Just a few months ago, the future of NASA seemed in doubt. But the space agency announced on Wednesday  a new rocket design that it says will be the centerpiece of a deep-space exploration program for decades to come. The Space Launch System could lift astronauts farther than ever before, making it eventually possible to journey to Mars. Neil deGrasse Tyson, astrophysicist at the American Museum of Natural History, host of Star Talk Radio, and author of the forthcoming book “Space Chronicles,” on NASA’s vision for the future of space exploration.