SpaceX is building an upgraded version of its Dragon cargo spacecraft to lift the International Space Station out of orbit for a controlled reentry and breakup in an uninhabited stretch of ocean when the lab is finally retired in the 2030s, NASA and company officials said Wednesday.
The ISS Deorbit Vehicle, or DV, will be a unique, custom-built spacecraft needed to ensure the space station reenters the atmosphere in just the right place and with the right orientation to ensure any debris that survives the 3,000-degree heat of reentry will crash harmlessly into the sea.
In late June, NASA awarded SpaceX a contract worth up to $843 million to build the deorbiter, which will be owned and operated by the space agency. The heavy-lift rocket needed to launch it has not yet been selected, but NASA Administrator Bill Nelson has asked Congress for a total of about $1.5 billion to carry out the deorbiter operation.
And it's no small feat. The space station's long axis, made up of multiple pressurized modules where visiting crews live and work, is 218 feet (66 meters) long. The lab's power and cooling structure, mounted at right angles to the long axis, stretches 300 feet (94 meters) from end to end — longer than an American football field.
The entire lab complex has a combined mass of 925,000 pounds and moves through space at about 17,100 mph, or 84 football fields per second.
To carefully reduce its altitude for a controlled reentry, the ISS DV will carry about 35,000 pounds of propellant feeding 46 Draco rocket engines, 30 of which will be mounted in an extended trunk section to perform most of the deorbiting maneuvers.
“When we make the decision to deorbit the station, we will launch the U.S. DV approximately a year and a half before the final reentry burn,” said Dana Weigel, ISS program manager at the Johnson Space Center.
“We will dock it to the forward port, do a series of checks and, when we are satisfied that everything seems to be in good condition and we are ready, we will allow the ISS to begin its descent.”
The final crew of the space station will remain on board until periodic thruster firings and increasing “drag” in the extreme upper atmosphere combine to lower the laboratory to an altitude of about 205 miles. That milestone will be reached about six months before the final reentry procedure.
When the unmanned International Space Station (ISS) reaches an altitude of about 225 kilometers, the U.S. DV “will do a series of burns to get us ready for that final deorbit,” Weigel said. “And then four days later, it will do the final reentry burn.”
The space station's large but relatively flimsy solar panels will break off and burn up first, along with antennas, radiator panels and other appendages.
Larger components — modules and the lab's massive power structure — will also break apart in the hellish high-speed descent, but chunks as big as a small car are expected to survive until splashdown in the ocean along a narrow, 1,200-mile-long “footprint.”
Remote areas of the South Pacific Ocean offer unpopulated landing zones, although an ultimate goal has not yet been specified.
To achieve a precision entry, “the deorbit vehicle will need six times the usable propellant and three to four times the power generation and storage of the current Dragon spacecraft,” said Sarah Walker, SpaceX’s director of Dragon mission management.
“It needs enough fuel on board not only to complete the main mission, but also to operate in orbit in association with the space station for about 18 months. Then, at the right time, it will perform a complex series of actions over several days to deorbit the International Space Station.”
Some kind of deorbiter is needed because even at the space station's current altitude of 417 kilometers, there is still traces of atmosphere. As the station flies through that tenuous material at nearly five miles per second, collisions with those particles act to slow the craft slightly in a phenomenon known as atmospheric drag.
Throughout the life of the program, periodic booster firings have been performed by engines on Russian modules or on docked Progress cargo ships to increase the laboratory's altitude as needed to compensate for drag effects. More recently, Northrop Grumman's Cygnus cargo ships have added a modest reboost capability.
Without those carefully planned shots, the station would eventually crash itself back into the lower atmosphere.
The station flies all points on Earth between 51.6 degrees north and south latitude, covering the entire planet from London to the tip of South America. In an uncontrolled reentry, station debris that survived the warming of entry could strike the surface anywhere in that zone.
While the chances of impact in a populated area are relatively small, nothing as massive as the space station has ever re-entered and fallen to Earth, and NASA isn't taking any chances.
NASA and its station partners — the European, Canadian, Japanese and Russian space agencies Roscosmos — planned from the start to deliberately propel the laboratory into the atmosphere at the end of its life to ensure its disintegration in an uninhabited stretch of ocean.
The original plan was to use thrusters on several Russian Progress cargo ships to lower the lab's altitude and prepare for a fall toward Earth.
“Early in the planning of the station, we had considered doing the deorbit using three Progress vehicles,” Weigel said. “But the Roscosmos segment was not designed to control three Progress vehicles at once, so that presented a bit of a challenge.
“In addition, the capacity was not exactly what we really needed for the size of the station. That is why we jointly agreed to ask the American industry to analyze what we could do on our part for deorbiting.”
Last year, NASA asked for proposals from industry and two companies responded: SpaceX and Northrop Grumman. The agency announced last week that SpaceX had won the contract.
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