Have you ever wondered when we will be able to travel to distant stars as easily as in science fiction stories? NASA Glenn’sMarc Millis, who has taken a break from Project Management for NASA’s Breakthrough Propulsion Physics (BPP) Project to return to conducting research, offers this assessment of the prospects for achieving the propulsion breakthroughs that would enable such far-future visions of interstellar travel.
This web site focuses on the propulsion related issues, explaining the challenges of interstellar travel, existing propulsion ideas, and the possibilities emerging from scientific literature that may one day provide the desired breakthroughs. To simplify the presentation for the general public, analogies to familiar science fiction are used. This site is intended for public audiences, whereas researchers might want to check out the Breakthrough Propulsion Physics Project web site.
A Look at the Scaling
The ideal interstellar propulsion system would be one that could get you to other stars as quickly and comfortably as envisioned in science fiction. Before this can become a reality, three scientific breakthroughs are needed: discovery of a means to exceed light speed, discovery of a means to propel a vehicle without propellant, and discovery of a means to power such devices. Why? – Because space is big, really, really, really big.
Space takes up a lot of space!
Interstellar distances are so astronomical (pun intended) that it is difficult to convey this expanse. Consider the following analogy: If the sun were the size of a typical, 1/2 inch diameter marble, the distance from the sun to the Earth, called an “Astronomical Unit (AU)” would be about 4 feet, the Earth would be barely thicker than a sheet of paper, and the orbit of the Moon would be about a 1/4 inch in diameter. On this scale, the closest neighboring star is about 210 miles away. That’s about the distance from Cleveland to Cincinnati.
To help put this in perspective, consider that it takes light over 8 minutes to cover that 4 ft “Astronomical Unit” mentioned before. Light is the fastest thing that we know to exist! Just imagine… How long will take you to travel 210 miles if it takes you over 8 minutes to travel just 4 feet? Actually, the answer is 4.2 years. Our nearest neighboring star, Proxima Centauri, is 4.2 Light Years away.
The Voyager spacecraft left the solar system at 37,000 miles per hour. At that speed, it would take Voyager 80,000 years to reach Proxima Centauri.
Speed: Getting there in a reasonable time – an obvious challenge
The most obvious challenge to practical interstellar travel is speed. Our nearest neighboring star is 4.2 Light Years away. Trip times to reach our nearest neighboring star at conventional speeds would be prohibitively long. At 55 miles-per-hour for example, it would take over 50 million years to get there! I don’t think even the twinkies in the glove box would survive that long. At a more typical spacecraft speed, for example the 3-day trip time that it took the Apollo spacecraft to reach the moon, it would still take over 900 thousand years. I still don’t think the twinkies will make it. And even if we consider the staggering speed of 37-thousand miles-per-hour, which was the speed of the NASA Voyager spacecraft as it left our solar system years ago, the trip would still take 80,000 years. Maybe the twinkies would make it, but there would be nothing left on board to eat them. In conclusion, if we want to cruise to other stars within comfortable and fundable time spans (say, less than a term in Congress), we have to figure out a way to go faster than light.
Mass: Rockets use too much propellant – a less obvious challenge
A less obvious challenge is overcoming the limitations of rockets. The problem is fuel, or more specifically, rocket propellant. Unlike a car that has the road to push against, or an airplane that has the air to push against, rockets don’t have roads or air in space. Today’s spacecraft use rockets and rockets use large quantities of propellant. As propellant blasts out of the rocket in one direction, it pushes the spacecraft in the other — Newton’s third law. The farther or faster we wish to travel, the more propellant we’ll need. For long journeys to neighboring stars, the amount of propellant we would need would be enormous and prohibitively expensive.
This chart highlights two critical features of a rocket, Thrust and Specific Impulse. Thrust is how much push a rocket can give. The higher up on the chart, the greater the push.
Specific Impulse can be thought of as a kind of fuel efficiency for rocket engines, analogous to the miles-per-gallon for cars. The farther right on the chart, the less propellant you’ll need. It really has to do with how fast the fuel blasts out of the rocket.
What you should notice is the red region. This is the range of rocket performance we can conceivably create with what we know today. And what we need for interstellar travel is in that desired region or even more fuel efficient.
Here are four examples of what it would take to send a canister about the size of a Shuttle payload (or a school bus) past our nearest neighboring star…and allowing 900 years for it to make this journey.
Well….If you use chemical engines like those that are on the Shuttle, well…, sorry, there isn’t enough mass in the universe to supply the rocket propellant you’d need.
So let’s step up to next possibilities, nuclear rockets with a predicted performance that’s 10 to 20 times better!
