Last Thursday was apparently garbage day at the International Space Station, which rid itself of a 2.9-ton pallet of used nickel-hydrogen batteries. It's the biggest mass of space junk the ISS has unleashed, and NASA wrote that the pallet is "safely moving away from the station and will orbit Earth between two to four years before burning up harmlessly in the atmosphere." As for how big, a NASA rep tells Gizmodo the hunk is "more than twice the mass of the Early Ammonia Servicing System tank jettisoned by spacewalker Clay Anderson during the STS-118 mission in 2007." Syfy explains this wasn't the original plan. Japanese H-II Transfer Vehicle (HTV) disposable crafts were engaged in replacing those old batteries with lithium-ion ones, and each HTV would fall back to Earth with a battery pallet aboard. The items would burn up during reentry, with any remnants falling in the South Pacific.
As for why this pallet was jettisoned from the ISS instead, blame the "ripple" effect of an emergency landing in 2018 that messed with the ISS' disposal schedule, reports Syfy. Spaceflight Now explains that astronaut Nick Hague was supposed to take part in the battery swaps, but his failure to reach the ISS at that time ultimately resulted in there being one pallet of batteries that didn't get retrieved by an HTV. "Bad Astronomy" blogger Phil Plait isn't convinced of the long-term success of this plan, however, and tweeted Thursday, "This strikes me (haha, a pun given the circumstances) as dangerous. It seems big and dense so unlikely to burn up completely." Astronomer Jonathan McDowell concurred in a response, tweeting, "Yes. On the other hand e.g. Tiangong-1 was 7500 kg, much bigger. But I would say given how dense EP9 is, it's concerning, albeit at the low end of concerning." The US Space Command will keep an eye on things. (Read more International Space Station stories.)