Interstellar Space Travel

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 Galaxy


We have found out in this class that the Universe is large, very large! Even traveling from planet to planet in our solar system takes a considerable amount of time. For example, a round trip to Mars will take a year or more with current technology. Going to the outer solar system and beyond requires new, higher speed, and highly efficient propulsion technology.

For example, right now the highest speeds that we have achieved for any of our space probes is that by the Voyager probes of about 62,000 km/hr (done with an assist using the gravity of Jupiter). How long does it take to tour the solar system at that speed? Well, Jupiter is 598,400,000 km away at its closest approach to Earth. If we could manage to achieve a speed of 62,000 km/hr without using Jupiter's gravity, it still would take more than 9,000 hrs to get to Jupiter (1 year and a few weeks). Not too bad. How about Pluto? Pluto never gets much closer than 39 AU (5.8 billion km). It would take this probe more than 10 years to get to Pluto. The nearest star is Alpha Centauri at 4.2 ly (4 x 1013 km, note that it has a low mass companion, "Proxima", on this side of the triple star system). This space probe would require 76,000 years to get to Alpha Centauri! Here is a table in terms you can grasp:

How fast a rocket can travel depends directly on the speed of the exhaust from the rocket and on the ratio of the mass of the fuel to the mass of the rocket (go here for a simple introduction to the physics of rockets). Another important concept to grasp is that the more fuel you can carry versus you rocket's weight the better (this is from the famous Tsiolkovsky rocket equation). One way to go faster is to have a higher exhaust velocity. Another way is to eliminate the need to carry fuel. We will examine both of these issues.

The chemical burning used in current rocket technology is not efficient enough to attain the high speeds necessary to make quicker trips to the outer solar system. To do this requires exhaust velocities of 10 to 100 times that of the LOX/LH2 (liquid oxygen and liquid hydrogen) propellant used by the Space Shuttle (and the Apollo Moon missions). The Space Shuttle engines have exhaust velocities of about 4.4 km/s.

As early as the late 50's, the US and USSR were investigating nuclear powered propulsion. In this scheme, a nuclear reactor would be used to heat a liquid propellant to a very high temperature and expel it out the back to provide thrust. The most efficient systems used liquid hydrogen, and could provide exhaust velocities of four or five times that of the LOX/LH2 motor. Working models of these motors were built, but never flew due to the fear of radiation contamination if the rocket failed. They are now seriously being considered for trips to Mars. Our launch technology is much more reliable than in the 50's and 60's, and thus putting one into orbit is not quite as threatening as it once was (though I am sure it will generate a lot of heat from some quarters!).[Here is a news story about a recent test of this technology.]

Such rockets could supply large amounts of thrust at reasonably high exhaust velocities for much longer intervals than chemical rockets. Unfortunately, to shield humans from the reactor would introduce more weight to a potential spacecraft--but the reactor could be placed on a light weight truss well away from the crew quarters, and not be a serious problem. Of course, any unmanned missions could easily use this technology for quicker trips through the solar system. A great benefit of nuclear engines is their long life-a nuclear reactor can supply power for very long voyages. Because of these benefits, NASA had restarted its research program on nuclear propulsion technology (though budget issues have seen it diminished). [Note that there was once a project to use nuclear bombs to power a rocket, go here.]

Other (currently realizable!) concepts for trips to the outer solar system and beyond take a different course: super high exhaust velocities, but at very low thrust levels. In this form of propulsion, the "motor" operates continuously, slowly building the speed of the spacecraft. The first working model of this concept is the "ion drive" tested on the "Deep Space 1" (DS1) probe. Such craft need to be put into space before they can actually go anywhere due to the very low thrusts provided by ion drives.

Ion drives work by accelerating charged atoms using an electric field:

In DS1, solar panels supplied the electricity that both ionized, and accelerated the xenon propellant. The xenon atoms are accelerated to a speed of 30 km/s. 30 km/s is equivalent to 108,000 km/hr! Nearly twice as fast as the speed that the Voyager probes attained.

DS1 carried 81.5 kilograms of xenon, enough to supply 20 months of constant acceleration. During the first test of this system, 11.5 kg of xenon was used, and it accelerated the speed of the spacecraft by 2500 km/hr. Using the entire fuel supply would impart an increase in speed of the spacecraft by 16,600 km/hr (4.5 km/s, about the Space Shuttle engine exhaust velocity with just 81.5 kg of xenon!).

Ion drives are highly efficient, and more powerful systems could easily be developed. They simply need a source of electricity. In the inner solar system, solar panels could supply this electricity. Further out, a small nuclear reactor could supply the necessary electricity. The current Dawn mission to study the asteroids Vesta and Ceres uses an ion drive.

Another currently realizable technology is idea of a "solar sail", using the momentum of the light emitted by the Sun to accelerate a small payload, as seen here:

Using a large mirror, the light emitted by the Sun can accelerate a spacecraft to very high speeds. The benefit of this type of propulsion is that you need to carry no fuel! The main drawback is the weak thrust supplied by a solar sail. For example, a one square kilometer sail only provides 2 lbs of thrust! But, the Japanese have deployed this technology and have shown that it works.

This amount of thrust is too small to get a useful payload anywhere. To make the system more efficient several alternatives have been proposed. One of these is to use a stationary laser, based on an airless body like the Moon, to focus an enormous quantity of light onto a small sail. This could accelerate the craft much more quickly.

You could also use a microwave beam to do the same thing. Microwave transmitters are much more efficient than lasers and we are already close to the power level needed for small space probes. A spacecraft with 10 x 10 meter sail and a 1 kg payload accelerated by a megawatt transmitter for 20 hrs could reach Pluto in three weeks. That's an average speed of 7.3 million miles per hour, or about 1% the speed of light! (How you stop it once it reaches its destination would be a big problem--remember you usually want to stop when you get somewhere, and that takes just as much energy!)