Well…it’s still not looking all that good. For a fission rocket you would need a BILLION SUPERTANKER size propellant tanks to get you there, and even with fusion rockets you would still need a THOUSAND SUPERTANKERS!
Even if we look at the best conceivable performance that we could engineer based on today’s knowledge, say an Ion engine or an antimatter rocket whose performance was 100 times better that the shuttle engines, we would need about ten railway tanker sized propellant tanks.
That doesn’t sound too bad, until you consider that we didn’t bring along any propellant to let us stop when we get to the other star system…or if we want to get there quicker than 9 centuries.
Once you add the desire to actually stop at your destination, or if you want to get there sooner, you’re back at the incredible supertanker situation again, even for our best conceivable rockets.
In conclusion, we’d really like to have a form of propulsion that doesn’t need any propellant! This implies the need to find some way to modify gravitational or inertial forces or to find some means to push against the very structure of spacetime itself.
Energy: – yet another challenge
Our third big challenge is energy. Even if we had a nonrocket space drive that could convert energy directly into motion without propellant, it would still require a lot of energy. Sending a Shuttle-sized vehicle on a 50 year one-way trip to visit our nearest neighboring star (subrelativistic speed) would take over 7 x 10^19 Joules of energy. This is roughly the same amount of energy that the Space Shuttle’s engines would use if they ran continuously for the same duration of 50 years. To overcome this difficulty, we need either a breakthrough where we can take advantage of the energy in the space vacuum, a breakthrough in energy production physics, or a breakthrough where the laws of kinetic energy don’t apply.
From Inspirations to Inventions
Fun Retrospectives & The Future
Right now we don’t even know if practical interstellar travel is possible. Just because we don’t know how to do something today, however, doesn’t mean that it is impossible. There is a historical pattern that has emerged where the grand visions of yesterday’s science fiction inspired today’s reality. Maybe the same will happen with today’s science fiction.
Here are examples:
To the Moon - A long time ago, Jules Verne wrote a story about sending people to the moon by blasting them out of a giant cannon. That story inspired a whole host of rocketry pioneers who pondered how to make such a journey a reality. Based on the science of their day, they were eventually able to create visions of how to achieve such a feat — using rockets instead of cannons. And, when all the conditions were right, these visions evolved into reality.
Now, we look back over 25 years to our landing people on the moon and bringing them back safely.
Access to space - This next example is about routine access to space. In this case the inspirations were from Buck Rogers and Flash Gordon stories with their rockets gallivanting across space as routinely as that era’s aircraft flew in the air. Again, such stories inspired rocketry pioneers who created visions, and reality followed. In this case the Space Shuttle.
To the stars - And now the inspirations behind this web page – Journeys to the stars. The idea of “Warp Drives” and “Hyper Space” date back to the 1930’s and are attributed to John Campbell. Another appropriate example is the 1956 movie “Forbidden Planet.” Its opening sequence has a prediction of how humanity conquers gravity and then goes on to discover how to exceed light speed. And a very well known example is, of course, Star Trek.
But today, we don’t even have the scientific principles from which to create the visions. First we need to develop such science and then we can engineer the visions and shape them into reality.
Contemplating an Inspiration
Because of its widespread familiarity, Let’s examine the Star Trek inspirations.
We would like to point out that although such science fiction vehicles and ones from other stories like Star Wars are inspirational, they are science fiction, not research guides for real science. By that we mean that there is not enough substance behind these vehicles and the techno-speak that is used to describe them that you could begin a research program. They are however, inspirational, and they do provide mental pictures that make it easier to contemplate how you might achieve such feats.
This inspiration shows some of the features we would need for our interstellar craft.
Faster than light propulsion,sometimes called “warp drive”, “hyperspace drive”, etc. The point is that we would like some form of propulsion that can get us there in comfortable timeframes. Also note that this propulsion does not use propellant — a crucial feature.
Control over gravity. The Trek vehicle has a variety of features that all imply that we have mastered the control of gravity or inertia: These things called “Inertial Dampers” on Star Trek are what help keep the crew from flying out of their seats when the ship maneuvers. The “deflectors” help move objects out of the way so that the ship doesn’t smash into them at damaging speeds. And “synthetic gravity” is so the crew can walk about normally. And in movies this makes their special effects budget much, much smaller.The control over gravitational forces could also be used to propel the vehicle without the need for rockets. It is in this propulsion role where the feat of gravity control would bring the most benefit. If we ever, or should I say more optimistically, when we conquer gravity, it will be an enormous breakthrough for space travel and for scientific and technological advances in general. That would usher in an exciting age for humanity.
Power generation. And to power the vehicle and its propulsion, some form of power generation is needed. In Trek, they talk a lot about Antimatter, so we have provided a status on that topic.
