SpaceX is building a souped-up version of its cargo Dragon spacecraft to drive the International Space Station out of orbit for a controlled re-entry and breakup over an uninhabited stretch of ocean when the lab is finally retired in the 2030 timeframe, NASA and company officials said Wednesday.
The ISS Deorbit Vehicle, or DV, will be a custom-built, one-of-a-kind spacecraft needed to make sure the space station re-enters the atmosphere at the precise place and in the proper orientation to insure any wreckage that survives the 3,000-degree heat of re-entry will crash harmlessly into the sea.
In late June, NASA awarded SpaceX a contract valued at up to $843 million to build the deorbit vehicle, 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 entire de-orbit operation.
And it is no trivial matter. The long axis of the space station, made up of multiple pressurized modules where visiting crews live and work, measures 218 feet long. The lab's solar array power and cooling truss, mounted at right angles to the long axis, stretches 310 feet from end to end, longer than a U.S. football field.
The entire lab complex has a combined mass of 925,000 pounds and it's moving through space at some 17,100 mph, or 84 football fields per second.
To carefully lower its altitude for a controlled re-entry, the DV will carry some 35,000 pounds of propellant powering 46 Draco rocket engines, 30 of which will be mounted in an extended trunk section to carry out the bulk of the deorbit maneuvers.
"When we do make the decision to deorbit station, we'll launch the U.S. DV about one-and-a-half years before the final re-entry burn," said Dana Weigel, the ISS program manager at the Johnson Space Center.
"We'll dock it to the forward port, we will do a series of checkouts and then once we're convinced that everything looks healthy and we're ready, we'll allow ISS to begin drifting down."
The final space station crew will remain on board until periodic thruster firings and ever increasing "drag" in the extreme upper atmosphere combine to lower the lab to an altitude of about 205 miles. That milestone will be reached about six months before the final re-entry procedure.
As the by-then-uncrewed ISS reaches an altitude of about 140 miles, the DV "will perform a series of burns to set us up for that final deorbit," Weigel said. "And then four days later, it will do the final re-entry burn."
The space station's large but relatively flimsy solar arrays will break off and burn up first, along with antennas, radiator panels and other appendages.
More massive components — modules and the lab's huge power truss — also will break apart in the hellish high-speed descent, but chunks as large as a small car are expected to survive all the way to ocean splashdown along a narrow 1,200-mile-long "footprint."
Remote areas of the South Pacific Ocean offer unpopulated splashdown zones, although a final target has not yet been specified.
To achieve a precisely targeted entry, "the deorbit vehicle will need six times the usable propellant and three to four times the power generation and storage of today's Dragon spacecraft," said Sarah Walker, a senior manager at SpaceX.
"It needs enough fuel on board not just to complete the primary mission but also to operate in orbit in partnership 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."
A deorbit spacecraft of some sort is needed because even at the space station's current altitude of 260 miles, trace amounts of the atmosphere still exist. As the station flies through that tenuous material at nearly 5 miles per second, collisions with those particles act to slow the craft every so slightly in a phenomenon known as atmospheric drag.
Over the life of the program, periodic thruster firings have been carried out by engines in Russian modules or attached Progress cargo ships to boost the lab's altitude as needed to offset the effects of drag. More recently, Northrop Grumman's Cygnus cargo ships have added modest reboost capability.
Without those carefully planned firings, the station eventually would crash back into the lower atmosphere on its own.
The station flies over every point on Earth between 51.6 degrees north and south latitude, covering the entire planet between London and the tip of South America. In an uncontrolled re-entry, station debris that survived entry heating could hit the surface anywhere in that area.
While the odds of impacts in a populated area are relatively small, nothing as massive as the space station has ever re-entered and fallen to Earth, and NASA is taking no chances.
NASA and its station partners — the European, Russian, Canadian and Japanese space agencies — planned from the beginning to deliberately drive the lab into the atmosphere at the end of its life to ensure breakup over an uninhabited stretch of ocean.
The original plan was to use thrusters in multiple Russian Progress cargo ships to lower the lab's altitude and set up a targeted fall to Earth.
"Early on in the station planning, we had considered doing the deorbit through the use of three Progress vehicles," Weigel said. "But the Roscosmos segment was not designed to control three Progress vehicles at one time. So that presented a bit of a challenge.
"And also, the capability wasn't quite what we really needed for the size of station. So we jointly agreed together to go have U.S. industry take a look at what we could do on our side for the deorbit."
Last year, NASA sought industry proposals and two companies responded: SpaceX and Northrop Grumman. The agency announced last week that SpaceX had won the contract.
Bill Harwood has been covering the U.S. space program full-time since 1984, first as Cape Canaveral bureau chief for United Press International and now as a consultant for CBS News.
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