The difficulties of interstellar space travel
The main challenge facing interstellar travel is the vast distances that
have to be covered. This means that a very great speed and/or a very
long travel time is needed. The time it takes with most realistic
propulsion methods would be from
decades to
millennia. Hence an interstellar ship would be much more severely exposed to the hazards found in
interplanetary travel, including
vacuum,
radiation,
weightlessness, and
micrometeoroids. The long travel times make it difficult to design manned missions. The fundamental limits of
space-time present another challenge.
Furthermore, it is difficult to foresee interstellar trips being justified for conventional economic reasons.
Required energy
A significant factor contributing to the difficulty is the energy
which must be supplied to obtain a reasonable travel time. A lower bound
for the required energy is the
kinetic energy K = ½ mv
2 where m is the final mass. If
deceleration
on arrival is desired and cannot be achieved by any means other than
the engines of the ship, then the required energy at least doubles,
because the energy needed to halt the ship equals the energy needed to
accelerate it to travel speed.
The velocity for a manned round trip of a few decades to even the
nearest star is thousands of times greater than those of present space
vehicles. This means that due to the square law,
millions of times as much energy is required. Accelerating one ton to
one-tenth of the speed of light requires at least 450 PJ or 4.5
×10
17 J or 125 billion kWh, not accounting for losses. This energy has to be carried along,
as
solar panels do not work far from the Sun and other stars.
There is some belief that the magnitude of this energy may make
interstellar travel impossible. It has been reported that at the 2008
Joint Propulsion Conference, where future space propulsion challenges
were discussed and debated, a conclusion was reached that it was
improbable that humans would ever explore beyond the Solar System.
Brice N. Cassenti, an associate professor with the Department of
Engineering and Science at Rensselaer Polytechnic Institute, stated “At
least 100 times the total energy output of the entire world [in a given
year] would be required for the voyage (to
Alpha Centauri)”
Interstellar medium
A major issue with traveling at extremely high speeds is that
interstellar dust and gas
may cause considerable damage to the craft, due to the high relative
speeds and large kinetic energies involved. Various shielding methods to
mitigate this problem have been proposed.
Larger objects (such as macroscopic dust grains) are far less common,
but would be much more destructive. The risks of impacting such objects,
and methods of mitigating these risks, have not been adequately
assessed.
Travel time
It can be argued that an interstellar mission which cannot be
completed within 50 years should not be started at all. Instead,
assuming that a civilization is still on an increasing curve of
propulsion system velocity, not yet having reached the limit, the
resources should be invested in designing a better propulsion system.
This is because a slow spacecraft would probably be passed by another
mission sent later with more advanced propulsion.
On the other hand, Andrew Kennedy has shown that if one calculates the
journey time to a given destination as the rate of travel speed derived
from growth (even exponential growth) increases, there is a clear
minimum in the total time to that destination from now.
Voyages undertaken before the minimum will be overtaken by those who
leave at the minimum, while those who leave after the minimum will never
overtake those who left at the minimum.
One argument against the stance of delaying a start until reaching
fast propulsion system velocity is that the various other non-technical
problems that are specific to long-distance travel at considerably
higher speed (such as interstellar particle impact, possible dramatic
shortening of average human life span during extended space residence,
etc.) may remain obstacles that take much longer time to resolve than
the propulsion issue alone, assuming that they can even be solved
eventually at all. A case can therefore be made for starting a mission
without delay, based on the concept of an achievable and dedicated but
relatively slow interstellar mission using the current technological
state-of-the-art and at relatively low cost, rather than banking on
being able to solve all problems associated with a faster mission
without having a reliable time frame for achievability of such.
Intergalactic travel
involves distances about a million-fold greater than interstellar
distances, making it radically more difficult than even interstellar
travel.
Interstellar distances
Astronomical distances are often measured in the time it would take a beam of
light to travel between two points. Light in a vacuum travels approximately 300,000 kilometers per second or 186,000 miles per second.
The distance from
Earth to the Moon is 1.3
light-seconds.
With current spacecraft propulsion technologies, a craft can cover the
distance from the Earth to the Moon in around eight hours (
New Horizons).
That means light travels approximately thirty thousand times faster
than current spacecraft propulsion technologies. The distance from Earth
to other planets in the solar system ranges from three light-minutes to
about four light-hours. Depending on the planet and its alignment to
Earth, for a typical unmanned spacecraft these trips will take from a
few months to a little over a decade.
