A spacecraft launched at a speed less than 8000 meters per second (approximately 17,898 miles per hour) will not achieve orbit and will fall back to Earth.

Here's why:

* Orbital Velocity: To achieve orbit, a spacecraft needs to reach a specific speed called orbital velocity. This velocity depends on the altitude of the orbit. For low Earth orbit (LEO), the required orbital velocity is around 7.8 km/s (17,500 mph).

* Gravity: Earth's gravity constantly pulls on everything near it. To stay in orbit, a spacecraft needs to be moving fast enough to constantly "fall" around the Earth.

* Insufficient Speed: Launching at a speed less than 8000 m/s means the spacecraft doesn't have enough horizontal velocity to overcome gravity's pull. This results in a ballistic trajectory where the spacecraft rises and then falls back to Earth like a projectile.

What happens to the spacecraft:

* Re-entry: As the spacecraft falls back, it will re-enter the Earth's atmosphere.

* Atmospheric Drag: The atmosphere will create friction, causing the spacecraft to heat up and slow down.

* Possible Impact: Depending on its trajectory and the spacecraft's design, it might crash into Earth or burn up completely during re-entry.

Important Note: 8000 m/s is a simplified value. The actual orbital velocity required depends on the specific altitude and shape of the orbit.