The kilometer per second, unit of speed
One kilometer per second is exactly 1 000 meters per second, that is 3 600 kilometers per hour. It is the unit in which orbits, escape velocities and spacecraft are counted, because at those speeds the kilometer per hour turns into an unreadable number.
It is the scale at which leaving becomes possible. Escaping Earth's gravity takes 11.186 kilometers per second from the surface, and staying in low orbit takes about 7.8; below that a craft comes back down. The Parker Solar Probe, launched in 2018, is the fastest object people have ever built, passing 190 kilometers per second near the Sun, which in kilometers per hour would be a seven-figure number that says nothing to anyone.
The kilometer per second in other units
| 1 km/s | 3,600 km/h |
| 1 km/s | 2,237 mph |
| 1 km/s | 1,000 m/s |
| 1 km/s | 1,944 kn |
| 1 km/s | 3,281 ft/s |
| 1 km/s | 0.621371 mi/s |
Frequently asked questions
How many km/h is one kilometer per second?
3 600 exactly, since there are 3 600 seconds in an hour. The conversion is the only one on this site where the factor is also the number of units in the divisor.
Why do space agencies use km/s rather than km/h?
Because the figures stay short and comparable. Earth's escape velocity is 11.2 in this unit and 40 270 in kilometers per hour; the first can be held in mind and set against the 2.4 of the Moon, the second cannot.
What is the fastest thing measured in km/s?
Among human-made objects, the Parker Solar Probe near the Sun, above 190. Among natural ones, light itself at 299 792.458 kilometers per second, which is exact by definition since the meter was tied to it.