When an orbital rocket is launched, it seems to lift off very slowly from the launchpad, but within minutes it is traveling at hypersonic speeds, raising the question of how fast rockets must travel to reach space and beyond.
A rocket must travel at a speed of approximately 28 000 km/h or 17 500 mph to achieve Low Earth Orbit and 40 000 km/h or 25 000 mph to break free from Earth’s gravity. It can further increase its speed to several hundred thousand miles per hour by using the gravitational pull from celestial bodies.
It is hard to believe that orbital launch vehicles are among the fastest human-made objects ever produced when observing how painfully slowly it seems to lift off from a launchpad during any significant rocket launch.
Yet, these large spacecraft pick up speed very quickly, and by the time it reaches orbit, a rocket is traveling many magnitudes the speed of the fastest rifle bullet. (Up to 7 times the speed of a modern high-velocity rifle cartridge traveling at 4000 km/h or 2500 mph.)
The following sections will describe how fast rockets actually travel, why they can’t travel any faster, and possible innovations to overcome obstacles for accelerating in space.
How Fast A Rocket Must Travel To Reach Earth Orbit
In order to leave the Earth’s atmosphere and establish an orbit around the planet, a rocket has to travel very fast. In fact, it has to reach and maintain a speed of approximately 28 000 km/h (17 500 mph) or 7.9 kilometers per second to stay in orbit.
Rockets need to achieve these high speeds since the Earth’s gravity extends for thousands of miles into space and will eventually drag all objects back to its surface without any form of countermeasure. For a spacecraft, this comes in the shape of forward momentum or speed.
A spacecraft, satellite, or any other object use a combination of its forward momentum (speed) and the Earth’s gravitational pull to remain in orbit around the planet. They need both forces to stay airborne.
A spacecraft traveling in a specific orbit around the Earth is always flying towards the horizon at a speed of 7.9 kilometers per second. At the same time, the Earth’s gravity is pulling the craft down.
By combining these two forces, a spacecraft can stay at a fixed altitude while orbiting the planet. Since the Earth is round, the vehicle will start to pull away from the Earth if it travels any faster. If it travels any slower, the planet’s gravity will start to pull it towards the surface.
As already stated in other publications, it would be fair to say that spacecraft and satellites essentially “fall around the Earth in orbit.” It is just their forward momentum or speed that stops them from being pulled back to the planet’s surface by the Earth’s gravity.
How Fast A Rocket Must Travel To Escape Earth’s Gravity
Maintaining an orbit around the Earth is one thing. Breaking free from it to travel to another celestial body or deeper into space is quite another.
For a spacecraft to break free from Earth’s gravity and travel deeper into space, it has to accelerate to a speed of 40 000 km/h (25 000 mph) or 11 kilometers per second.
Typically, a spacecraft will first establish an orbit around the Earth before burning its upper stage engines to accelerate the vehicle to escape velocity (the speed required to break free from Earth’s gravity) and put the craft on a trajectory to reach its destination.
For example, the Saturn V rocket that carried American astronauts to the Moon during the Apollo missions first established a parking orbit at an altitude of 190 km (118 miles) around the Earth before firing its third stage thrusters which put it on a trajectory to the Moon.
(This was done in part to perform multiple system checks to ensure all equipment is functioning correctly before putting the spacecraft on a Trans Lunar Injection orbit.)
After firing its third stage engines, the Apollo spacecraft accelerated to a speed of 40 319 km/h (25 053 mph), putting the vehicle on a trajectory for Trans Lunar Injection (TLI), after which the Moon’s gravity eventually assisted in capturing and pulling the craft closer.
Even at these meteoric speeds by Earth standards (the fastest commercial airliners travel at speeds of “merely” 950 km/h or 590 mph), they are still fairly pedestrian in terms of the vast distances that have to be covered to reach celestial bodies in and beyond our solar system.
