"Space is big. You just won't believe how vastly, hugely, mind-bogglingly big it is. I mean, you may think it's a long way down the road to the chemist's, but that's just peanuts to space."
Douglas Adams, The Hitchhiker's Guide to the Universe
But that's not all. Navigating in space isn't like driving on roads or flying through the air; you have the vacuum and orbital mechanics to consider. On Earth you can turn by simply moving a car's wheels, ship's rudder or aircraft's ailerons. But in space there is no friction and therefore no traction between a tyre and the road, or a plane's wing and the onrushing air.
On Earth the most efficient way to travel between two points is in a straight line, but in space it's achieved by increasing the circle of your orbit around Earth until it stretches into a giant ellipse that encompasses the other planet or moon you want to travel to. This can also involve using the mass of other solar system bodies to pull on the spacecraft and give you a 'gravitational assist' and speed you up.
The slowly expanding & then contracting orbit to place the Chandrayaan spacecraft in orbit around the moon
Credit: the Indian Space Research Organisation
But, as it currently costs between $5,000 and $10,000 just to launch a single kg of payload into space (not including the extra cost to get it beyond Earth orbit), time is often the trade-off to cost efficiency:
- Getting to the moon took the Apollo astronauts 3 and a half days and the next planned return to the moon in the 2020s will take just as long.
- It would take 6 months to get to Mars (some 142 times further than the moon when favourably aligned).
- It took 16 months to fire Voyager 1 the half a billion miles to get to Jupiter in 1979 and a trip to visit Saturn's intriguing moon Titan can take 7 years!
Unless you're going to take a surplus of heavy and expensive fuel with you to slow you down when you get there, it also takes longer to rocket yourself to a planet that to simply sail on by as you've got to get yourself into that object's gravity by a) planning your arrival at that body at the right speed to get trapped in its gravity well or b) firing rockets in the opposite direction (brakes don't work in space - you remember the friction/traction problem earlier?). And we still have to rely on conventional rockets which currently gives us our best bang for the buck in terms of controllable and storable energy output for space travel.
Size comparison of large US launch systems (past, current & in development)
The image above shows the launchers used or soon to be in use to get 'payload' into space. Only the Space Shuttle and Saturn V have ever launched human explorers, though the Falcon 9 and Space Launch System (SLS) are actively moving towards lofting humans, and Atlas & Delta have plans on paper.
What will be clear form the chart is the size of the rockets needed to get ever bigger payloads into space. But you might be surprised at how much of the rocket is purely fuel tanks. Apart from the Space Shuttle, which has a unique (and dangerous) piggy-back design, almost all of the other designs are mostly fuel and rocket engines to actually get into space: the red circles highlight the important bit - the 'stuff' that gets put into space by the rest of the stack. This can be satellites in Earth orbit, probes to other planets or humans. The fuel required for the Space Shuttle weighed 20 times more than the parts that made their way into Earth orbit.
Now, the more weight you want to loft, the more fuel you'll need, and therein lies a diminishing return - the bigger you build a rocket to carry the fuel, the thicker the rocket walls have to be, the more rocket engines you'll need and these all have weight penalties. That's why a child's toy rocket accelerates away from the ground enormously fast and a NASA rocket takes many seconds just to clear the launch tower. That's also the reason that rockets are so incredibly expensive.
But rockets are currently our only technology for getting into space and getting anywhere in a reasonable amount of time, once in space. Though a 'reasonable amount of time' is a movable feast too: If we send robots to explore the solar system then we can happily wait months or years for them to reach their destination. With humans, however, the hazards of cosmic radiation, micrometeorites, naturally occurring illnesses and possible psychosis means that getting them to their destination as quickly as possible greatly reduces the risks to the fragile human body in such a hostile environment.
So let's take a look at the progression of speed over the last one hundred years and project forward for another hundred.
Back in 1913, the fastest any human could travel (without falling off a cliff!) was 60 mph on the fastest steam train. Steam gave us a tremendous energy density compared with the next best thing: quite literally horsepower or manpower.
Due to the technological advances brought about by war, by 1945 we'd released the energy of refined oil in the form of petroleum and Kerosene to create jet engines that advanced our maximum possible speed to 584mph. This was a technological step-change but it couldn't compete with the next advance that the same war had nurtured and used to devastating effect over London.
The race to capture as many German plans and scientists at the Peenemuend rocket facility by the United States, Russia, France and Great Britain in 1945, allowed the wealthier countries to develop nuclear warhead delivery systems - but also manned spaceflight. The father of the Apollo programme, Werner Von Barun, was a German Technical Director who saw the abuse of rockets as a means to the end of advancing space travel. At the end of the war he mode his plans to surrender to the allied country that would allow him to pursue his interplanetary dream: "we're terrified of the Russians.. and we don't really like the French. The British can't afford us, so that leaves the Americans", Von Braun later recounted as his deliberations.
As you can see from the graph above, rocketry changed everything and provided the first sufficient energy release from a compact fuel to let mankind 'slip the surly bonds of Earth'.
