Interstellar Space Travel

We have found out in our last discussion that the solar system is large. Traveling from planet to planet takes a considerable amount of time. Even a round trip to Mars, a nearby planet, 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.

Back at the beginning of the semester we discussed the Tsiolkovsky equation, how the final velocity of a rocket was tied directly to the speed of the exhaust of a rocket (and also to the ratio of the mass of the fuel to the mass of the rocket). One way to go faster is to have a higher exhaust velocity. Another way is to eliminate the need to carry fuel.

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 propellant used by the Shuttle.

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!).

Such rockets could supply large amounts of thrust at reasonably high exhaust velocities. 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.

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!

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 1500 mph. Using the entire fuel supply imparts an increase in speed of the spacecraft by 10,000 mph (4.5 km/s).

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.

Another currently realizable technology is a variation of the solar sail idea: using the momentum of the light emitted by the sun to accelerate a small payload. 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!

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!

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 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 was the Cassini mission. 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.

The nearest star, Alpha Centauri, is at a distance of 882,900 AU! To talk about such distances, we need use a much large distance unit: the light year. As stated in the notes for the last class, a light year is the distance light travels in one year: (3.0 X 10⁵ km/s)(3.15 X 10⁷ s/yr) = 9.15 X 10¹² km. Alpha Centauri is 4.3 light years (ly) distant.

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.

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 envision a spacecraft that is about 300 metric tons, and would be 44 ft long. The ISP of this engine would be 130,000 s (LOX/LH2 has an ISP of 450s). This would be sufficient for interstellar space travel.

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.

The 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!!!

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 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 ($1.5 trillion), or compare it to the total of the budgets of the "G7" nations.

To establish colonies on the moon and Mars would probably require trillions of dollars (spread over many decades). The US economy is currently (GDP) at $9 trillion, but that is only about a quarter of the global economy. 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, 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 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.

Are there other planetary systems out there? As you have probably heard, in the past few years astronomers have discovered a number of planets orbiting other stars-in fact, the current number of known planets orbiting other stars is 28! It is likely that just about every single star out there has planets of some form in orbit-the process of star formation is not 100% efficient, so the remnant material that condenses into planets will probably always accompany the formation of stars.

Long before we have the means to travel to the stars, we will already have identified which ones we will want to visit. In the next few decades, advanced space-based telescopes will be built which will allow us to actually observe the planets around other stars-we will be able to determine which ones are most like the Earth. It may even be possible to determine if life is present on these planets by analyzing the chemistry of their atmospheres! By this time we will have developed the technique of terraforming to a level that even rather inhospitible planets can be made livable.