Sub-orbital spaceflight

A rocket can fly very high. It reaches outer space.

But it does not stay there. The rocket falls back down. It does not go around the Earth in a circle.

This trip is called a sub-orbital flight. Some rockets use this to test new things.

People can even go on these trips. They fly up and then come home.

These flights can be very short. Some trips only last a few seconds.

It is a fast way to see the sky.

A sub-orbital spaceflight is a special trip to space. The spacecraft goes very high. It reaches the Kármán line. This is a line about 100 km above sea level.

But the craft does not stay in space. It does not go around the Earth in a circle. It does not become a satellite. Instead, it falls back down to Earth.

Some rockets use these trips to test new parts. These parts might be used for bigger trips later. Other vehicles are made only for this way of flying. Some carry people. Others are uncrewed, like sounding rockets.

These flights can be very fast or very slow. Some trips only last a few seconds. One probe called Pioneer 1 stayed up for 43 hours.

When the craft comes back, it must go through the air. This is called reentry. The craft gets hot as it hits the air. This heat changes based on how fast the craft moves.

A sub-orbital spaceflight is a special journey into the stars. The spacecraft reaches outer space, but it does not stay there. Instead, its path always crosses back through the surface of the Earth. This means the craft cannot complete a full circle around our planet. It will not become an artificial satellite or reach escape velocity.

One way to think of it is like a very high jump. You go up into the air, but gravity eventually pulls you back down. A common goal is to reach the Kármán line. This line is about 100 km above sea level. It is the point where a craft must fly very fast to stay up.

How these flights work depends on how much speed they use. To save fuel, the high parts of the flight often happen while the rockets are off. This part of the trip is called free-fall. During free-fall, the craft follows a curved path called an ellipse.

If the craft moves very fast horizontally, it can travel a long way. An intercontinental ballistic missile can travel 5,500 km or more. These long trips might reach altitudes over 1,300 km. When the craft returns, it must go through the air. This is called atmospheric reentry. The speed of the flight determines how much heat the craft feels.

Humans have been testing these paths for a long time. The first sub-orbital vehicles to reach space were ballistic missiles. A famous example is the German V-2 rocket. It reached space on October 3, 1942. Later, the United States and the USSR developed even longer-range missiles.

Scientists also used these flights to test tools for future missions. Some flights were meant to go to the Moon but failed. For example, the Pioneer 1 probe stayed in a sub-orbital path for 43 hours. These tests helped us learn how to fly much further.

Many different machines have made these trips. The X-15 was a famous aircraft used for testing. It could fly up to 100 km and then glide back to Earth.

In 1961, the Mercury-Redstone rocket carried people on sub-orbital flights from Cape Canaveral. Other famous trips include the SpaceShipOne flights in 2004 from Mojave. More recently, Blue Origin has flown many missions from Corn Ranch. These flights use different rockets to reach different heights. Each flight provides new data about how things move in space.

Sub-orbital flights connect what we know about airplanes to space travel. An airplane stays in the atmosphere to fly. A satellite stays in space by moving very fast in a circle. A sub-orbital flight sits right in the middle of those two ideas.

It uses a rocket to get high, but it lacks the speed to stay up. This makes it a great way to study how things behave in low gravity. It is a stepping stone for the much bigger journeys we take to other worlds.

A sub-orbital spaceflight is a specific type of journey into space. The spacecraft reaches the edge of outer space, but its path always intersects the surface of the planet from which it launched. Because of this trajectory, the craft cannot complete a full orbital revolution. It will not become an artificial satellite, nor will it reach escape velocity. A common example is a craft that reaches the Kármán line and then falls back to Earth. The Kármán line is located about 100 km above sea level.

This boundary was chosen because a vehicle would need to fly faster than orbital speed to stay up using aerodynamic lift at that height.

To understand how these flights work, we must look at the physics of the path. During the high part of the flight, rockets are often turned off to save fuel. This phase is known as free-fall. Even though the craft is moving upward, it is technically in free-fall. The trajectory follows an elliptic orbit described by the orbit equation. However, the perigee distance—the closest point to the center of the Earth—is less than the radius of the Earth. This means the ellipse physically intersects the Earth, which is why the craft cannot stay in orbit.

Different types of flights require different amounts of energy, measured as delta-v. Delta-v is an astrodynamical measure of the change in velocity needed for a maneuver. If the goal is simply to reach space, a craft needs a delta-v of about 1.4 km/s to reach 100 km. This is much less than the 9.2 km/s needed for a low Earth orbit at 300 km. Some flights cover horizontal distances, which increases the required speed. An intercontinental ballistic missile (ICBM) can travel 5,500 km or more. These long-range flights can reach maximum altitudes of more than 1,300 km.

History shows that sub-orbital flight was often used for testing or military purposes. The first vehicles to reach space were ballistic missiles. On October 3, 1942, the German V-2 rocket became the first ballistic missile to reach space. Following this, the United States and the USSR developed much longer-range missiles.

Some early space probes also ended up on sub-orbital paths due to failures. NASA's Pioneer 1 was intended to reach the Moon, but it followed a sub-orbital trajectory instead. It reentered the Earth's atmosphere 43 hours after its launch.

Many unique vehicles have been designed specifically for sub-orbital missions. The X-15 was a famous experimental aircraft used for testing between 1958 and 1968. It was launched from a B-52 mothership to reach altitudes of approximately 100 km.

Other vehicles include uncrewed sounding rockets and crewed tourist spacecraft. In 2004, SpaceShipOne completed several successful flights from the Mojave Air and Space Port. More recently, Blue Origin has conducted many missions from Corn Ranch. These flights help researchers understand how objects behave in different environments.

Flight duration can vary wildly depending on the mission profile. A simple vertical flight might last only a few minutes. For example, a flight with a maximum speed of 1 km/s might have a total free-fall time of 3 minutes and 20 seconds. In contrast, an intercontinental flight has a different timeline. The boost phase lasts about 3 to 5 minutes, while the midcourse free-fall phase lasts about 25 minutes. The reentry phase for an ICBM takes about 2 minutes. Some missions can even last for days, such as the failed Pioneer 1 mission.

Sub-orbital flight serves as a vital connection between atmospheric flight and orbital flight. While airplanes use the atmosphere for lift, and satellites use high speeds to stay in orbit, sub-orbital flights sit between them. They use rockets to reach space but lack the horizontal speed to stay there. This makes them perfect for studying low gravity.

They also allow scientists to test how spacecraft handle atmospheric reentry. As a craft returns, it experiences aerodynamic heating. The heat level depends on the maximum speed; a craft moving at 7 km/s experiences much more heat than one moving at 1 km/s.