Space Station’s Ongoing Battle Against Atmospheric Drag
The International Space Station (ISS) has been orbiting Earth for over 26 years, 250 miles (400 kilometers) above the planet’s surface. Despite this impressive height, the station experiences drag from Earth’s atmosphere, which gradually pulls it lower due to collisions with oxygen molecules and other gases.
To maintain its orbital position, NASA and its partners regularly execute reboost maneuvers. Traditionally, these are accomplished using the ISS’s weaker internal thrusters or via Russia’s Progress spacecraft and Northrop Grumman’s Cygnus. However, starting in September, NASA will incorporate SpaceX’s Dragon vehicle into this process for the first time.
Launching the Reboost System
On Sunday at 2:45 a.m. ET, SpaceX’s Dragon successfully launched to the ISS, transporting more than 5,000 pounds of supplies in what was otherwise a routine commercial resupply mission. Notably, this mission also included a propellant system for reboost operations, hidden away in the Dragon’s trunk.
This boost kit will help maintain the altitude of the ISS through planned burns during the fall, effectively nudging the station back up in its orbit. While docked at the ISS, the Dragon will utilize a propellant system that is separate from the one intended for its own engines. Instead, this boost kit fuels two Draco engines housed in the spacecraft’s trunk, designed using existing propellant systems.
Engine Specifications and Functionality
The Dragon’s main engines are not positioned correctly for the reboost maneuvers. Thus, additional rear-facing engines, aligned with the ISS’s velocity vector, are employed. These engines connect to propellant tanks containing hydrazine and nitrogen tetroxide, which ignite upon contact. During a reboost operation, the engines will fire, adjusting the ISS’s altitude in low Earth orbit.
Demonstration and Future Plans
In November 2024, NASA and SpaceX tested Dragon’s abilities for reboost maneuvers, successfully adjusting the ISS’s orbit by 7/100 of a mile at its highest point and 7/10 of a mile at its lowest. “By testing the spacecraft’s ability to provide reboost and, eventually, attitude control, NASA’s International Space Station Program will have multiple spacecraft available to provide these capabilities for the orbital complex,” stated NASA at the time.
The Dragon spacecraft will remain attached to the ISS until December, marking the longest duration for a cargo mission, to perform these reboost maneuvers. The current boost kit represents a smaller model of a system SpaceX is developing for the ISS’s final deorbit procedure.
With the retirement of the ISS anticipated by 2030, NASA plans to use a Dragon spacecraft to carry out a series of deorbit burns, lowering the station’s altitude until it disintegrates upon re-entry into the Earth’s atmosphere. Until then, the ISS can expect a helpful nudge from the Dragon.
