SpaceX is building an upgraded version of its Dragon cargo spacecraft to deorbit the International Space Station for a controlled re-entry and breakup over an uninhabited patch of ocean. The laboratory is scheduled to be permanently decommissioned by 2030, NASA and company officials said Wednesday.
The ISS Deorbit Vehicle, or DV, is a purpose-built, one-of-a-kind spacecraft designed to ensure that the space station re-enters the atmosphere in the right place and direction, ensuring that any debris that survives the 3,000-degree heat of re-entry will crash safely into the ocean.
In late June, NASA awarded SpaceX a contract worth 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 about $1.5 billion in total to carry out the entire deorbit operation.
And it's no trivial matter. The space station's long axis, made up of multiple pressurized modules where visiting crews live and work, is 218 feet long. The lab's solar array power and cooling truss, mounted perpendicular to the long axis, extends 310 feet from end to end, longer than an American football field.
The entire laboratory complex has a total mass of 925,000 pounds and is moving through space at about 17,000 mph, or 84 football fields per second.
To gently lower the altitude for a controlled re-entry, the DV will carry about 16,800 kilograms of fuel. This fuel will power 46 Draco rocket engines, 30 of which will be mounted in an extended fuselage to perform most of the exit maneuvers.
"When we make the decision to leave the space station, we will launch the American DV about a year and a half before the last burn in the atmosphere," he said. Dana Weigel the ISS program manager at the Johnson Space Center.
"We dock it to the forward port, do some checks, and once we're confident everything looks healthy and we're ready to go, we'll float the ISS down."
The final crew of the space station will remain aboard until periodic booster explosions and ever-increasing "drag" in the extreme upper atmosphere lower the lab to an altitude of about 205 miles. That milestone will be reached about six months before the final reentry procedure.
As the then-unmanned ISS reaches an altitude of about 140 miles, the DV will "perform a series of burns to get us ready for that final deorbit," Weigel said. "And then, four days later, it will perform the final re-entry burn."
The space station's large but relatively fragile solar panels will be the first to break off and burn up, along with antennas, radiator panels and other appendages.
Heavier components - modules and the lab's massive power truss - will also break apart during the hellish, high-speed descent, but pieces the size of a small car are expected to survive all the way to splashdown in the ocean via a narrow 1,200-mile (1,930-kilometer) "footprint."
Remote areas of the South Pacific offer uninhabited landing zones, although no definitive target has yet been determined.
To leave space with precision, "the vehicle will require six times the usable propellant and three to four times the power generation and storage of the current Dragon spacecraft," said Sarah Walker, a senior manager at SpaceX.
"It needs enough fuel on board to not only complete the primary mission, but also to operate in orbit for approximately 18 months in conjunction with the space station. Then, at the appropriate time, it will perform a complex series of actions over several days to deorbit the International Space Station."
Some sort of deorbit spacecraft is needed, because even at the space station's current altitude of 260 miles, there are still traces of the atmosphere. As the station flies through that thin material at nearly 5 miles per second, collisions with those particles cause the spacecraft to slow down slightly, a phenomenon known as atmospheric drag.
Periodic thruster firings have been performed over the life of the program by engines in Russian modules or attached Progress cargo ships to increase the lab's altitude as needed to compensate for the effects of drag. More recently, Northrop Grumman's Cygnus cargo ships have added a modest reboost capability.
Without these carefully planned launches, the space station would eventually crash back into the lower atmosphere.
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 the event of an uncontrolled re-entry, any debris from the station that survives the heat of re-entry could be deposited on the surface anywhere in that region.
Although the chances of an impact in a populated area are relatively small, nothing as big as the space station has ever returned and crashed to Earth. NASA is taking no chances.
NASA and its space station partners - the European, Russian, Canadian and Japanese space agencies - planned from the beginning to deliberately send the laboratory into the atmosphere at the end of its life, ensuring it would disintegrate over an uninhabited patch of ocean.
The original plan was to use thrusters in multiple Russian Progress cargo ships to lower the laboratory's altitude and perform a directed fall to 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 handle three Progress vehicles at the same time. So that was quite a challenge.
"And on top of that, the capacity wasn't quite what we really needed for the size of the station. So we mutually agreed to have the American industry look at what we could do from our side for the deorbit."
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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