NASA’s Space Launch System can propel payloads at up to an incredible 17,500 mph; Voyager probes that have received gravity assist travel even faster.
Recently, the Parker Solar Probe became the fastest human-made object when it approached within 4 million miles of the sun’s surface with Venus providing gravity assistance. But how fast exactly was this journey?
Escape Velocity
Escape velocity is the maximum speed at which an object can escape gravity and ascend into space without collapsing back down again. An object must reach this velocity for freedom from its gravitational pull that keeps all objects on Earth revolving around the Sun.
When discussing celestial bodies such as planets and their natural satellites, escape velocity is usually discussed relative to their centres (rather than moving surfaces). When discussing spacecraft trajectories such as those which involve gravity-assisted flybys of large celestial bodies, however, escape velocity is typically stated relative to an object’s speed relative to gravitationally captured bodies rather than what we perceive to be visible surfaces of these objects.
Due to variations in gravitational acceleration (i.e. g), which depends on an object’s mass and radius, an escape velocity should be expressed as its speed of reaching infinity without falling back: v escape = GMm/r
Cruise Velocity
Cruise velocity refers to the maximum distance covered with an given amount of fuel by an aircraft in one flight. It depends on forces acting upon it in its horizontal plane: drag and lift. Thrust and weight also play roles, though their effects tend to be minor during cruise.
Star Trek fans might recall that “cruising speed” in Star Trek refers to a warp factor which could be maintained for extended periods without compromising efficiency, such as with the Delta Flyer where its warp nacelles were tuned to provide an efficient warp factor 3 cruising speed.
The SR-71 Blackbird typically cruised at Mach 3.2 when flying missions altitudes, although acceleration could occur quickly as needed to meet mission profiles. From its perspective, however, there was little difference in fuel consumption and range between Mach 3.2 and top speed; indeed anecdotal accounts indicate that damage to engines wouldn’t have been an issue had speed been unnecessary for its mission profile.
Orbital Velocity
Orbital velocity refers to the speed necessary to keep an artificial or natural satellite orbiting Earth, where its mass tends to keep it moving along a straight trajectory; however, due to gravitational force pulling it down into an elliptical or circular path. If its velocity falls below or equals that of orbital velocity it will fall back down toward earth and be consumed by heat.
At an altitude of approximately 17,000 miles, satellites require orbital velocity of about 17000 mph to maintain proper orbits. To accelerate travel to Alpha/Proxima Centauri more quickly, spacecraft employ the Slingshot Maneuve (Powered Gravity Assist), or SGA for short. To accomplish this maneuver faster it involves close passing the point of closest approach periapsis before using their engines at that location to increase relative velocity; increasing orbital velocity. By increasing orbital velocity the spacecraft maintains proper orbits.
Cosmic Velocity
The initial cosmic velocity is the minimum speed needed to attain escape velocity from any celestial body and is also used as an indicator for galaxies leaving our local cluster (and therefore leaving our galaxy cluster).
This value can be calculated using the cosmological constant and radius of an celestial body, or by comparing its orbital period to light travel time over its distance. It can provide useful comparisons among cosmic bodies.
There has been some discussion and confusion surrounding this number due to its difficulty of interpretation without extensive background knowledge. A rocket needs to reach this speed to remain in a circular orbit above Earth’s surface; however, due to air resistance it cannot achieve that goal. Numerical calculations of cosmic velocity reveal some surprising and non-intuitive results.