When does a rocket launch into space?

A rocket can't lift off at just any time. Earth rotates, and the pad only passes through the target orbit's plane once or twice a day. To reach the International Space Station the window is practically instantaneous: a margin of seconds. For a geostationary satellite it usually lasts an hour or two, and for an interplanetary probe the window is measured in weeks, recurring every 26 months in Mars's case, when the geometry between the planets allows the trip with the least propellant. Within the window, liftoff also requires winds, clouds, and electrical activity within limits, a safety range clear of ships and aircraft, and a rocket whose thousands of sensors show no anomaly during the countdown.

After ignition, the vehicle climbs vertically for a few seconds and then tilts its trajectory toward the horizon to build horizontal speed, which is what actually sustains an orbit. At around one minute it passes through the zone of maximum dynamic pressure, where the structure is under the greatest stress. Around the three-minute mark it crosses 100 km in altitude, the Kármán line conventionally used as the boundary of space; around there the first stage separates and the payload fairing is jettisoned, no longer needed once there's no air to protect against.

Being in space isn't the same as being in orbit. To avoid falling back down, the second stage keeps accelerating to about 7.8 km/s (28,000 km/h), usually achieved eight to ten minutes after liftoff for low Earth orbit. A sounding rocket, by contrast, passes 100 km for a few minutes and comes back down: it has been in space without having orbited. Missions to higher orbits or to the Moon add further engine burns, sometimes hours later.