Space travel under constant acceleration is a hypothetical method of space travel that involves the use of a propulsion system that generates a constant acceleration rather than the short, impulsivethrusts produced by traditional chemical rockets. For the first half of the journey the propulsion system would constantly accelerate the spacecraft toward its destination, and for the second half of the journey it would constantly decelerate the spaceship.[1] Constant acceleration could be used to achieve relativistic speeds,[2][3][4] making it a potential means of achieving human interstellar travel. This mode of travel has yet to be used in practice.
Constant-thrust and constant-acceleration trajectories both involve a spacecraft firing its engine continuously. In a constant-thrust trajectory,[5] the vehicle's acceleration increases during thrusting period, since the use of fuel decreases the vehicle mass. If, instead of constant thrust, the vehicle has constant acceleration, the engine thrust decreases during the journey.
The spacecraft must flip its orientation halfway through the journey and decelerate the rest of the way, if it is required to rendezvous with its destination (as opposed to a flyby).
Interstellar travel
This plot shows a ship capable of 1-g (10 m/s or about 1.0 ly/y ) "felt" or proper acceleration can travel vast distances, although is limited by the mass of any propellant it carries.
The distance traveled, under constant proper acceleration, from the point of view of Earth as a function of the traveler's time is expressed by the coordinate distancex as a function of proper timeτ at constant proper acceleration a. It is given by:[8][9]
where c is the speed of light.
Under the same circumstances, the time elapsed on Earth (the coordinate time) as a function of the traveler's time is given by:
Feasibility
A limitation of constant acceleration is adequate fuel. Constant acceleration is only feasible with the development of fuels with a much higher specific impulse than presently available.
There are two broad approaches to higher specific impulse propulsion:
Higher efficiency fuel (the motor ship approach). Two possibilities for the motor ship approach are nuclear and matter–antimatter based fuels.
Drawing propulsion energy from the environment as the ship passes through it (the sailing ship approach). One hypothetical sailing ship approach is discovering something equivalent to the parallelogram of force between wind and water which allows sails to propel a sailing ship.
Picking up fuel along the way — the ramjet approach — will lose efficiency as the space craft's speed increases relative to the planetary reference. This happens because the fuel must be accelerated to the spaceship's velocity before its energy can be extracted, and that will cut the fuel efficiency dramatically.
A related issue is drag. If the near-light-speed space craft is interacting with matter that is moving slowly in the planetary reference frame, this will cause drag which will bleed off a portion of the engine's acceleration.
A second big issue facing ships using constant acceleration for interstellar travel is colliding with matter and radiation while en route. In mid-journey any such impact will be at near light speed, so the result will be dramatic.
Interstellar traveling speeds
If a space ship is using constant acceleration over interstellar distances, it will approach the speed of light for the middle part of its journey when viewed from the planetary frame of reference. This means that the effects of relativity will become important. The most important effect is that time will appear to pass at different rates in the ship frame and the planetary frame, and this means that the ship's speed and journey time will appear different in the two frames.
Planetary reference frame
From the planetary frame of reference, the ship's speed will appear to be limited by the speed of light — it can approach the speed of light, but never reach it. If a ship is using 1 g constant acceleration, it will appear to get near the speed of light in about a year, and have traveled about half a light year in distance. For the middle of the journey the ship's speed will be roughly the speed of light, and it will slow down again to zero over a year at the end of the journey.
As a rule of thumb, for a constant acceleration at 1 g (Earth gravity), the journey time, as measured on Earth, will be the distance in light years to the destination, plus 1 year. This rule of thumb will give answers that are slightly shorter than the exact calculated answer, but reasonably accurate.
Ship reference frame
Plot of velocity parameters and times on the horizontal axis, versus position on the vertical axis, for an accelerated twin roundtrip to a destination with ΔxAB=10c /α ~10 light years away if α~9.8 m/s .
From the frame of reference of those on the ship the acceleration will not change as the journey goes on. Instead the planetary reference frame will look more and more relativistic. This means that for voyagers on the ship the journey will appear to be much shorter than what planetary observers see.
At a constant acceleration of 1 g, a rocket could travel the diameter of our galaxy in about 12 years ship time, and about 113,000 years planetary time. If the last half of the trip involves deceleration at 1 g, the trip would take about 24 years. If the trip is merely to the nearest star, with deceleration the last half of the way, it would take 3.6 years.[10]
In fiction
The spacecraft of George O. Smith's Venus Equilateral stories are all constant acceleration ships. Normal acceleration is 1 g, but in "The External Triangle" it is mentioned that accelerations of up to 5 g are possible if the crew is drugged with gravanol[11] to counteract the effects of the g-load.
"Sky Lift" is a science fiction short story by Robert A. Heinlein, first published 1953. In the story, a torchship pilot lights out from Earth orbit to Pluto on a mission to deliver a cure to a plague ravaging a research station.[12]
Spacecraft in Joe Haldeman's 1974 novel The Forever War make extensive use of constant acceleration; they require elaborate safety equipment to keep their occupants alive at high acceleration (up to 25 g), and accelerate at 1 g even when "at rest" to provide humans with a comfortable level of gravity.
