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The Physics of Interstellar Travel

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Summary: Many people wonder when we will be able to travel to distant solar systems as easily as envisioned in science fiction. This essay will explain the challenges of interstellar travel, the prospects and limitations of existing propulsion ideas, and the prospects emerging from science that may one day provide the breakthroughs needed to enable practical interstellar voyages. Analogies to familiar science fiction are used to simplify notions such as the 'warp drive'. It will show the step-by-step approach towards discovering the ultimate breakthroughs needed to revolutionize space travel and enable human journeys to other star systems - credible progress towards incredible possibilities.

Introduction

Many people wonder when we will be able to travel to distant solar systems as easily as envisioned in science fiction. This essay will explain the challenges of interstellar travel, the prospects and limitations of existing propulsion ideas, and the prospects emerging from science that may one day provide the breakthroughs needed to enable practical interstellar voyages. Analogies to familiar science fiction are used to simplify notions such as the 'warp drive'. It will show the step-by-step approach towards discovering the ultimate breakthroughs needed to revolutionize space travel and enable human journeys to other star systems - credible progress towards incredible possibilities

Abstract

This essay shows the beginnings of interstellar travel and its developments as it shadows advancing and emerging science. Reasons for the difficulties of interstellar travel are explained as well as possible ways of bypassing them. It gives examples of past projects and ideas as well as those which are being worked on for future uses. Three missions which are presently running are described and information is supplied about them. Last, a list of main contributors to todays understanding of science is given. Difficulties

The propellant problem

The first challenge is propulsion, specifically propellant mass. Unlike aircraft that can use the air as their reaction mass, rockets need to bring along their own reaction mass, propellant, with them. By blasting propellant out the back, rockets push spacecraft. The problem is quantity. Propellant needs rise exponentially with increases in payload, destinations, or speed.

Ideally, a space drive would not need any propellant. A few researchers have begun studying how to achieve this, searching for something else in space to push against, perhaps even by pushing against the very structure of space-time itself, or by finding a way to modify gravitational or inertial forces. Examples of this include warp drives, radiation sails and worm holes.

The need for speed

The next and more obvious challenge is speed. Our nearest neighbouring star is about 26 trillion miles away. That is more than four years away at the speed of light, and light speed (3*108) is about 17,000 times faster than the Voyager spacecraft.

Although the search for a non-propellant space drive would dramatically improve this speed situation, some researchers have even contemplated bypassing the light speed limit for interstellar travel.

We know that it is impossible to break the light speed limit, so how is it possible to travel faster than light? The trick is to get past the light speed limit by distorting the fabric of space-time itself to create "wormholes," which are shortcuts in space-time, or by using "warp drives," which are moving segments of space-time.

Looking for energy

The last challenge is energy. Even if we had a space drive that could convert energy directly into motion, it would still require a lot of energy. Sending a shuttle-sized vehicle on a 50-year, one-way trip to the nearest star would require 70 trillion (7*1019) joules of energy - the equivalent of running the space shuttle's engines continuously for that same 50 years. This amount is roughly the same as the total output of a nuclear power plant. 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 do not apply.

For warp drives and wormholes, the energy situation is much, much worse. Creating a 3-foot-wide wormhole, something with the mass of Jupiter would have to be converted into negative energy. To overcome these difficulties, a few breakthroughs in energy production would be needed.

To find out if we can actually begin making progress towards these grand ambitions, NASA established the Breakthrough Propulsion Physics Program in 1996. The program has supported conference sessions, workshops and Internet sites to foster collaborations and to identify affordable research. 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. This is necessary because space is big - really big.

Interstellar distances are so astronomical (pun intended) that conveying this expanse is difficult. Consider the following analogy: If the sun were the size of a typical, 1.25cm diameter marble, the distance from the sun to the Earth, called an "Astronomical Unit (AU)" would be about 220cm, the Earth would be barely thicker than a sheet of paper, and the orbit of the Moon would be about a 0.5cm in diameter. On this scale, the closest neighbouring star is about 336km away. 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 aeroplane that has the air to push against, rockets do not have 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 (conservation of momentum). The further or faster we wish to travel, the more propellant we will need. For long journeys to neighbouring stars, the amount of propellant we would need would be enormous and prohibitively expensive.

Examples of ideas

Project Orion

The first example is from the 1950's-60's, Project Orion - which offered to use nuclear bombs for a constructive purpose - space travel. The proposal was that about five bombs per second would be dropped from the back of the vessel and detonated to propel it 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

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 journey time. (Barnard's star is about 6 Light Years away.) In this case it used micro fusion explosions that relied on obtaining the appropriate fuel isotope (deuterium) from Jupiter's upper atmosphere. It scoops this up on its way out of the solar system - a very difficult manoeuvre.

