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The surprising carbon footprint of space travel.

image credit: Image courtesy of Unsplash

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.

Emission from different modes of transport.

Other considerations

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! 

Rocket Launch

 

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

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

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

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

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

John Armstrong's picture

Thank John for the Post!

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Discussions

Matt Chester's picture
Matt Chester on Jun 17, 2020 4:32 pm GMT

Really compelling topic and well explained, John-- thanks for this! Brought back memories of when I wrote an article looking into the relative fuel efficiency of that Tesla Roadster that Elon Musk and SpaceX sent into space :)

John Armstrong's picture
John Armstrong on Jun 17, 2020 4:49 pm GMT

Thanks Matt. Its an interesting topic.... although the carbon is dwarfed by transport in general there is an opportunity to be greener simply by generating the oxygen from renewable sources.

I suspect some rocket scientists out there may be able to improve on my maths! 

John Armstrong's picture
John Armstrong on Jun 17, 2020 4:52 pm GMT

Thats a great article! Do they know where the roadster is now? 

Bob Meinetz's picture
Bob Meinetz on Jun 17, 2020 5:44 pm GMT

John, they know precisely where the roadster is.

https://www.whereisroadster.com/

Now you do too!

Matt Chester's picture
Matt Chester on Jun 17, 2020 9:19 pm GMT

Precisely the resource I used in my calculations-- you'll actually see that the constantly updated measurements actually link back to my analysis :)

It has achieved a fuel economy of 9,265.6 miles per gallon (3,939.2 km/liter, 0.02539 liters/100 km), assuming 126,000 gallons of fuel.

If all EVs got that kind of mileage, we'd see a heck of a lot more Teslas, Leafs, and Bolts on the highways!

Bob Meinetz's picture
Bob Meinetz on Jun 18, 2020 4:46 am GMT

"If the battery was still working, Starman has listened to Space Oddity 234,372 times since he launched in one ear, and to Is there Life On Mars? 315,807 times in his other ear."

Except there's no sound in interplanetary space...poor Starman might as well be deaf.

The only film that gets it right is 2001: a Space Odyssey. A massive explosion at the end is made all the more dramatic by a soundtrack of stone cold silence.

John Armstrong's picture
John Armstrong on Jun 18, 2020 9:42 am GMT

Thats brilliant. I hadn't seen that before...

Thanks!

Bob Meinetz's picture
Bob Meinetz on Jun 17, 2020 5:41 pm GMT

"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."

John, a few points:

  • Kerosene has an emissions intensity of 3kgCO2e/kg, so more correctly: "the CO2e generated from the combustion of kerosene and oxygen during the launch was 465 tonnes."
  • You're not including the carbon footprint of the production of kerosene from extraction / refinining / transportation ≈ +32% of combustion emissions, or 149 tonnes CO2e.
  • Total CO2e emission footprint of the launch is 1115 + 149 = 1,264 tonnes.

"The annual carbon footprint of '278' average world citizens. In all honesty I’d have expected it to be far greater."

Not sure what units you're using here, but the average per capita annual CO2e footprint is 4.8 tonnes/person (2017).

My son is an engineer at Spacex, so for that launch everyone in our household was sitting on pins and needles (one thing for a rocket full of satellites to blow up, another thing altogether with astronauts on board). The job's not over yet, but a big sigh of relief on May 30 here at the Meinetz household!

John Armstrong's picture
John Armstrong on Jun 18, 2020 9:55 am GMT

Bob

Thankyou for the great comments! 

1. Great point on carbon costs of production of Kerosene.. let me check out that 32% number and we can add that and update the math!

2. The total launch carbon 1115 Tonnes so divided by the figure of around 4.8 you get to my number...? Sorry did I miss something? (I need to go back to the reference as we will be a little higher than 4.8 now)

Thats fantstic to have a family member in SpaceX! Its such an exciting project.... could he tell us if the oxygen used is linked to a zero carbon PPA (power purchase agreement) for offshore wind? That would take a huge chunk out of the carbon impact.

Thanks!

John

 

Bob Meinetz's picture
Bob Meinetz on Jun 18, 2020 2:46 pm GMT

John, I couldn't find any hard numbers for emissions from extraction / refining / transportation of kerosene. One source I found had from 24%-40% of the CO2 from combustion, so I chose 32% as an average.

I misunderstood the point you were making about the footprint of the launch in relation to 278 world citizens - my bad.

Thanks for the info on liquid oxygen, that was an eye-opener for me. I'll ask me son whether it's linked to a PPA or not, but I doubt it. Here in California the environmental value of 100%-renewable PPAs is controversial. To use electricity only from renewable sources to power a grid is impossible, so generators engage in a shell game of "renewable energy certificates" (RECs), which can be bought by fossil fuel producers to 'offset' their emissions. With RECs, 2 generation facilities receive credit for generating clean electricity, but only one is renewable - so the actual electricity produced is 50% renewable, with the other half having the carbon footprint of whatever fossil plant bought the REC.

If the oxygen was liquified in Florida it had a carbon footprint of 720 lbsCO2e/MWh, about twice as high as California, based on the average emissions intensity of both states. But in reality, carbon intensity is extremely time-dependent. If the oxygen was liquified in the middle of the night in Florida, when 4 nuclear reactors at Turkey Point and St. Lucie are churning out 4.3 billion watts of clean electricity, it's likely less carbon-intensive to liquify oxygen there, than here. "Renewable energy certificates", notwithstanding.

John Armstrong's picture
John Armstrong on Jun 19, 2020 5:40 pm GMT

Thanks Bob... I agree on the green PPA... it ends up a bit arms length greenwash! 

Mark Silverstone's picture
Mark Silverstone on Jun 19, 2020 2:33 pm GMT

Really interesting. Thanks for posting. I would have guessed that the carbon footprint was much more.

John Armstrong's picture
John Armstrong on Jun 19, 2020 5:40 pm GMT

Mark

I would too... thats why I thought I would share my calculations... am I missing something?!

John

Mark Silverstone's picture
Mark Silverstone on Jun 22, 2020 7:09 pm GMT

Sorry John - I did not mean to suggest that you were missing something.  Looks good to me. Let´s launch more - Back to the moon and beyond!

John Armstrong's picture
John Armstrong on Jun 23, 2020 11:43 am GMT

Mark - no problem... its a genuine question. I think i should probably include some carbon in the calculation for refining and transporting the aviation fuel... also I suspect there may be higher losses in moving cooled oxygen around. 

I have also been wondering if a number of other gases are created with higher climate change infleunce - i.e. NOx etc.... the combustion can't be that complete. 

I think its important for engineers to share their numbers and be open to challenge.... nobody is perfect!

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