An alternative concept is to fly an absorbing, carbon-fiber sail very close to the sun, where it would heat up to 3600 degrees, and accelerate the craft. An acceleration of 14 g's might be achievable.

In the last few years, a number of spacecraft (like the Voyager probes) have used a "free" source of energy to increase their speeds: "gravitational slingshots". The close passage of a probe by a planet that increases its speed so that the travel time is reduced. The most recent of these were the Cassini and New Horizons missions. Future expeditions to the outer solar system could also use this method, and then use one of the other power sources to accelerate to an even higher velocity.

None of the methods we have discussed so far, however, will supply sufficient acceleration to navigate interstellar space. The distances in the solar system are large, but compared to the distances between stars they are insignificant. Back to Alpha Centauri, the nearest star. If we could travel at the speed of light, we could get there in 4.3 years. Obviously, we need to travel at very high velocities to make it anywhere in a human lifetime. For example, if we could achieve a speed of 0.1c (where "c" is shorthand for the speed of light), we could reach Alpha Centauri in 43 years. 0.1c is 30,000 km/s, 1000 times the exhaust velocity of the DS1 ion drive. The interesting thing is that as you go faster, and faster, time appears to slow down. This is Einstein's "Special Theory of Relativity". The effect is known as time dilation. If you go 50% the speed of light, your trip will appear to you to only take 87% as long as light would take to make the same trip. At 0.9c, the trip only takes 44% as long. Thus, a trip to alpha Centauri at 0.9c would actually take you 4.7 years to get there (as seen by an Earth based observer), but appear to the passengers to have only taken 2 years!

To achieve such high velocities will require dramatic technological advances. Modified nuclear fission propulsion systems can supply about 20 times the thrust of a LOX/LH2 engine. This is too low for interstellar travel. A nuclear fusion engine, however, can provide about ten times more energy.

Current designs (e.g., project Daedalus) for a fusion engine (hydrogen is fused into helium, like in a star), such as shown here,

envision a spacecraft that is about 300 metric tons, and would be 44 ft long. The exhaust speed of this engine could be more than 1000 times (v ~ 10,000 km/s) that of a classical LOX/LH2 rocket. This would be sufficient for interstellar space travel. But it would be difficult to carry enough fuel to make this trip. One way around this is to scoop-up your fuel (interstellar hydrogen) on your way, such as shown here:

There is an even more exotic propulsion fuel: antimatter. Every subatomic particle known in physics has an "antiparticle". For example, an electron has a charge of -1, its antiparticle (the "positron") has the same mass, but a charge of +1. When antimatter comes in contact with matter both are annihilated producing energy.

This process is the most efficient/energetic in the universe, governed by Einstein's famous formula: E = mc2. The c2 in this equation is the speed of light squared! The annihilation of one single gram of antihydrogen with normal hydrogen releases as much energy as the burning of 23 Space Shuttle External Fuel Tanks full of LOX/LH2!!!

One gram! How much is one gram? About the weight of a pen cap:

It is estimated that 10 grams (0.3 ounces) of antiprotons would be enough fuel to send a manned spacecraft to Mars in one month.

Using such an efficient fuel would supply 100X the thrust of a fusion motor. The problem is that antimatter is hard to create, and hard to store. It is so rare, that a single gram (1/453 of a pound) of antimatter is worth (costs) $62.5 trillion! Right now, the world's particle accelerators can only create one billionth of a gram per year (equivalent to 10 grams of LOX/LH2).

It will be a long time before we can realistically expect to use antimatter, or even fusion, as a source of propulsion for an interstellar spacecraft. But we have plenty of time!

In the next 30 yrs or so, a manned mission to Mars is likely to occur. The cost of such a mission will be in the $40 to 50 billion range. The costs of more ambitious efforts, like a self-sustaining colony on the Moon will require even more money. It is obvious that a single country, even one as rich as ours, will not be able to afford those enterprises on its own. These future missions will have to become international endeavors. But the costs of these missions are actually quite small when you consider the size of the budget of the US government ($3.6 trillion in 2010), or compare it to the total of the budgets of the "G7" nations. Note that the entire economies of the world "produced" about $58 trillion dollars of goods and services ("GDP") in 2009, the US economy was $14 trillion.

To establish colonies on the Moon and Mars would probably require trillions of dollars (spread over many decades). Certainly, if the richer countries of the world pooled their resources, such projects would be quite feasible.

Why should we build these colonies, or even attempt interstellar travel? If the human population of Earth continues to expand at its current rate (it recently passed 7 billion), as shown here,

we will eventually have to leave this planet or we will exhaust its resources. Starting colonies on the Moon or Mars are one way to obtain more resources and room for a growing population.

But this is not the only reason. Humans have always sought to explore beyond the next horizon, and certainly, the generations that follow us will use this as one reason to continue to push the sphere of human influence to the nearby stars.

Any interstellar spacecraft that we can currently envision will have to be very large-it must be completely self-sufficient for hundreds of years. It cannot resupply itself with raw materials until it reaches another planetary system. Of course, maybe there is some other way to travel in our Universe that we currently do not know about. This is where science meets science fiction--such as the "Warp Drive" used in the Star Trek series/movies. People are contemplating such ideas, and to read more about them, go here. We are still at our infancy in understanding how nature works, especially as it pertains to quantum mechanics, and general relativity. Who knows what may or may not be possible given a million years of research by human beings, or their descendants.