SPACEX SUCCESSFULLY LANDS ROCKET AFTER LAUNCH OF SATELLITES INTO ORBIT.

By KENNETH CHANGDEC. 21, 2015

People living along the central Atlantic coast of Florida have for decades enjoyed the spectacle of rockets headed for space. On Monday night, they were treated to a new sight that may become common: a rocket coming back down to a gentle landing.

“It really felt like it was right on top of us,” Elon Musk, the chief executive of Space Exploration Technologies Corporation of Hawthorne, Calif., or SpaceX for short, said during a telephone news conference afterward.

With the rush of sound from the rocket engines, Mr. Musk, who was at the launch site in Florida, said he initially thought the landing was a failure, ending with an explosion. But then he heard from mission control that the booster was standing, in one piece.

For SpaceX, the 8:29 p.m. liftoff of the Falcon 9 rocket from Cape Canaveral Air Force Station was a threefold success.

First, it marked the company’s return to flight after half a year. In SpaceX’s last launch attempt, a rocket taking supplies to the International Space Station disintegrated. (This time, the rocket was carrying a commercial payload: 11 small data-relay satellites for Orbcomm of Rochelle Park, N.J.)

Second, SpaceX’s upgraded design for its Falcon 9 rocket worked flawlessly. The liquid oxygen was chilled to minus 340 degrees Fahrenheit, about 40 degrees colder than on earlier flights, and the kerosene fuel was cooled to 20 degrees instead of 70 degrees.

Mr. Musk explained last week via Twitter that the lower temperatures improved performance of the rocket engines.