Yuri Gagarin achieved the orbital speed of 17,200 mph when he became the first human in space and thereby, the first to orbit the Earth on 12th April 1961. This record was overtaken when the US left a close Earth orbit for the moon with Apollo 8 in December 1968. Apollo 10 in 1969 still holds the record for the fastest humans have ever travelled at 24,791 mph. Their rocket engines propelled them towards the Earth and the Earth's gravity combined to assist the incoming speed.
After the Apollo endeavours, the Space Race was over and humanity has remained in Low Earth Orbit ever since - and therefore constrained to the required orbital speed of 17,240 mph.
However, the NASA plans to send men to Mars in the 2030s, using a beefed-up version of the Space Launch System currently in development, would require humans to attain the epic speed of 30,000 miles per hour.
And that's where it stops. The end of the road for increasing speeds using only rocket technology. You could build a rocket the size of a mountain to get men to Jupiter's moon Europa in a few months, but the cost, likelihood of system failure and ludicrousness of this approach makes it untenable. You could add rockets to another technology to give you a 'quick push' or slow down. But what technologies are there to get us to explore the whole solar system in months rather than years/decades, and what if we want to explore those new planets we've discovered around Alpha Centauri and WISE J104915?
Nuclear propulsion is being investigated by NASA to make a module for the SLS that would double its efficiency, and a Nuclear Thermal Rocket was built and tested on the ground in the 1950s. This would speed up trips to Mars, asteroids and the moon but would require a close look at the Outer Space Treaty to ensure it is in compliance. However this technology is still in its infancy in terms of being advanced enough to trial in space. Of course, a launch failure involving a nuclear payload would be unthinkable unless NASA can find a way of making a reactor that can withstand the forces of falling from the sky during an explosion - a much lighter version of the impervious containers used for transporting nuclear material on Earth.
A nuclear propulsion concept for Mars
Ion drives are often talked out as a possible alternative to rockets and, given enough time to operate, they can attain immense speeds. By accelerating a stream of electrically charged particles (ions) from the engine, ion drives use the minute equal & opposite force to gradually propel it forward. As the fuel (noble gases) last a very long time, compared to rocket fuel, they can fire for many months attaining eventual high speeds. NASA's successful Dawn spacecraft to Vesta and Ceres reached 4,048 mph expending around 70kg of its fuel, xenon gas. For human spaceflight, this would have to be scaled up significantly and could only work once in space as there isn't the rapid energy release to launch a craft.
A Stationary Plasma Thruster (ion propulsion)
Credit: O. A. Mitrofanova, R. Yu. Gnizdor, V. M. Murashko,A. I. Koryakin, A. N. Nesterenko
Solar sails have a similar problem in that they increase your speed incrementally but can, theoretically, achieve great speeds. Solar sails rely on the gentle flow of the sun's photons to hit an incredibly light material stretched out into a giant sail. This then subtly pushes a craft away from the sun. To assist a spacecraft with enough weight to carry humans a sail would need to be miles in diameter and it's still doubtful from the exploratory Japanese tests in space whether they could even achieve speeds greater than current rocket technology.
Antimatter propulsion would solve many problems but also raise a few more. When matter comes into contact with antimatter they annihilate one another in a burst of energy that converts far more mass to energy than anything else we're aware of. That makes it a great fuel. But... how do we use it? How do we store it and prevent it from coming into contact with matter? Magnetic chambers may be able to store antimatter but then we still have to decide how to convert it into thrust. An early NASA nuclear propulsion project suggested blasting nuclear bombs behind a spacecraft with a protective blast shield to protect the crew! Apart from the problems of rapid acceleration and the risks of faults developing, this was wisely seen as a bad idea. But, if harnessed safely - rather than putting clumps of matter and antimatter together behind the ship with a blast shield for protection - antimatter could take a ship to more than half the speed of light (>90,000 miles per second or a round trip to Proxima Centauri in just over 16 years), making interstellar travel possible if we can keep the crew sane for that duration!
But then we need to create or capture antimatter. It exists in space but isn't easy to detect and collect en-route to the next star system, and our biggest atom smashers, while being the most effective antimatter factories we have, only create millionths of a gram - frustratingly, we'd need a few kilos of the stuff.
So put simply, we need a new technological revolution or two. Not a depressing diagnosis - we had quite a few of those in just the last century.
But, to put a positive spin on this problem, we can return to the graph above: in 1969 humans could travel 416 times faster than they could in 1913. This was not due to an incremental increase in the performance of our engineering of known science, it was an exponential increase due to revealing new engineering concepts that weren't yet known. If we can maintain the same increase in the speeds we can achieve over the next 100 years we would be looking at space travel at 10,400,000 mph in 2113. If we continue on an exponential curve our speeds could be pushing the theoretical limits of what's possible in terms of speed - the brick wall, the speed of light.
It's doubtful any of us will see 2113 (and uncertain whether or not we'll continue to make these technological step changes), but we astronomers are dreamers so, while you or I might not make it to the Oort Cloud, where would you go - Mars, Titan, Europa?