60,000 kilometres per second may be the practical (!) speed limit for space travel

Imagine we could accelerate continuously at 1 g — what we're comfortable with on good old terra firma — to the midpoint of our voyage, and decelerate continuously at 1 g until we arrive at our destination. It would take a day to get to Mars, a week and a half to Pluto, a year to the Oort Cloud, and a few years to the nearest stars.

The fastest that human spacecraft are likely to achieve in the twenty-first century, I think, is 300 kilometres per second.

The history of technology will record that for the first time a machine of human construction, a five-and-a-half-ton missile, covered a distance of a hundred and twenty miles with a lateral deflection of only two and a half miles from the target. Your names, my friends and colleagues, are associated with this achievement. We did it with automatic control. From the artilleryman's point of view, the creation of the rocket as a weapon solves the problem of the weight of heavy guns. We are the first to have given a rocket built on the principles of aircraft construction a speed of thirty-three hundred miles per hour by means of rocket propulsion. Acceleration throughout the period of propulsion was no more than five times that of gravity, perfectly normal for maneuvering of aircraft. We have thus proved that it is quite possible to build piloted missiles or aircraft to fly at supersonic speed, given the right form and suitable propulsion. Our automatically controlled and stabilized rocket has reached heights never touched by any man-made machine. Since the tilt was not carried to completion our rocket today reached a height of nearly sixty miles. We have thus broken the world altitude record of twenty-five miles previously held by the shell fired from the now almost legendary Paris Gun.
The following points may be deemed of decisive significance in the history of technology: we have invaded space with our rocket and for the first time--mark this well--have used space as a bridge between two points on the earth; we have proved rocket propulsion practicable for space travel. To land, sea, and air may now be added infinite empty space as an area of future intercontinental traffic, thereby acquiring political importance. This third day of October, 1942, is the first of a new era in transportation, that of space travel. . . .
So long as the war lasts, our most urgent task can only be the rapid perfection of the rocket as a weapon. The development of possibilities we cannot yet envisage will be a peacetime task. Then the first thing will be to find a safe means of landing after the journey through space...

I shall assume that such a species will... develop a self-replicating universal constructor with intelligence comparable to the human level—such a machine should be developed within a century, according to the experts...—and... combined with present-day rocket technology would make it possible to explore and/or colonize the Galaxy in less than 300 million years, for an initial investment less than the cost of operating a 10 MW microwave beacon for several hundred years, as proposed by SETI...

[T]hat any intelligent species which develops... interstellar communication will also have... rocketry... is... a consequence of the principle of mediocrity... (that our own evolution is typical)... [T]he human species developed rockets 600 years before... radio waves...

[T]he problem of interstellar travel has been reduced to... transporting a von Neumann machine to another stellar system. This can be done even with present-day rocket technology.

Rocket tech applied to a car opens up revolutionary possibilities.

[A]n intelligent species with the technology for interstellar communication would necessarily develop the technology for interstellar travel, and this would automatically lead to the exploration and/or colonization of the Galaxy in less than 300 million years.

If we had a starship, we could move near a black hole, stay there for a half hour and then come out. Outside hundreds of years could have gone. So we could... come away from the black hole into the far future easily.

Hunter has pointed out that by using a Jupiter swingby to approach the Sun and then giving a velocity boost at perihelion, a solar system escape velocity... is possible with present-day chemical rockets... [M]ost other stars should have planets (or companion stars) with characteristics sufficiently close to... the Jupiter-Sun system to use this launch strategy in reverse to slow down in the other solar system.

He had steeled himself just a little for the Jump through hyper-space, a phenomenon one did not experience in simple interplanetary trips. The Jump remained, and would probably remain forever, the only practical method of travelling between the stars. Travel through ordinary space could proceed at no rate more rapid than that of ordinary light (a bit of scientific knowledge that belonged among the items known since the forgotten dawn of human history), and that would have meant years of travel between even the nearest of inhabited systems. Through hyper-space, that unimaginable region that was neither space nor time, matter nor energy, something nor nothing, one could traverse the length of the Galaxy in the interval between two neighboring instants of time.

Everything is already moving so very fast, but you need a great deal more speed than this to escape the earth's gravitational pull. Seven miles per second. More fuel, please.

If so, we might be able to use them for rapid travel around the galaxy or travel back in time. Of course, we have not seen anyone from the future (or have we?) but I discuss a possible explanation for this.