Virgin Galactic may finally reach space tomorrow, with the company's second SpaceShipTwo spacecraft, VSS Unity, currently being prepared for its fourth powered test flight. If weather and technical issues permit, the spaceplane and its crew of two will conduct the longest yet in-flight engine burn that will send the craft at supersonic speeds on a suborbital trajectory to reach an altitude of about 62 mi (100 km).

Four years ago, the first SpaceShipTwo broke up during a test flight when the tailplanes deployed prematurely, resulting in the death of the co-pilot. With this incident apparently in mind, Virgin Galactic is placing a supreme emphasis on safety. It says that the flight will not take place or will not attempt to reach space unless all the conditions are right. Otherwise, this will just be one more phase in what the company calls an open-ended, incremental flight test program.

If the flight does take place, VSS Unity will be pushing the envelope on altitude, air speed, loads, and thermal heating. The hybrid rocket engine will be set on an extended burn partly to try for a new speed and altitude record for the spaceplane, but also to gather data on the craft's thermal dynamics and handling at supersonic speeds.

In addition, the two pilots will experience a long period of weightlessness as the craft goes ballistic and reaches apogee or the highest point of its trajectory before it begins to fall back to Earth.

Another goal of the flight will be to simulate the weight distributions that the craft will experience in the air during commercial and passenger flights. This will be done using four research payloads provided by the NASA Flight Opportunities Program as ballast.