The International Space Station (ISS) has been circling the Earth since November 1998. Even though it’s in space, it still feels some drag from the atmosphere. This isn’t a lot, but it’s enough that the ISS needs periodic boosts to stay in orbit. Without these “reboosts,” the station would eventually fall back to Earth.
The U.S. and Russia were aware of this when they began building the ISS. Early modules like Zarya and Zvezda had thrusters designed to counteract this drag. Over time, Zarya’s thrusters were taken offline, and its fuel was integrated into Zvezda, allowing for greater fuel efficiency.
Visiting spacecraft regularly top off the fuel tanks. According to a NASA report, over 40,000 kilograms of propellants had been transferred to the ISS by 2019. Notable vehicles, like the Russian Progress spacecraft, typically docked with Zvezda to provide these boosts. The Progress has a long history—dating back to 1978—for resupplying space stations like Mir long before the ISS was built.
Recently, SpaceX’s Cargo Dragon joined the lineup of spacecraft capable of boosting the ISS. This capability will not only help keep the ISS in orbit, but it also adds valuable data for its future decommissioning.
Historically, Russian Progress vehicles have been the go-to for reboosts. Although European Space Agency’s Automated Transfer Vehicle (ATV) also served this purpose, its operation was limited. The ATV could carry more cargo but only completed five missions from 2008 to 2014.
On the American side, boosting the ISS is a bit more complicated. The entire station needs to rotate to align correctly with the thrusters of shuttles—mostly retired since 2011. SpaceX’s Cygnus spacecraft also performed reboosts. The newest version, Cygnus XL, is expected to start missions soon.
SpaceX’s Cargo Dragon was modified specifically for reboosting. Extra propellant tanks and rear-thrusting Draco engines were added to make this possible. This innovation is essential as SpaceX will develop a “Deorbit Vehicle” to safely re-enter the ISS into Earth’s atmosphere, likely using similar technology.
Overall, the ability to boost the ISS highlights a significant shift in space logistics, especially as international relations evolve. The ISS, once a symbol of cooperation, is now facing an uncertain future, but its operational advancements are paving the way for what’s next in space exploration.
For more information, you can check out the NASA report on in-space propellant transfer, which dives deeper into the challenges and achievements in orbital fuel resupply.
