The Challenge of Interstellar Flight

The Challenge of Interstellar Flight

Kelvin F.Long

In the early 1970s the US Space Agency NASA launched the Pioneer 10 and 11 probes. These were 258 kg spacecraft with a science package each of less than 30 kg. Both probes were sent to the outer solar system and have surpassed the 100 AU limit. A unique feature of the Pioneer probes was a 120 gram 349 square cm bold plaque which was a message to any intelligent life that may find our distant ambassadors. The plaque featured a depiction of a male and female human as well as a chart depicting the location of our solar system, using pulsar signatures as a reference. It also depicted symbols to illustrate our knowledge of the atom. In the late 1970s NASA sent out the Voyager 1 and 2 spacecraft, on a grand tour of our solar system, particularly the gas giants Jupiter and Saturn. Each spacecraft was around 722 kg each and both are nearing the 100 AU mark. The Voyager probes carry a golden record of sounds and images carrying the message: “This is a present from a small, distant world, a token of our sounds, our science, our images, our music, our thoughts and our feelings. We are attempting to survive our time so we may live into yours”. This is profound and heartfelt words. One wonders what another intelligent species coming across these distant messengers would make of these brave attempts into the void. Would they look back upon the solar system from whence they came and see a glorious solar system wide economy in the making as we go further still, or will they look upon barren worlds, once populated by a promising but short lived species?

The Pioneer and Voyager probes travel at a speed of something like 17 km/s or between 2-3 AU/year. This is a snails pace in a vast galaxy and it would take them tens of thousands of years to reach the nearest stars. That said, they do effectively represent the first interstellar precursor missions to travel to the boundaries of our solar system. The Pioneer program cost around $350 million for both spacecraft, which is equivalent to around $1,350 million/ton or $6.1 million/AU. The Voyager spacecraft cost around $865 million for both spacecraft which is equivalent to around $1,198 million/ton or $7.01 million/AU. Considering all the amazing things these mission achieved, this was money well spent. NASA and other space agencies such as the ESA should be sending more such missions out into the depths of space. But where should they go and how?

Others have discussed the possibility of an interstellar precursor mission out to 200 AU. This would seem a logical next step, past the Kuiper belt and perhaps to explore some of those distant dwarf planets. Indeed, the New Horizons mission is currently on its way to the dwarf planet Pluto. But we should go further still. We should then attempt a mission to between 500 – 1,000 AU. This is a good distance from Earth, gets us near to the Oort Cloud to allow much valuable science and more importantly we will push the propulsion technology to enable more ambitious missions into the Oort Cloud at 2500 AU and beyond. We already have various propulsion technologies capable of performing these missions although they are at various stages of technological readiness. With some moderate investment and an organised program they can be matured. We are talking about Nuclear Thermal Rockets, Solar Sails, Ion Drives, Plasma Engines and Nuclear-Electric Engines. When you have this many options on the table there really is no excuse not to keep trying. The universe beckons us to come and join it. This requires courage and a unified sense of purpose.

Interstellar Exploration Possibilities

If we can send a probe to another star system, what might we learn? Well, for a start, learning about how another solar system formed gives us an important comparison for our own solar system evolution. Having many different planets to study will also tell us about the possibilities of life. One of the issues that astronomers have to deal with is the accuracy of parallax measurements. Studying a star close up will away give us a reliable normalisation for all other measurements of distant stars in the universe, even at galactic distances.

The most tantalising possibility is the discovery of not just life but intelligent life. Imagine if we could make contact with a species from another world that had its own culture, music, literature, spiritual values, science. The plurality would enrich our own and add to our joint knowledge of the universe. We may find that their values are so alien to us to be abhorrent. How we deal with that will be a test of our own culture and levels of tolerance. Indeed, one could argue that the human race is not yet ready for such an encounter. Just look at how we treat each other. The world has massive poverty and inequality, resources are not well distributed, people are victimized because of their racial or sexual types. The scientific and religious communities have become significantly diverged when in fact they are both important parts of humanity. Global conflicts continue and the superpower states of the developed world cannot find it in themselves to work together but instead prefer to see each other as competitors. But, we should be optimistic and hope that eventually their interests will converge, in time.

In the wonderful book ‘Pal Blue Dot’ Carl Sagan said: “By the time we’re ready to settle even the nearest other planetary systems, we will have changed. The simple passage of so many generations will have changed us. The different circumstances we will be living under will have changed us. Prostheses and genetic engineering will have changed us. Necessity will have changed us. We’re an adaptable species. It will not be we who reach Alpha Centauri and the other nearby stars. It will be a species very like us, but with more of our strengths and fewer of our weaknesses, a species returned to circumstances more like those for which it was originally evolved, more confident, farseeing, capable, and prudent – the sorts of beings we would want to represent us in a Universe that, for all we know, is filled with species much older, much more powerful, and very different”.

In terms of humanity moving outwards and colonisation the solar system and beyond, it is generally believed this would occur in a set of phases as follows:

·       Reach earth orbit.

·       Colonise the inner solar system.

·       Colonise the outer solar system.

·       Launch interstellar precursor missions.

·       Colonise exosolar space (e.g. Oort cloud).

·       Launch (unmanned) interstellar missions.

·       Launch (manned) interstellar missions.

Before this programme of missions can begin and a specific propulsion scheme chosen however, significant justification must be made. This is in several areas:

·       Physics – does it work in theory?

·       Engineering – is it practical?

·       Science – what is the science drivers for the mission?

·       Infrastructure – is the support/launch/assembly structure in place?

·       Politics – is a politically directed programme authorized?