Ideas Based On What We Know
The following section has a brief description of some ideas that have been suggested over the years for interstellar travel, ideas based on the sciences that do exist today.
The first example is from the 1950’s-60’s, Project Orion – which offered to use nuclear bombs for a constructive purpose — space travel.
About 5 bombs per second are dropped out the back and detonated to propel the craft along. A huge shock plate with shock absorbers make up the base of the craft. Experiments using conventional explosives were conducted to demonstrate the viability of this scheme. Although this vehicle was conceived to take a crew to Mars, it can also be considered for sending smaller probes to the stars. This project ended with the nuclear test ban treaty in the 60’s.
Project Daedalus, British Interplanetary Society
In the late 1970’s the British Interplanetary Society revisited the Orion propulsion concept, but at a more reasonable scale and for in-space use only. Project Daedalus was a design study for sending a probe past Barnard’s star with a 50 year trip time. (Barnard’s star is about 6 Light Years away.) In this case it used micro fusion explosions which relied on obtaining the appropriate fuel isotope from Jupiter that it scooped up on its way out of the solar system — tricky.
Bussard Interstellar Ramjet
Well, rather than bring your fuel along, why not get it as you go. This Bussard Interstellar Ramjet concept, from the 1960’s, relies on scooping up the lonely protons that drift in interstellar space, and then somehow getting them to fuse to make a nuclear rocket. There are a variety of limitations to this concept, such as how many protons can be scooped up, the drag created from scooping them, and, not to mention, the feat of getting these protons to engage in nuclear fusion for a rocket.
Robert Forward’s interstellar laser sails
Light sails are another possibility. Rather than use rockets, why not use light. When light strikes an object, it pushes on it ever so slightly. Use lots of light over a very large area, and the forces get noticeable. That is the idea here. Robert Forward proposed using a 10-million-gigawatt laser to shine through a thousand kilometer Fresnel lens onto a thousand kilometer sail. With these numbers, it is claimed that one could send a thousand-ton vehicle with crew to our nearest star in 10 years!
What’s the catch? That 10-million-gigawatt laser. That power level is ten thousand times more than the power used on all the Earth today.
So, Forward revised the concept to more reasonable power levels. This time it only has a 10-gigawatt microwave laser (still a feat unto itself), and this time the vehicle is a frail 16 grams of fine wires spread over just one kilometer. The sail has all its sensors and stuff built right into its array of wires.
This and similar concepts are still under investigation. Significant advances are still required, however, before we can create such systems and before we have a sufficiently robust space program that could put them in space.
Ideas Based On What We’d Like To Achieve
The following section has a brief description of some ideas that have been suggested over the years for interstellar travel, ideas based on the sciences that do exist today.
Worm Hole transportation
Just when you thought it was confusing enough, those physicist had to come up with wormholes. Here’s the premise behind a “wormhole.”
Although Special Relativity forbids objects to move faster than light within spacetime, it is known that spacetime itself can be warped and distorted. It takes an enormous amount of matter or energy to create such distortions, but distortions are possible, theoretically. To use an analogy: even if there were a speed limit to how fast a pencil could move across a piece of paper, the motion or changes to the paper is a separate issue. In the case of the wormhole, a shortcut is made by warping space (folding the paper) to connect two points that used to be separated. These theories are too new to have either been discounted or proven viable. And, yes, wormholes do invite the old time travel paradox problems again.
Here’s one way to build one:
First, collect a whole bunch of super-dense matter, such as matter from a neutron star. How much?- well enough to construct a ring the size of the Earth’s orbit around the Sun. Then build another ring where you want the other end of your wormhole. Next, just charge ‘em up to some incredible voltage, and spin them up to near the speed of light — both of them.
No problem? Well if you could do all that, and notice you already had to be where you wanted to go to, I’m sure you could think of more clever ways to travel. Don’t expect any wormhole engineering any time soon. There are other ideas out there too – ideas that use “negative energy” to create and to keep the wormhole open.
Alcubierre’s “Warp Drive”
Here’s the premise behind the Alcubierre “warp drive”: Although Special Relativity forbids objects to move faster than light within spacetime, it is unknown how fast spacetime itself can move. To use an analogy, imagine you are on one of those moving sidewalks that can be found in some airports. The Alcubierre warp drive is like one of those moving sidewalks. Although there may be a limit to how fast one can walk across the floor (analogous to the light speed limit), what about if you are on a moving section of floor that moves faster than you can walk (analogous to a moving section of spacetime)? In the case of the Alcubierre warp drive, this moving section of spacetime is created by expanding spacetime behind the ship (analogous to where the sidewalk emerges from underneath the floor), and by contracting spacetime in front of the ship (analogous to where the sidewalk goes back into the floor). The idea of expanding spacetime is not new. Using the “Inflationary Universe” perspective, for example, it is thought that spacetime expanded faster than the speed of light during the early moments of the Big Bang. So if spacetime can expand faster than the speed of light during the Big Bang, why not for our warp drive? These theories are too new to have either been discounted or proven viable.