Bussard Interstellar Ramjet

An idea which allows the craft to collect its fuel as it travels. 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 idea, such as how many protons can be scooped up, the size of the 'scoop', 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, a small amount of pressure is exerted. Use a lot of light over a very large area, and the forces become noticeable. Here, Robert Forward proposed using a 10 million gigawatt laser to shine through a thousand kilometre Fresnel lens onto a thousand kilometre sail. On this scale, it is claimed that one could send a thousand-ton vehicle with a crew to our nearest star in 10 years! The snag is the 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 kilometre. The sail has all its sensors and technical systems built right into its array of wires.

The laser problem would be bypassed by building a sail (made of foil only, say, 20 atoms thick) which is pushed by the sun's radiation. The problem with this is that the sail receives less push the further it is from the sun and the sails would be difficult to manufacture and keep in good condition.

Wormholes

Here is the premise behind a 'wormhole'. Although Special Relativity forbids objects to move faster than light within space time, it is known that space time itself can be warped and distorted. It takes an enormous amount of matter or energy to create such distortions (a heavy mass such as a planet can cause a small, but noticeable curve in the path of a light ray), but distortions are possible. To use an analogy: even if there were a speed limit to how fast a pencil could move across a sheet 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, wormholes invite the old time travel paradox problems again.

Here is an over simplified way to build one: First, collect a lot (enough to construct a ring the size of the Earth's orbit around the Sun) of super-dense matter, such as matter from a neutron star. Then build another ring where you want the other end of your wormhole. Next, charge them up to a massive voltage, and spin them up to near the speed of light - both of them. The problems - well if it was possible to do all that, you notice that you already had to be where you wanted to go to, there are certainly more clever ways to travel. Wormhole engineering cannot be expected 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 is the theory 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. The warp drive idea is something like a conveyor belt, similar to those you find at many airports. By expanding space-time behind the starship and contracting it in front, a segment of space-time moves and carries the ship with it. The starship itself still moves slower than light within its space-time, but when you add the "conveyor belt" effect; the apparent motion exceeds the speed of light. There are numerous difficulties with these concepts, however. 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 a warp drive? These theories are too new to have either been discounted or proven viable. Other problems: First, to create this effect, an immense ring of negative energy has to be wrapped around the ship. 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. It is still unknown if this could happen. And fourth, if all the previous issues weren't complicated enough, these concepts evoke the same time-travel paradoxes as the wormhole concepts.

Negative mass propulsion

It has been shown that it 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.

The radiation 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.

The radiation diode sail

Has the same effect of a diode or one-way mirror, space radiation passes through one direction and reflects from the other. This creates a net difference in radiation pressure, forcing the craft foreward.

The radiation induction sail

Equivalent 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.

The pitch drive

This concept entertains the possibility that somehow, a localized slope in scalar potential can be induced across the vehicle which causes forces on the vehicle. 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. What's happening now

The Voyager missions

Mission objective

The mission objective of the Voyager Interstellar Mission (VIM) is to extend the NASA exploration of the solar system beyond the neighbourhood of the outer planets to the outer limits of the Sun's sphere of influence, and possibly beyond. This extended mission is continuing to characterise the outer solar system environment and search for the heliopause boundary, the outer limits of the Sun's magnetic field and outward flow of the solar wind. Penetration of the heliopause boundary between the solar wind and the interstellar medium will allow measurements to consist of the interstellar fields, particles and waves unaffected by the solar wind.

The previous missions

The VIM is an extension of the Voyager primary mission that was completed in 1989 with the close flyby of Neptune by the Voyager 2 spacecraft. Neptune was the final outer planet visited by a Voyager spacecraft. Voyager 1 completed its planned close flybys of the Jupiter and Saturn planetary systems while Voyager 2, in addition to its own close flybys of Jupiter and Saturn, completed close flybys of the remaining two gas giants, Uranus and Neptune.

At the start of the VIM, the two Voyager spacecraft had been in flight for over 12 years having been launched in August (Voyager 2) and September (Voyager 1), 1977. Voyager 1 was at a distance of approximately 40 AU (Astronomical Unit - mean distance of Earth from the Sun, 150 million kilometres) from the Sun, and Voyager 2 was at a distance of approximately 31 AU. Voyager 1 is escaping the solar system at a speed of about 3.5 AU per year, 35 degrees out of the ecliptic plan to the north, in the general direction of the Solar Apex (the direction of the Sun's motion relative to nearby stars). Voyager 2 is also escaping the solar system at a speed of about 3.1 AU per year, 48 degrees out of the ecliptic plane to the south.

Voyager 1 is now the most distant human-made object in space.

