I've been reading a few books about space travel and humans living off Earth recently, the latest two being:

1. Beyond - Our Future In Space by Chris Impey,
2. Blue Mars by Kim Stanley Robinson.

And these have got me thinking about our near term prospects for travelling both within the solar system and beyond. This is a fascinating area, and one where experts know so much more than me, but I just wanted to pull together some of my thoughts on this from reading these (and other) excellent books and resources.

The Big Picture

Humans love to explore, Chris Impey's book has a really good chapter on this where he discusses the human journey from sub-saharan Africa. Where I didn't feel so comfortable was on some of his arguments around the biological determinism of some people's risk aversion vs others. He may have painted a sound picture of the science but I'd like to investigate more - I'm always very sceptical of claims like this. Anyway, the point he makes is that we, as humans, are always looking for the next frontier and that naturally this now means we are looking to explore the planets and stars.

In the medium to long term, it's also clear that being a multi-planetary species will increase our odds of long-term survival, since we (to borrow a software analogy) won't have a 'single point of failure'. We'll be more robust as a species to any extinction level events since these will almost always only occur at the planetary level.

In the nearer term, space also means big business and potentially a huge resource source for future endeavours. In Beyond, Impey covers this point a few times, highlighting ideas around mining asteroids for resources etc. What Impey is excellent at doing in this book is always caveating these ideas with reasonable calculations of required energy and financial expenditure. It's so easy to assume "we'll go mine the moon for Helium" without realising that it probably isn't worth the huge cost required.

Stanley Robinson's book's are also excellent on this point. There is no waving of a technological magic wand that makes colonisation (and later terraforming) of Mars look easy. Everything is painstakingly shown to be a series of small wins, each with (well explained) incremental improvements in technology and reasonable expenditures of time and energy. The only time where there is a bit of a 'hand wave' explanation for a piece of technology is in the 'gerontological treatments' developed in the first of the Mars trilogy books (Red Mars). These treatments allow for the large extension of human lifetime and create a whole host of interesting psychological, political and even economic considerations both on Mars and Earth, but as mentioned are not exactly 'near term' technology by the sounds of things. Then again, rapid progress in medicine and the current thrust for innovation in gerontology may prove me wrong in the near future, but I am definitely no expert in this area (this interview with Laura Deming in the FT was really interesting).

 

Big Rockets (Some Assembly Required)

I am always fascinated by questions around visiting and colonising other planets because to me, as alluded to above, it just seems so damn hard. Impey highlights this fact with some interesting calculations around the required energy and fuel expenditure needed to get to the nearest stars.

For example, the first calculation is one done by NASA [1], apparently to get to a school bus size payload to Alpha Centauri in 900 years (Alpha Centauri is 4.37 light years from the Sun, so this is about equivalent to travelling at 0.48% the speed of light) would require more chemical rocket fuel than there is mass in the universe .... So no chance there really.

The next rung up in complexity / speculative tech becomes fission or fusion based pulse propulsion engines,  but apparently even a fusion engine would still require 10¹¹ kg of fuel, or roughly "1000 supertankers worth", to get to our nearest stellar neighbour in under a millennium. So again pretty damn hard.

Even when Impey considers matter-antimatter rockets (loved by science fiction aficianados), and assuming 100% conversion to kinetic energy of the fuel, we're still talking about "the energy consumption of the entire United States for six months" to get a 2k ton space shuttle to Alpha Centauri in approx. fifty years .... As I said, travelling to another star is really damn hard.

Ok, so what happens if we rein in our ambition a bit and consider planetary rather than interstellar travel. This is where the Mars Trilogy comes in. When the 'first hundred' explorers set off on the Ares spacecraft, they are aiming to reach Mars using a Type II Hohmann transfer orbit in about 300 days. This is not far off what's typically deemed the time it takes to get to Mars using current technology, given a launch during the appropriate window, which can give you a travel time from between 9 months to 1.5 years [2]. So this means that it's definitely feasible to get to the Red Planet in a reasonable timescale. It still won't be easy due to having to deal with the psychological and physical challenges of travelling for that long in space but lets leave that to the side for the moment.

If we stick with that potential technology stack for the moment, how long will it take to explore the rest of the solar system?

A simple way to do this is to simply use the travel times for different probes we have sent across the solar system as a good base estimate. Missions with personnel may take different times for a variety of reasons but let's assume we can use these probe travel times as a useful approximation. This article lists a good number of bodies we've sent probes to, but here are some highlights:

1. Venus - 15 months (Magellan mission). This is interesting as Venus is closer to us than Mars in terms of distance from the Sun, but this mission must have taken this long due to the choice of transfer orbit and departure window.
2. Jupiter - 6 years (Galileo mission).
3. Neptune - 12 years (Voyager missions). The voyager missions were designed to study the 4 gas giant planets (Voyager 1 Jupiter and Saturn, Voyager 2 Uranus and Neptune). Calculations in 1965 revealed that there would be a once in 176 year alignment of the planets that meant a single spacecraft could technically explore all four of the gas giants. Voyager 2 was launched first and then Voyager 1 launched a few months later in 1977. Voyager 2 was so named because it would reach Jupiter and Saturn after Voyager 1. The full timelines and a lot more information about the Voyager missions are given here in a pretty amazing set of resources from NASA.

It should be clear from these numbers than using our current technology, or something close to it, to explore the solar system will still be, yup you guessed it, pretty damn hard!

One ray of light coming from the Mars Trilogy books is (spoiler alert!), when late in Blue Mars 'fusion pulse propulsion' technology is developed (similar to that mentioned above in the discussion of Beyond). As I stated above, this still makes getting to nearby stars pretty hard but, as explored in the booked, it would act to drastically shrink the subjective size of the solar system. In the book, travel time between the planets goes from months and years to weeks and days, and drastically reduces or removes the need for complex transfer orbits. This acts to accelerate the development and intensify the political manoeuvering of the different colonies throughout the solar system, leading to a period of rapid expansion of the human races civilisational footprint known as the Accelerando. Now, don't get me wrong, the prospect of creating a pulsed fusion propulsion system is not one we can currently entertain for the near future, since we haven't even been able to generate sustainable net energy gain in a fusion reactor. This doesn't mean it is not attainable in the next few decades however, and if it does become feasible, it is clear that the implications for solar system travel are potentially more transformative than for the prospective of interstellar travel.

Conclusion

In summary, I'd recommend reading 'Beyond' by Chris Impey and the Mars Trilogy by Kim Stanley Robinson. Even if they tell you that space travel on the interplanetary and interstellar scales is pretty damn hard, they are still great reads packed with interesting science and fun speculation about what's possible in humanity's near future.

Notes:

[1] I've linked to the page quoted as a source in 'Beyond', but it looks like the page no longer has the original calculation (it seems to have been archived).

[2] More information about transfer times to Mars in this article: https://www.space.com/24701-how-long-does-it-take-to-get-to-mars.html