Nearly a third of that weight is equipment for science that furthers NASA's ability to explore space: a capsule that records how spacecraft break up on re-entry, an experiment that tests how fires behave in microgravity, a shiny new LED-powered plant growing habitat.
But also tucked into the spacecraft's cargo hold are 38 CubeSats, diminutive satellites that are cheaper and easier to assemble than the behemoths rockets usually take into orbit. The ISS is a major launch platform for CubeSats—it has ejected more than 100 small payloads from companies and universities into orbit in the past few years. Those small sats will rely on new launch providers when the ISS retires in less than a decade. But this launch, from Cape Canaveral Air Force Station in Florida, will be business as usual; you can watch the livestream (now also in glorious 3D!) when it starts at 10 am Eastern, with the launch scheduled for 11:11 am.