For example, the time it takes to travel to the Moon, our only satellite and closest celestial body, takes approximately 3 days with current technology. And the Moon is just 384 400 km (238 855 miles) from Earth.
Space agencies like SpaceX and NASA are targeting Mars as the next destination for human spaceflight. However, with current technology, it will take approximately 9 months to travel to the planet, which is an average distance of 225 million km (140 million miles) from Earth.
At present, returning humans to the Moon is a massive undertaking for the most advanced launch vehicle manufacturer. Putting a human on the surface of Mars is currently just a theoretical possibility, and this is not even close to the furthest reaches of our solar system.
Needless to say, physically exploring anything beyond our solar system is but a pipedream. To put it in perspective, the star nearest to Earth, Proxima Centauri, is 4.25 light-years away from Earth. That is a distance of 40 208 000 000 000 kilometers.
Currently, the fastest (uncrewed) spacecraft ever created, the Parker Solar Probe, will orbit the Sun at a speed of 700 000 km/h (430 000 mph) on its closest approach. But even at this speed, it will take more than 6 500 years to reach our nearest neighbor.
The question often raised is why engineers don’t just design spacecraft that can accelerate to achieve the speeds that would cover these vast distances in a much shorter period. Unfortunately, as the following section will illustrate, it is not that simple.
Why A Rocket Cannot Travel Any Faster
When lifting off, more than 80% of a rocket’s mass consists of fuel. The vast majority of this fuel is expended just to get the launch vehicle out of the Earth’s atmosphere and into Low Earth Orbit.
This great fuel expenditure is primarily due to the energy required for a rocket to lift several tons of vehicle & fuel, push through the drag created by the thick atmosphere surrounding the planet, and break free from the Earth’s gravitational pull.
The fuel left in the spacecraft after reaching space is used for orbital maneuvering, attitude adjustments, and putting the vehicle on the correct trajectory to reach the mission objective and, if required, return to the planet’s surface.
Accelerating to orbit alone requires a multistage rocket, which means a launch vehicle consists of more than one stage, each with its own propulsion system and fuel. Each stage is ignited in a specific sequence to allow the rocket to reach space.
(The Saturn V rocket that carried astronauts to the moon was a three-stage launch vehicle. Learn more about rocket staging and why it is necessary for an orbital rocket to reach space in this article.)
As a result, there is very little fuel left in a spacecraft for the long acceleration needed to come even close to the speeds required to drastically reduce the time to reach other planets in our solar system or explore deep space beyond it.
Companies like SpaceX are exploring the idea of refueling launch vehicles in space with its Starship Program, but at the time of writing, this was still in the early development phase.
But even if refueling in space were a reality, it would still not be possible to accelerate a crewed spacecraft to the speed of light or even close to it due to multiple factors, as the following section will illustrate.
Why Spacecraft Can Not Accelerate To The Speed Of Light
The previous section touched on difficulties launch vehicles have to overcome to escape the Earth’s thick atmosphere and gravity to put a heavy launch vehicle and payload into orbit and the amount of propellant expended in the process.
However, even if a rocket can be fully fueled in space, there are a number of other obstacles that it needs to overcome to enable the spacecraft to accelerate to anything remotely close to traveling at the speed of light.
For example, the structure of a rocket, which is already made as light as possible to reduce mass, needs to be extremely strong to withstand the immense forces this type of acceleration will put on the vehicle.
(Learn more about the four primary sections or structures that a rocket consists of as well as their function in this article.)
And even if it was possible for the structure to be strong enough to withstand the forces generated by such a sustained acceleration, the amount of additional mass required to strengthen the spacecraft will work against and counteract its ability to gain speed.
Another potential problem is navigation. Rockets have sophisticated guidance systems that allow them to maneuver & keep it on a specific trajectory. In most cases, the destination and trajectory to reach it are well defined, with all possible obstacles in the way clearly identified.