In the Known Space universe, constructed by Larry Niven, Earth uses constant acceleration drives in the form of Bussard ramjets to help colonize the nearest planetary systems. In the non-known space novel A World Out of Time, Jerome Branch Corbell (for himself), "takes" a ramjet to the Galactic Center and back in 150 years ships time (most of it in cold sleep), but 3 million years passes on Earth.
In The Sparrow, by Mary Doria Russell, interstellar travel is achieved by converting a small asteroid into a constant acceleration spacecraft. Force is applied by ion engines fed with material mined from the asteroid itself.
In the Revelation Space series by Alastair Reynolds, interstellar commerce depends upon "lighthugger" starships which can accelerate indefinitely at 1 g, with superseded antimatter powered constant acceleration drives. The effects of relativistic travel are an important plot point in several stories, informing the psychologies and politics of the lighthuggers' "ultranaut" crews for example.
In the novel 2061: Odyssey Three by Arthur C. Clarke, the spaceship Universe, using a muon-catalyzed fusion rocket, is capable of constant acceleration at 0.2 g under full thrust. Clarke's novel "Imperial Earth" features an "asymptotic drive", which utilises a microscopic black hole and hydrogen propellant, to achieve a similar acceleration travelling from Titan to Earth.
The UET and Hidden Worlds spaceships of F.M. Busby's Rissa Kerguelen saga utilize a constant acceleration drive that can accelerate at 1 g or even a little more.
Ships in the Expanse series by James S. A. Corey make use of constant acceleration drives, which also provide artificial gravity for the occupants.
Explorers on the Moon, one of the Adventures of Tintin series of comic albums by Hergé, features a crewed Moon rocket with an unspecified 'atomic rocket motor'. The ship constantly accelerates from takeoff to provide occupants with consistent gravity, until a mid-way point is reached where the ship is turned around to constantly decelerate towards the Moon.
The Lost Fleet, written by John G. Hemry under the pen name Jack Campbell, is a military science fiction series which various ships of all sizes utilize constant acceleration propulsion to travel distances within star systems. Taking into account relativistic effects on space combat, communication, and timing, the ships work in various formations to maximize firepower while minimizing damage taken. The series also features the use of Jump Drives for travel between stars using gravitational jump points as well as the use of Hypernets, which utilizes quantum entanglement and probability wave principles for long distance travel between massively constructed gates.
Related Research Articles
Interstellar travel is the hypothetical travel of spacecraft between star systems. Due to the vast distances between the Solar System and nearby stars, interstellar travel is not practicable with current propulsion technologies.
Interplanetary spaceflight or interplanetary travel is the crewed or uncrewed travel between stars and planets, usually within a single planetary system. In practice, spaceflights of this type are confined to travel between the planets of the Solar System. Uncrewed space probes have flown to all the observed planets in the Solar System as well as to dwarf planets Pluto and Ceres, and several asteroids. Orbiters and landers return more information than fly-by missions. Crewed flights have landed on the Moon and have been planned, from time to time, for Mars, Venus and Mercury. While many scientists appreciate the knowledge value that uncrewed flights provide, the value of crewed missions is more controversial. Science fiction writers propose a number of benefits, including the mining of asteroids, access to solar power, and room for colonization in the event of an Earth catastrophe.
Spacecraft propulsion is any method used to accelerate spacecraft and artificial satellites. In-space propulsion exclusively deals with propulsion systems used in the vacuum of space and should not be confused with space launch or atmospheric entry.
Solar sails are a method of spacecraft propulsion using radiation pressure exerted by sunlight on large surfaces. A number of spaceflight missions to test solar propulsion and navigation have been proposed since the 1980s. The first spacecraft to make use of the technology was IKAROS, launched in 2010.
In physics, the twin paradox is a thought experiment in special relativity involving twins, one of whom takes a space voyage at relativistic speeds and returns home to find that the twin who remained on Earth has aged more. This result appears puzzling because each twin sees the other twin as moving, and so, as a consequence of an incorrect and naive application of time dilation and the principle of relativity, each should paradoxically find the other to have aged less. However, this scenario can be resolved within the standard framework of special relativity: the travelling twin's trajectory involves two different inertial frames, one for the outbound journey and one for the inbound journey. Another way to understand the paradox is to realize the travelling twin is undergoing acceleration, which makes them a non-inertial observer. In both views there is no symmetry between the spacetime paths of the twins. Therefore, the twin paradox is not actually a paradox in the sense of a logical contradiction.
A magnetic sail is a proposed method of spacecraft propulsion where an onboard magnetic field source interacts with a plasma wind to form an artificial magnetosphere that acts as a sail, transferring force from the wind to the spacecraft requiring little to no propellant as detailed for each proposed magnetic sail design in this article.
