With a growing number of Earth-like exoplanets
discovered in recent years, it is becoming increasingly frustrating
that we can’t visit them. After all, our knowledge of the planets in our
own solar system would be pretty limited if it weren’t for the space
probes we’d sent to explore them.
The problem is that even the nearest stars are a
very long way away, and enormous engineering efforts will be required to
reach them on timescales that are relevant to us. But with research in
areas such as nuclear fusion and nanotechnology advancing rapidly, we
may not be as far away from constructing small, fast interstellar space
probes as we think.
Scientific and societal case
There’s a lot at stake. If we ever found evidence
suggesting that life might exist on a planet orbiting a nearby star, we
would most likely need to go there to get definitive proof and learn
more about its underlying biochemistry and evolutionary history. This
would require transporting sophisticated scientific instruments across
But there are other reasons, too, such as the cultural rewards we would get from the unprecedented expansion of human experience.
And should it turn out that life is rare in our galaxy, it would offer
opportunities for us humans to colonize other worlds. This would allow
us to spread and diversify through the cosmos, greatly increasing the
long-term survival chances of Homo sapiens and our evolutionary descendants.
Five spacecrafts—Pioneers 10 and 11, Voyagers 1 and 2, and New Horizons—are
currently leaving the solar system for interstellar space. However,
they will cease to function many millennia before they approach another
star, should they ever get to one at all.
Clearly, if starships are to ever become a
practical reality, they will need to be based on far more energetic
propulsion technologies than the chemical rockets and gravitational
sling shots past giant planets that we use currently.
To reach a nearby star on a timescale of decades
rather than millennia, a spacecraft would have to travel at a
significant fraction—ideally about 10%—of the speed of light (the
Voyager probes are traveling at about 0.005%). Such speeds are certainly
possible in principle—and we wouldn’t have to invent new physics such
as “warp drives,” a hypothetical propulsion technology to travel faster than light, or “wormholes” in space, as portrayed in the movie Interstellar.
Top rocket-design contenders
Over the years, scientists have worked out a
number of propulsion designs that might be able to accelerate space
vehicles to these velocities (I outline several in this journal article).
While many of these designs would be difficult to construct today, as
nanotechnology progresses and scientific payloads can be made ever
smaller and lighter, the energies required to accelerate them to the
required velocities will decrease.
The most well thought through interstellar
propulsion concept is the nuclear rocket, which would use the energy
released when fusing together or splitting up atomic nuclei for
Spacecraft using “light-sails” pushed by lasers
based in the solar system are also a possibility. However, for
scientifically useful payloads this would probably require lasers
concentrating more power than the current electrical generating capacity
of the entire world. We would probably need to construct vast solar
arrays in space to gather the necessary energy from the sun to power
Another proposed design is an antimatter rocket.
Every sub-atomic particle has an antimatter companion that is virtually
identical to itself, but with the opposite charge. When a particle and
its antiparticle meet, they annihilate each other while releasing a huge
amount of energy that could be used for propulsion. However, we
currently cannot produce and store enough antimatter for this to work.
Interstellar ramjets, fusion rockets using
enormous electromagnetic fields as a ram scoop to collect and compress
interstellar hydrogen for a fusion drive are another possibility, but
these would probably be yet harder to construct.
The most well developed proposal for rapid interstellar travel is the nuclear-fusion rocket concept described in the Project Daedalus study,
conducted by the British Interplanetary Society in the late 1970s. This
rocket would be capable of accelerating a 450 tonne (496 ton) payload
to about 12% of the speed of light (which would get to the nearest star
in about 36 years). The concept is currently being revisited and updated
by the ongoing Project Icarus study.
Unlike Daedalus, Icarus will be designed to slow down at its
destination, permitting scientific instruments to make detailed
measurements of the target star and planets.
All current starship concepts are designed to be
built in space. They would be too large and potentially dangerous to
launch from Earth. What’s more, to get enough energy to propel them we
would need to learn to collect and manage large amounts of sunlight or
mine rare nuclear isotopes for nuclear fusion from other planets. This
means that interstellar space travel is only likely to become practical
once humanity has become a spacefaring species.
The road to the stars therefore begins here—by
gradually building up our capabilities. We need to progressively move on
from the International Space Station to building outposts and colonies
on the Moon and Mars (as already envisaged in the Global Exploration Roadmap).
We then need to begin mining asteroids for raw materials. Then, perhaps
sometime in the middle of the 22nd century, we may be prepared for the
great leap across interstellar space, and reap the scientific and
cultural rewards that will result.
This post originally appeared at The Conversation. Follow @ConversationUS on Twitter.