However, if spacecraft travel at a mere fraction of the speed of light, especially through the vast space beyond our solar system, navigation becomes infinitely more complex and dangerous. Space may be vast & empty but is still littered with unidentified smaller objects.
Objects like small rogue comets & asteroids are often difficult to identify at a distance and, at the velocity a spacecraft will travel, it may be impossible to detect. Even if a craft is able to identify an object, it may be impossible to react & avoid it in time at such a high velocity.
But there are two factors that currently makes traveling close to the speed of light practically and theoretically impossible:
- Special Relativity
- Limitations Of The Human Body
Both these factors are part theory part fact, but enough real-life scenarios have proven them to be a reality, and any one of them by themselves makes the idea of space travel at light speed a little more than wishful thinking at the moment.
Throughout this article, the amount of fuel spent during a rocket’s launch, as well as the mass it has to carry into space, has been mentioned. It also highlighted the fact that space agencies are in the process of devising plans to refuel launch vehicles in orbit.
Combined with the fact that spacecraft do not have to deal with huge gravitational forces or a thick atmosphere in space, the argument can be made that a fully fueled rocket in orbit is free to accelerate without much restraint.
Unfortunately, this is where the theory of Special Relativity comes in. A spacecraft still has a mass that has to be moved, no matter where it finds itself. Even in space, a certain amount of energy has to be expended to move an object with a significant amount of mass.
According to the theory of Special Relativity, an object’s mass will continue to increase as it accelerates, and as it approaches the speed of light, its mass will grow to infinity.
This means the amount of energy required to propel this growing mass forward will also grow to infinity. In other words, no amount of fuel will be enough to allow a spacecraft to reach the speed of light.
Limitations Of The Human Body
When it comes to crewed spaceflight, a much bigger problem arises that dwarfs the technical difficulties of accelerating a rocket to speeds that will make interstellar travel possible. And that problem is the frailty and limitations of the human body.
When any object accelerates, it generates a force that is measured in G’s, where a g-force of 1 equals the strength with which the Earth’s gravity pulls all objects to its surface. All humans experience this force at all times, whether we sleep, work, sit, or exercise.
However, when an object accelerates at a rate that generates a force greater than 1 g, it starts to place additional stress on the human body. As acceleration increases, so does the g-force, which causes blood to start flowing away from a person’s brain to the feet.
If this continues, a person will start to develop symptoms like tunnel vision, followed by a blackout, loss of consciousness, and ultimately death if the acceleration continues to persist. The average human being can withstand a maximum of 9 g’s but only for a few seconds.
(Trained fighter pilots can sustain g-forces of 9 g’s for longer periods, but modern aircraft like an F-16 are capable of generating much higher g-forces than even they can handle.)
As a result, the amount of acceleration that will be experienced if a spacecraft “jumps” to light speed almost instantaneously, as portrayed in science-fiction movies, will cause the human body to be crushed and a person instantly killed.
Even at an acceleration at 9 g’s, the maximum force a human can handle for a very short time, it will take a rocket 19 days to reach half the speed of sound. By this time, anyone on board would have passed away from the sustained acceleration.
Until some form of mechanism or means of protecting the human body against these forces can be developed, this will always be a limiting factor that has to be taken into consideration.
A rocket travels fast, very fast. At an escape velocity of 40 000 km/h or 25 000 mph, it is faster than a high-velocity rifle bullet and faster than any fighter aircraft on the planet.
There are limitations to the speed they can reach, though, as this article illustrated. The technology currently available, the limitations described by the theory of Special Relativity, and the frailty of the human body make reaching and traveling at lightspeed impossible.
Maneuvers like Gravity Assist (using celestial bodies) and new technologies like solar sails (that use solar rays to propel a spacecraft forward very much like the wind drives a sailship) all help increase a rocket’s speed while traveling through space.
It remains to be seen if current limitations can be overcome and how fast that will allow rockets to travel in the future.
This article was originally published on headedforspace.com. If it is now published on any other site, it was done without permission from the copyright owner.