA and ESA officials have said, the risk of injury or death from a spacecraft reentry is quite low. Falling space debris has never killed anyone. According to ESA, the risk of a person getting hit by a piece of space junk is about 65,000 times lower than the risk of being struck by lightning.
This circumstance is unique in the type and origin of the space debris, which is why NASA purposely cast it away on an uncontrolled trajectory back to Earth.
The space station's robotic arm released the battery cargo pallet on March 11, 2021. Since then, the batteries have been adrift in orbit, circling the planet about every 90 minutes. Over a span of months and years, low-Earth orbit is self-cleaning thanks to the influence of aerodynamic drag. The resistance of rarefied air molecules in low-Earth orbit gradually slowed the pallet's velocity until, finally, gravity pulled it back into the atmosphere Friday.
The cargo pallet, which launched inside a Japanese HTV cargo ship in 2020, carried six new lithium-ion batteries to the International Space Station. The station's two-armed Dextre robot, assisted by astronauts on spacewalks, swapped out aging nickel-hydrogen batteries for the upgraded units. Nine of the old batteries were installed on the HTV cargo pallet before its release from the station's robotic arm.
Delayed consequences
NASA's analysis suggested the discarded battery pallet would burn up on reentry, but this is not how the agency prefers to dispose of trash in low-Earth orbit. During NASA satellite launches, the agency requires launch providers to reserve enough propellant to remove the rocket's upper stage from orbit or put it on a trajectory where it won't interfere with other spacecraft.
After delivering supplies to the International Space Station, cargo ships depart the outpost with tons of trash and surplus equipment. SpaceX's Dragon capsule brings this cargo back to Earth intact, while automated supply ships from Northrop Grumman and Russia dispose of trash with targeted destructive reentries over the ocean.
Japan's HTV cargo freighters served a similar role on nine flights to the ISS. Four of the Japanese resupply missions delivered 24 of the upgraded lithium-ion batteries to the station as part of a mid-life modernization of the lab's electrical system.
Officials originally planned to dispose of most of the old batteries on the same HTV spacecraft that launched the new batteries. Some of the old batteries remained on the station for long-term storage as spares.
The batteries were changed out robotically, with help from spacewalking astronauts, while the HTV remained attached to the space station. The original idea was to remove the new batteries from the HTV cargo pallet, put the old batteries in their place, then robotically load the battery tray back into the HTV before departure. After leaving the station, the HTVs reentered the atmosphere over the South Pacific Ocean. Most of the spacecraft and battery hardware burned up, and any surviving debris fell into a remote part of the sea.
NASA followed this plan until 2018 when a Soyuz launch failure prevented NASA astronaut Nick Hague and Russian commander Alexey Ovchinin from reaching the space station. The crewmen safely landed in Kazakhstan, but Hague was supposed to help install the new batteries delivered to the station a few weeks before by an HTV cargo ship.
With Hague still on the ground, the station's robotic arm removed the battery pallet from the HTV and put it in a temporary storage location until a new team of astronauts was ready to install them. Then the robot arm released the Japanese supply ship to conclude its mission. This threw off the cycle of replacing new batteries with old batteries, and each subsequent HTV mission departed the station with the cargo pallet left behind by the previous HTV.
But the HTV program ended in 2020, and none of the station's other cargo freighters are designed to accommodate the battery pallet, which is about twice as tall as a standard refrigerator. A new Japanese cargo spacecraft, the HTV-X, still hasn't flown, so there was no vehicle to dispose of the final tray of nine batteries.
Without any propulsion of its own, the cargo pallet couldn't steer itself toward reentry over an unpopulated area. The space station's orbit takes it between 51.6 degrees north and south latitude. "Large uncertainties, primarily driven by fluctuating levels of atmospheric drag, prevent more precise predictions at this time," ESA said Thursday. "The closer we get to the expected reentry window, the better the concerned region can be geographically constrained."
Early Friday, the reentry prediction from US Space Command narrowed to a window of six hours and then to four hours. Space Command later confirmed in a Trajectory Impact Prediction (TIP) message that the pallet decayed from orbit, or reentered the atmosphere, at 2:29 pm EST (1929 UTC).