·       Business – is a commercial programme support?

·       Economics – is the funding in place?

·       Sociological – does the proposed mission have societal support?

·       Motivations – is their compelling reasons for making the trip?

·       Biological – have the life support systems been demonstrated?

The Technical Issues

When addressing the interstellar travel problem we can look at it from the standpoint of velocity, distance and time. These are related nicely by a simple equation:

Time = Distance / Velocity

In order to reduce the mission duration of a probe to another star there is only two options: (1) high velocity and (2) shorten the distance. Let’s examine both of these briefly.

(1) High Velocity. In order to do this one must either accelerate rapidly or accelerate slowly but over a long period. Both require energy, kinetic energy. How does one induce high kinetic energy, by the reaction of materials or substances which generate a large energy release in a controlled way. This is the rocket propellant or fuel and is discussed below. But we note that some materials will release more than others and so to get more “bang for your buck” it is a design aim to choose more energetic materials. But the most energetic materials are difficult to react.

(2) This could mean go for a mission target that is closer. However, this is not what we really are talking about but instead breakthroughs in physics which enable us to effectively shorten the distance between two points. We are instead referring to the bending of space-time in either a worm hole or a warp drive type effect. This is the stuff of scientific conjecture but it is a subject worthy of study.

The nearest star system is Centauri A/B and its companion Proxima Centauri, located 13,000 AU from the binary pair. This star system is located 4.3 light years away which is around 270,000 AU. In order to travel this distance in around a century the vehicle would need to hit a cruise speed of around 2700 AU/year. No small challenge when we consider that historical interstellar precursor missions like the Pioneer and Voyager probes travelled at a mere 2-3 AU/year. And in order to complete this journey in one century it would likely require engines with an exhaust velocity of order 8,000 – 10,000 km/s. To put this nearest star system in perspective, if the Moon is 1 cm away from the Earth, then the Sun is 4 m away and Centauri A is around 1,000 km away.

In terms of making the journey, the (unmanned) modes of travel can be split up into the following divisions:

LOW SPEED PROBE: <0.01c (slow mission taking centuries)

MODERATE SPEED PROBE: 0.01c<0.05 (faster mission taking of order a century)

HIGH SPEED PROBE: 0.05<c<0.15 (even faster mission taking decades)

VERY HIGH SPEED PROBE: 0.15<c<0.5 (rapid acceleration mission taking of order a decade)

ULTRA HIGH SPEED PROBE: 0.5<c<0.9c (relativistic ship taking years)

FASTER THAN LIGHT PROBE: >c (superluminal ship taking days, weeks or months)

Similarly, looking at colonisation vessels, the (manned) modes of travel can be split up into the following divisions:

·       SLOW BOAT: 0.01<c<0.1 (Population = 100s at start and 1,000s at end)

·       SLOW SHIP: 0.005<c<0.01 (Population = 1,000s at start and 10,000s at end)

·       WORLD SHIP: <0.005c (Population = 10,000s at start and 100,000s at end)


The chemically based rockets like Saturn V which took men to the Moon, or the Space Transportation System (Space Shuttle) are inspiring works of engineering complexity and adequate for accessing Earth or Lunar space. But if you want to send people much further then more energetic fuels are required. The laws of physics limit the energy release of a chemical reaction to around 10 eV, where 1 electron Volt is equal to 1.609 times ten to the power of minus 19 Joules of energy. This means that the maximum theoretical performance of chemical based fuels is around 500 km/s. Chemical propulsion and therefore conventional rocket technology will not help us achieve star travel, except in perhaps the construction of the starship itself. So, we must look for alternative fuels.The list below compares different fuel types in terms of their energy release. We can see from this that thankfully we do have other options. These form the basis of most interstellar propulsion systems.

Chemical: ~13 MJ/kg

Nuclear Fission: ~82 million MJ/kg (This includes fission/fusion hybrids and antimatter catalysed fusion)

Nuclear Fusion: ~347 million MJ/kg Pure Antimatter: ~90 billion MJ/kg

Vacuum/Dark Energy: amount yet unknown.

Matter/antimatter: amount yet unknown.

There are also more exotic options for crossing space such as by using worm holes and warp drives. The subject of worm hole theory was first initiated in a paper by Albert Einstein titled “The Particle Problem in the General Theory of Relativity”, published in Physical Review in 1935. It has since been progressed by others such as Kip Thorne and even Carl Sagan in his novel ‘Contact’. The subject of warp drive theory was discussed in the science fiction literature for some time (e.g. Star Trek) but first became an academic field of study in a 1994 paper published in Classical & Quantum Gravity by Miquel Alcubiere titled “The Warp Drive: Hyper-fast Travel Within General Relativity”. Currently, these methods of crossing space remain scientific conjecture but if they ever advance to full scientific theorems, validated by experiments, this could change the way we interact with the universe and its inhabitants. It’s also a lot of fun just to study the mathematics and physics of these concepts, which present causality issues for would be time travellers. No doubt, Dr Who knows all about the physics at work here.


On this page we have discussed some of the background material relevant to Interstellar Studies. If you are new to this field then we hope this has provided you with some background material on where to begin your studies. There is a lot to learn, from understanding the technical problems, to the various solutions, propulsion or other technologies, to reading the vast literature that exists. And it’s not easy. To understand the field properly requires a good grounding in physics, mathematics and engineering. Applying these subjects to Interstellar Studies is what we may refer to as the application of ‘extreme aerospace engineering’. Jump into this playground of knowledge and see what you can come up with. You may just invent a new Starship technology.

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