Gravity. More than the name of a killer movie, it's likely something we take for granted every single day. After all, nearly everything we do is reliant on the idea that stuff stays in place when we stop holding it. Astronauts don't have that luxury, however, and when even simple tasks take a ton of effort, something relatively complex like using a 3D printer is even harder. Why would astronauts need one of those? Well, because stuff breaks in space, and replacing a busted part isn't as simple as hitting Home Depot -- just ask the crew of Apollo 13. To help get around that, the folks at Made in Space have designed a 3D printer that circumvents the lack of Earth's gravity when used in orbit. Instead of molten filament essentially "stacking" on itself to form an object like it does planetside, according to The Verge, the Zero-G Printer liquid's surface-tension holds a widget together.
Update: No launch tonight! Weather conditions forced a postponement. According to NASA, the next launch window is tomorrow night, on the 21st at about 1:52 AM ET.