Riddle: It's the size of a small microwave, and it may alleviate the need for NASA astronauts to wait for resupply ships to arrive at the International Space Station to get some essential items.

Answer: A 3-D printer -- the first ever to be flown to space. And it could change the way NASA does business aboard the space station.

The 3-D Printing In Zero-G Technology Demonstration (3-D Printing In Zero-G), led out of NASA's Marshall Space Flight Center in Huntsville, Alabama, provided a Small Business Innovation Research (SBIR) award to Made In Space Inc. to build the first 3-D printer for operation in microgravity. It is scheduled to launch to the station aboard the SpaceX-4 resupply mission.

The project is supported by three NASA customers: the International Space Station Technology Development Office at the agency's Johnson Space Center in Houston; the Advanced Exploration Systems division within the Human Exploration and Operations Mission Directorate at NASA Headquarters in Washington; and the Game Changing Development Program within the Space Technology Mission Directorate, also at NASA Headquarters.

Researchers hope to show a 3-D printer can work normally in space and produce parts equitable to those printed on the ground. It works by extruding heated plastic, which then builds layer upon layer to create three-dimensional objects.

Testing this on the station is the first step toward creating a working "machine shop" in space. This capability may decrease cost and risk on the station, will be critical when space explorers venture far from Earth and will create an on-demand supply chain for needed tools and parts.

If the printer is successful, it will not only serve as the first demonstration of additive manufacturing in microgravity, but it also will bring NASA and Made In Space a big step closer to evolving in-space manufacturing for future missions to destinations such as an asteroid and Mars.

Long-term missions would benefit greatly from onboard manufacturing capabilities. Data and experience gathered in this demonstration will improve future 3-D manufacturing technology and equipment for the space program, allowing a greater degree of autonomy and flexibility for astronauts.

"I remember when the tip broke off a tool during a mission," recalls NASA astronaut TJ Creamer, who flew aboard the space station during Expedition 22/23 from December 2009 to June 2010. "I had to wait for the next shuttle to come up to bring me a new one. Now, rather than wait for a resupply ship to bring me a new tool, in the future, I could just print it."

So, if something breaks, like a wrench for instance, how long will it take to print one? It depends on the size and complexity of the part.

Depending on these factors, it can take anywhere from 15 minutes to an hour to print a part on the station. The computer-aided design model, which serves as the instructions, can be pre-loaded on the printer or uplinked from the ground to the station printer. It requires minimal crew time as it can be operated primarily from ground control at Marshall's Operations Support Center.

"This means that we could go from having a part designed on the ground to printed in orbit within an hour to two from start to finish," said Niki Werkheiser, NASA's 3-D print project manager. "The on-demand capability can revolutionize the constrained supply chain model we are limited to today and will be critical for exploration missions."

Not only will the printer decrease cost, but it also will decrease risk -- and increase efficiency.

"NASA is great at planning for component failures and contingencies; however, there's always the potential for unknown scenarios that you couldn't possibly think of ahead of time," said Ken Cooper, the principal investigator at Marshall for 3-D printing.

"That's where a 3-D printer in space can pay off. While the first experiment is designed to test the 3-D printing process in microgravity, it is the first step in sustaining longer missions beyond low-Earth orbit."

Lessons learned from this 3-D demonstration will be used for the next generation printer known as the Additive Manufacturing Facility (AMF). The AMF will be a commercial printer that will enable not only NASA to print needed parts, but will also set a precedent as the first facility ever to provide anyone on Earth -- including academia and industry from around the world -- the opportunity to manufacture parts in space.

The program has spurred a stellar K-12 educational outreach initiative coined Future Engineers. Future Engineers announced the first 3-D Printing Challenge at the recent White House Maker Faire, which will allow students to design items that could be selected for print on the station.

In April, the Made In Space 3-D printer successfully passed all flight certification and acceptance testing at Marshall, and the flight hardware was turned over for flight integration. Very soon, the astronauts aboard the space station will get to try out this innovative technology first-hand in a whole new environment: space.