The Bussard ramjet is a theoretical method of spacecraft propulsion for interstellar travel. A fast moving spacecraft scoops up hydrogen from the interstellar medium using an enormous funnel-shaped magnetic field ; the hydrogen is compressed until thermonuclear fusion occurs, which provides thrust to counter the drag created by the funnel and energy to power the magnetic field. The Bussard ramjet can thus be seen as a ramjet variant of a fusion rocket.
Time dilation is the difference in elapsed time as measured by two clocks, either because of a relative velocity between them, or a difference in gravitational potential between their locations. When unspecified, "time dilation" usually refers to the effect due to velocity.
In spaceflight, an orbital maneuver is the use of propulsion systems to change the orbit of a spacecraft. For spacecraft far from Earth, an orbital maneuver is called a deep-space maneuver (DSM).
Artificial gravity is the creation of an inertial force that mimics the effects of a gravitational force, usually by rotation. Artificial gravity, or rotational gravity, is thus the appearance of a centrifugal force in a rotating frame of reference, as opposed to the force experienced in linear acceleration, which by the equivalence principle is indistinguishable from gravity. In a more general sense, "artificial gravity" may also refer to the effect of linear acceleration, e.g. by means of a rocket engine.
Intergalactic travel is the hypothetical travel between galaxies. Because the Milky Way and its closest neighbors are separated by millions of light-years, any such venture would also require millions of years based on current physics. Thus, intergalactic travel is impossible within the human lifetime. The technology required to travel between galaxies is far beyond humanity's present capabilities, and currently only the subject of speculation, hypothesis, and science fiction.
MagBeam is the name given to an ion propulsion system for space travel initially proposed by Professor Robert Winglee of the Earth and Space Sciences Department at the University of Washington for the October 2004 meeting of the NIAC. MagBeam is different from a traditional electrostatic ion thruster in several ways, the primary one being that instead of the fuel and propulsion system being part of the payload craft, they are instead located on a platform held in orbit. It has also been suggested that the technology could be used to reduce the amount of space debris in orbit around Earth.
Bell's spaceship paradox is a thought experiment in special relativity. It was first described by E. Dewan and M. Beran in 1959 but became more widely known after John Stewart Bell elaborated the idea further in 1976. A delicate thread hangs between two spaceships initially at rest in the inertial frame S. They start accelerating in the same direction simultaneously and equally, as measured in S, thus having the same velocity at all times as viewed from S. Therefore, they are all subject to the same Lorentz contraction, so the entire assembly seems to be equally contracted in the S frame with respect to the length at the start. At first sight, it might appear that the thread will not break during acceleration.
Tau Zero is a hard science fiction novel by American writer Poul Anderson. The novel was based upon the short story "To Outlive Eternity" appearing in Galaxy Science Fiction in 1967. It was first published in book form in 1970. The book is a quintessential example of "hard sci-fi", as its plot is dominated by futuristic technology grounded in real physics principles. It was nominated for the Hugo Award for Best Novel in 1971.
TAU was a proposed uncrewed interstellar probe that would go to a distance of one thousand astronomical units from the Earth and Sun by the NASA Jet Propulsion Laboratory in 1987 using tested technology. One scientific purpose would be to measure the distance to other stars via stellar parallax. Studies continued into 1990, working with a launch in the 2005–2010 timeframe.
In relativity, proper velocityw of an object relative to an observer is the ratio between observer-measured displacement vector and proper time τ elapsed on the clocks of the traveling object:
The Valkyrie is a theoretical spacecraft designed by Charles Pellegrino and Jim Powell. The Valkyrie is theoretically able to accelerate to 92% the speed of light and decelerate afterward, carrying a small human crew to another star system.
Field propulsion is the concept of spacecraft propulsion where no propellant is necessary but instead momentum of the spacecraft is changed by an interaction of the spacecraft with external force fields, such as gravitational and magnetic fields from stars and planets. Proposed drives that use field propulsion are often called a reactionless or propellantless drive.
Breakthrough Starshot is a research and engineering project by the Breakthrough Initiatives to develop a proof-of-concept fleet of light sail interstellar probes named Starchip, to be capable of making the journey to the Alpha Centauri star system 4.34 light-years away. It was founded in 2016 by Yuri Milner, Stephen Hawking, and Mark Zuckerberg.
The halo drive is a hypothetical method of spacecraft propulsion proposed by David Kipping. It is named for the ring of light that would result from its operation.
References
↑ Haloupek, William (2013). Calculations for Science Fiction Writers/Space Travel with Constant Acceleration - The Nonrelativistic Case. Smashwords Edition. ISBN9781301789078.
↑ Edwin F. Taylor & John Archibald Wheeler (1966 - first edition only) Spacetime Physics (W.H. Freeman, San Francisco) ISBN0-7167-0336-X, Chapter 1, Exercise 51, pp. 97–98: "Clock paradox III" (pdf).
↑ C. Lagoute and E. Davoust (1995) The interstellar traveler, Am. J. Phys.63:221–227
↑ Heinlein, Robert A. (1953). Sky Lift. Greenleaf Publishing Company. p.164.
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