Any other sticky issues?
Yes… First, to create this effect, you’ll need a ring of negative energy wrapped around the ship, and lots of it too. It is still debated in physics whether negative energy can exist. Classical physics tends toward a “no,” while quantum physics leans to a “maybe, yes.” Second, you’ll need a way to control this effect to turn it on and off at will. This will be especially tricky since this warp effect is a separate effect from the ship. Third, all this assumes that this whole “warp” would indeed move faster than the speed of light. This is a big unknown. And fourth, if all the previous issues weren’t tough enough, these concepts evoke the same time-travel paradoxes as the wormhole concepts.
Negative mass propulsion
It has been shown that is theoretically possible to create a continuously propulsive effect by the juxtaposition of negative and positive mass and that such a scheme does not violate conservation of momentum or energy. A crucial assumption to the success of this concept is that negative mass has negative inertia. Their combined interactions result in a sustained acceleration of both masses in the same direction. This concept dates back to at least 1957 with an analysis of the properties of hypothetical negative mass by Bondi, and has been revisited in the context of propulsion by Winterberg and Forward in the 1980’s.
Regarding the physics of negative mass, it is not known whether negative mass exists or if it is even theoretically allowed, but methods have been suggested to search for evidence of negative mass in the context of searching for astronomical evidence of wormholes.
Millis’s hypothetical “Space Drives”
A “space drive” can be defined as an idealized form of propulsion where the fundamental properties of matter and spacetime are used to create propulsive forces anywhere in space without having to carry and expel a reaction mass. Such an achievement would revolutionize space travel as it would circumvent the need for propellant. A variety of hypothetical space drives were created and analyzed by Millis to identify the specific problems that have to be solved to make such schemes plausible. These hypothetical drives are just briefly introduced here. Please note that these concepts are purely hypothetical constructs aimed to illustrate the remaining challenges. Before any of these space drives can become reality, a method must be discovered where a vehicle can create and control an external asymmetric force on itself without expelling a reaction mass and the method must satisfy conservation laws in the process.
[Note: This section is excerpted from Millis' "Challenge to Create the Space Drive," in the AIAA Journal of Propulsion and Power, Vol.13, No.5, pp. 577-582, Sept.-Oct. 1997. This 6 page report uses 7 hypothetical space drive concepts to highlight the unsolved physics and candidate next steps toward creating a propellantless space drive. It also contains figures for each concept which are not currently available electronically.]
Hypothetical Differential Sail: Analogous to the principles of an ideal radiometer vane, a net difference in radiation pressure exists across the reflecting and absorbing sides. It is assumed that space contains a background of some form of isotropic medium (like the vacuum fluctuations or Cosmic Background Radiation) that is constantly impinging on all sides of the sail.
Hypothetical Diode Sail: Analogous to a diode or one-way mirror, space radiation passes through one direction and reflects from the other creating a net difference in radiation pressure.
Hypothetical Induction Sail: Analogous to creating a pressure gradient in a fluid, the energy density of the impinging space radiation is raised behind the sail and lowered in front to create a net difference in radiation pressure across the sail.
Hypothetical Diametric Drive: This concept considers the possibility of creating a local gradient in a background scalar property of space (such as gravitational potential) by the juxtaposition of diametrically opposed field sources across the vehicle. This is directly analogous to negative mass propulsion. The diametric drive can also be considered analogous to creating a pressure source/sink in a space medium as suggested with the Induction Sail.
Hypothetical Pitch Drive: This concept entertains the possibility that somehow a localized slope in scalar potential is induced across the vehicle which causes forces on the vehicle. In contrast to the diametric drive presented earlier, it is assumed that such a slope can be created without the presence of a pair of point sources. It is not yet known if and how such an effect can be created.
Hypothetical Bias Drive: This concept entertains the possibility that the vehicle alters the properties of space itself, such as the gravitational constant, G, to create a local propulsive gradient. By modifying Newton’s constant to have a localized asymmetric bias, a local gradient similar to the Pitch Drive mechanism results.
Hypothetical Disjunction Drive: This concept entertains the possibility that the source of a field and that which reacts to a field can be separated. By displacing them in space, the reactant is shifted to a point where the field has a slope, thus producing reaction forces between the source and the reactant. Although existing evidence strongly suggests that the source, reactant, and inertial mass properties are inseparable, any future evidence to the contrary would have revolutionary implication to this propulsion application.
So, can we do it?