What is the carbon footprint of space travel?

What is the carbon footprint of space travel?

Who couldn’t have been wowed by the recent incredible crewed Space X launch on the 30th May? Two astronauts successfully left the earth’s atmosphere to dock with the international space station on a semi re-usable rocket. This was clearly an incredible achievement and returned the USA to the forefront of space travel. In addition this significant step moves us closer to manned flights to mars which I believe will very likely happen within the next decade. I love the idea of NASA outsourcing the 'easy stuff' so that they can focus on the bigger prize of Mars!

Watching the launch got me thinking about the carbon footprint and environmental impact of shifting the Dragon capsule 400 kilometers into space to meet the International Space Station (ISS). I was surprised how difficult it is to answer the carbon footprint question – and more worryingly how dubious the maths was where people have had a go - so I've pulled together various numbers from across the internet to try and get a feel for the number - my calculations are below so feel free to challenge the underlying logic.

Calculating the carbon footprint

The Falcon 9 rocket is powered by 9 Merlin engines. The Merlin engines generate about 1.7 million pounds of thrust at full power, consuming a mix of super-chilled kerosene and cryogenic liquid oxygen propellants. Around 155 tonnes of the cooled liquid Kerosene is consumed during a launch along with 362 tonnes of liquid oxygen. That’s a lot of fuel sat right underneath our two astronauts. Not only is high grade aviation fuel being burned but also a lot of oxygen is being used up in the combustion process. So whats the carbon footprint of the launch?

• Kerosene has a carbon intensity 3Kg carbon per Kg of Kerosene [I]. So, the carbon generated from the kerosene used in the launch is 465 tonnes.

• The Oxygen used is produced from a cryogenic process which uses electricity to chill air to release the oxygen. Assuming the storage and transportation is relatively efficient and grid electricity is used to produce the oxygen then the carbon emitted in producing the oxygen is a further 650 tonnes (see calculation below).

Therefore the total carbon footprint from the Kerosene and Oxygen is around 1115 Tones. The annual carbon footprint of '278' average world citizens. In all honesty I’d have expected it to be far greater.

There is an opportunity for the oxygen to be made using zero carbon electricity - but given no-one is shouting about it I doubt this is happening (I'd happily be corrected!).

Comparing that to conventional flight; a Boeing 747 burns about 4 liters of fuel a second; flying from London to New York in total uses around 70 tonnes with a carbon footprint of around 210 Tonnes of Carbon each way. Comparing that to our launch then we are using around the equivalent of 5 return transatlantic flights.

Another measure is emissions per passenger/per km traveled – which for the recent trip to the ISS of just two onslaughts is about 700kg/km (I’ve assumed 400km each way with no fuel burned on the return). That compares to 0.133kg/km for domestic flight or 177kg/km for car travel [ii]. This improves significantly one the Dragon capsule has a full compliment of 7 astronauts.

Its probably silly to compare space travel with rail and air - however it does show just how much energy comparatively is being used and consequently how much carbon emitted.

Other considerations

I think the above may be a little higher as you add losses in transportation and production of both fuels. I've struggled to find data on this however they are likely to increase footprint further. A huge step in decarbonising space travel would be to generate and transport all that oxygen using green zero carbon electricity!

There are some other interesting impacts of space travel regarding where the emissions happen for example soot in the upper atmosphere and depletion of the ozone layer [v]. I haven't gone into these here as they are super complex and there doesn't seem to be too much clarity in the science on the impact. They are however becoming increasingly important as the number of rocket launches increases.

Conclusion

Annually globally there are around 100 space launches a year – however with space tourism and increasing numbers of satellite launches this is pegged to rise to well over 1000 [iii]. If we take our carbon number for our launch then we get to a carbon footprint for space travel of around 3.1 million tonnes in only a few years time - along with damage to the ozone layer along with soot in the upper atmosphere. As space travel expands and Mars looks increasingly possible it will become more important to manage the footprint here on earth of our aspirations to explore our solar system.

If you want to see real power turn the volume up and click play on this amazing video of a merlin engine being tested! 

and if you want to see it again - here is the launch!

Some Maths!

There isn't a lot of data freely available on the carbon intensity of liquid oxygen. So I've taken an example 300kW[iv] oxygen plant using 300kw of electricity to produce 2 tonnes in a day. To produce one tonne of liquid oxygen you need about 3.6MWHr of electricity. To produce the 362 Tonnes of liquid oxygen needed for the launch you must therefore need 1300MWHr of electricity. Average grid carbon intensity in the US is 0.5 tonnes of Carbon Dioxide per MWHr. Therefore the carbon generated in producing the oxygen for launch is about 650 tonnes if they use 'standard' US grid electricity. Where the oxygen is made really matters here - if its made in the sunbelt of California it would be far less than if its using electricity from coal. Geography really matters when it comes to carbon intensity! 

[i] www.engineeringtoolbox.com/co2-emission-fuels-d_1085.html

[ii] www.bbc.co.uk/news/science-environment-49349566

[iii] www.space.com/elon-musk-starship-spacex-flights-mars-colony.html

[iv] advancedtech.airliquide.com/liquid-oxygen-lox-plant

[v] www.sciencefocus.com/space/are-space-launches-bad-for-the-environment/