Deep Space 1

Deep Space 1 is powered by a prototype ion propulsion drive which has been developed by NASA.The engine delivers 0.044kg of thrust at full power (roughly the weight of an A4 sheet of paper). This may not seem like much but it offers ten times the thrust of conventional chemical thrusters for a given amount of fuel. These thrusters can increase the speed of DS1 to about 16,000 kph. The fuel that powers the thrusters is 399kg of Xenon (Xe).

How it works (see diagram on next page): 1 Xe is released into a chamber ringed by magnets (these increase the efficiency of the ionisation progress). 2 Electrons are fired from a cathode ray tube (similar to the ones used in televisions). 3 The electrons knock electrons from the Xe atoms making them positively charged Xe+ ions. 4 Charged grids generate an electrostatic pull which 'yanks' the ions past the grid at 99200 kph 5 To stop the Xe+ ions from coming back into the chamber, a neutralising cathode ray tube gives the Xe+ and extra electron, neutralising it.

The DS1 ion propulsion drive

Acceleration of the ship

1 To find Xe mass ejected per second:

f = (m2/t) * v2

Where f = force acting on ship m2/t = mass of Xe eject per second v2 = speed of Xe

Force acting on ship = 0.486 N Total mass = 486.32kg Speed of Xe ejected = 99200 kph = 27556 m/s Let x = Xe mass per second

0.486 = x * 27556 x = 0.486/27556 x = 1.76*10-5 kg/s

2 Atoms of Xe per second:

moles = mass/ar

Where ar = the atomic mass

moles = 1.76*10-5/131.3 moles = 1.34*10-7 mole = 6.02*1023 atoms no. of atoms = (6.02*1023) * 1.34*10-7 no. of atoms of Xe ejected a second = 8.07*1016

3 To find the acceleration of the ship:

(m2/t) * v2 = m1 .(v/t)

Where: m1 = craft mass (inc. propellant) v/t = acceleration of ship m2/t = Xe mass ejected per second v2 = speed of Xe Let acceleration be x

1.76*10-5 * 27556 = 486.32 * x x = 1.76*10-5 * 27556/486.32 x = 9.97*10-4 ms-2

Acceleration of DS1 = 9.97*10-4 ms-2 Lists of some intriguing emerging physics

Science and technology are continuing to evolve. In just the last few years, there have been new, intriguing developments in the scientific literature. Although it is still too soon to know whether any of these developments can lead to the desired propulsion breakthroughs, they do provide new clues that did not exist just a few short years ago. A snapshot of just some of the possibilities is listed below:

1988; Morris and Thorne: Theory and assessments for using wormholes for faster-than-light space travel. 1988; Herbert: Book outlining the loopholes in physics that suggest that faster-than-light travel may be possible. 1989; Puthoff: Theory extending Sakharov's 1968 work to suggest that gravity is a consequential effect of the vacuum electromagnetic zero point fluctuations. 1992; Podkletnov and Nieminen: Report of superconductor experiments with anomalous results - evidence of a possible gravity shielding effect. 1994; Haisch, Rueda, and Puthoff: Theory suggesting that inertia is a consequential effect of the vacuum electromagnetic zero point fluctuations. 1994; Alcubierre: Theory for a faster-than-light "warp drive" consistent with general relativity. 1996; Eberlein: Theory suggesting that the laboratory observed effect of sonoluminescence is extraction of virtual photons from the electromagnetic zero point fluctuations.

Surveys & Workshops:

1972; Mead Jr.: Identification and assessments of advanced propulsion concepts. 1982; Garrison, et al.: Assessment of ultra high performance propulsion. 1986; Forward: Assessment of the technological feasibility of interstellar travel. 1990; NASA Lewis Research Centre: Symposium "Vision-21: Space Travel for the Next Millennium." 1990; British Aerospace Co.: Workshop to revisit theory and implications of controlling gravity. 1990; Cravens: Assessment of alternative theories of electromagnetics and gravity for propulsion. 1991; Forward: Assessment of advanced propulsion concepts. 1994; Bennett, et al.: NASA workshop on the theory and implications of faster-than-light travel. 1994; Belbruno: Conference assessing: "Practical Robotic Interstellar Flight: Are We Ready?" 1995; Hujsak & Hujsak: Formation of the "Interstellar Propulsion Society."

Theory:

1988; Forward; Winterberg: Further assessments of Bondi's 1957 theory regarding hypothetical negative mass and its propulsive implications. 1984; Forward: Conceptual design for a "vacuum fluctuation battery" to extract energy from electromagnetic fluctuations of the vacuum based on the Casimir effect (predicted 1948, measured 1958 by Sparnaay). 1994; Cramer, et. al.: Identification of the characteristics of natural wormholes with negative mass entrances that could be detectable using existing astronomical observations. 1996; Millis: Identification of the remaining physics developments required to enable " space drives," including the presentation and assessment of seven different hypothetical "space drive" concepts.