A rocket can reach space without reaching orbit. The difference between suborbital and orbital flight is mainly one of speed, not altitude. For research teams and companies, knowing which of the two they need can save a great deal of money.
Reaching space versus staying there
The boundary of space is commonly placed at 100 kilometres altitude, known as the Kármán line. A suborbital rocket climbs above this altitude and then falls back to Earth. To stay in orbit, a spacecraft must also move sideways fast enough that it keeps falling around the Earth instead of back to it. In low Earth orbit, this requires a speed of roughly 7.8 kilometres per second.
What suborbital flights offer
- Several minutes of weightlessness: useful for microgravity experiments and technology tests.
- Access to the upper atmosphere and near space: sounding rockets carry instruments to study the ionosphere, the aurora and the Sun.
- Lower cost and fast turnaround: payloads can often be recovered and flown again.
Examples
NASA and other agencies have flown sounding rockets for decades. The cover image shows a NASA funded sounding rocket launching into an aurora over Alaska to study how the aurora forms. Commercial suborbital vehicles also carry research payloads and, in some cases, passengers.
When an orbital launch is needed
Any mission that needs to operate for more than a few minutes, such as Earth observation, communications or long duration experiments, requires an orbital launch. Suborbital flights are well suited to testing components before an orbital mission, for example to raise the technology readiness of a new instrument.
Frequently asked questions
What is the difference between a rocket launching into space and reaching orbit?
Reaching space means climbing above the commonly used boundary of 100 km altitude. Reaching orbit also requires enough horizontal speed to keep falling around the Earth instead of back to it. According to ESA, satellites in low Earth orbit travel at approximately 7.8 km per second. A suborbital rocket reaches space but not this speed, so it falls back.
How long does a suborbital flight stay in space?
Only a few minutes, which is enough for short microgravity experiments and measurements in the upper atmosphere.
Related reading
- How Are Satellites Launched Into Space? From the Launch Pad to Orbit
- Delta-v Explained: The Currency of Spaceflight
- LEO, MEO and GEO Satellites Compared: Altitudes, Uses and Examples
Sources
Cover image: A NASA funded sounding rocket launches into an aurora over Venetie, Alaska, on 3 March 2014 for the GREECE mission. Credit: NASA (GSFC_20171208_Archive_e001188), public domain.