The massive explosion of an unmanned rocket in Virginia has raised fresh concerns about NASA’s reliance on private contractors. The rocket, which used a Soviet-era engine, was made by Orbital Sciences, which has a $1.9 billion contract with NASA to make deliveries to the International Space Station. Since retiring its shuttle fleet in 2011, NASA has relied on private companies to supply the space station. Another Orbital rocket engine reportedly suffered an explosion in Mississippi in May. The executive vice president of the company, Frank Culbertson, has warned the public that the rocket which exploded on Tuesday “had a lot of hazardous equipment, hazardous materials on board.”