Stats: Delta 4 history and backlog (updated March 30)

A NASA spacecraft that will fly closer to the sun than any mission before will launch on a Delta 4-Heavy rocket in 2018, requiring a boost from the largest active launcher in the U.S. inventory and repeated flybys around Venus to reach an operating post inside the solar atmosphere.

The problem for NASA and Boeing is that the Starliner’s service module, which houses the helium lines, thrusters and other critical systems, is discarded before re-entry and burns up in the atmosphere. Engineers will not be able to study the hardware after the fact and as a result, they want to collect as much data as possible before NASA astronauts Butch Wilmore and Sunita Williams head home.

A second unpiloted test flight of Boeing’s Starliner crew capsule — ordered after an initial demonstration mission fell short of reaching the International Space Station — is now scheduled for launch from Cape Canaveral in August or September, leaving little margin to conduct the spaceship’s first flight with astronauts before the end of the year.