The nearest known star to the
Sun is
Proxima Centauri, which is 4.23 light-years away. However, there may be undiscovered
brown dwarf systems that are closer.
The fastest outward-bound spacecraft yet sent,
Voyager 1,
has covered 1/600th of a light-year in 30 years and is currently moving
at 1/18,000th the speed of light. At this rate, a journey to Proxima
Centauri would take 72,000 years. Of course, this mission was not
specifically intended to travel fast to the stars, and current
technology could do much better. The travel time could be reduced to a
few millennia using
lightsails, or to a century or less using
nuclear pulse propulsion.
A better understanding of the vastness of the interstellar distance to
one of the closest stars to the sun, Alpha Centauri A (a Sun-like star),
can be obtained by scaling down the
Earth-Sun distance
(~150,000,000 km) to one meter. On this scale the distance to Alpha
Centauri A would still be 271 kilometers or about 169 miles.
However, more speculative approaches to interstellar travel offer the possibility of circumventing these difficulties.
Special relativity offers the possibility of shortening the travel time: if a starship with sufficiently advanced engines could reach
velocities approaching the speed of light, relativistic
time dilation
would make the voyage much shorter for the traveler. However, it would
still take many years of elapsed time as viewed by the people remaining
on Earth, and upon returning to Earth, the travelers would find that far
more time had elapsed on Earth than had for them. (For more on this
effect, see
twin paradox.)
General relativity
offers the theoretical possibility that faster-than-light travel may be
possible without violating fundamental laws of physics, for example,
through
wormholes, although it is still debated whether this is possible, in part, because of
causality concerns. Proposed mechanisms for
faster-than-light travel within the theory of General Relativity require the existence of
exotic matter.
Communications
The
round-trip delay time
is the minimum time between an observation by the probe and the moment
the probe can receive instructions from Earth reacting to the
observation. Given that information can travel no faster than the
speed of light, this is for the
Voyager 1
about 32 hours, near Proxima Centauri it would be 8 years. Faster
reaction would have to be programmed to be carried out automatically. Of
course, in the case of a manned flight the crew can respond immediately
to their observations. However, the round-trip delay time makes them
not only extremely distant from but, in terms of
communication,
also extremely isolated from Earth (analogous to how past long distance
explorers were similarly isolated before the invention of the
electrical telegraph).
Interstellar communication is still problematic — even if a probe
could reach the nearest star, its ability to communicate back to Earth
would be difficult given the extreme distance.
Prime targets for interstellar travel
There are 59 known stellar systems within 20 light years from the
Sun, containing 81 visible stars. The following could be considered
prime targets for interstellar missions:
| Stellar system |
Distance (ly) |
Remarks |
| Alpha Centauri |
4.3 |
Closest system. Three stars (G2, K1, M5). Component A similar to our sun (a G2 star). Alpha Centauri B has one confirmed planet. |
| Barnard's Star |
6.0 |
Small, low luminosity M5 red dwarf. Next closest to Solar System. |
| Sirius |
8.7 |
Large, very bright A1 star with a white dwarf companion. |
| Epsilon Eridani |
10.8 |
Single K2 star slightly smaller and colder than the Sun. Has two asteroid belts, might have a giant and one much smaller planet, and may possess a solar system type planetary system. |
| Tau Ceti |
11.8 |
Single G8 star similar to the Sun. High probability of possessing a solar system type planetary system. |
| Gliese 581 |
20.3 |
Multiple planet system. The unconfirmed exoplanet Gliese 581 g and the confirmed exoplanet Gliese 581 d are in the star's habitable zone. |
Existing and near-term astronomical technology is capable of finding
planetary systems around these objects, increasing their potential for
exploration.
Manned missions
The mass of any craft capable of carrying humans would inevitably be substantially larger than that necessary for an unmanned
interstellar probe. For instance, the first space probe,
Sputnik 1, had a payload of 83.6 kg, while spacecraft to carry a living passenger (
Laika the dog),
Sputnik 2,
had a payload six times that at 508.3 kg. This underestimates the
difference in the case of interstellar missions, given the vastly
greater travel times involved and the resulting necessity of a
closed-cycle life support system.
As technology continues to advance, combined with the aggregate risks
and support requirements of manned interstellar travel, the first
interstellar missions are unlikely to carry earthly life forms.
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