On May 30^{th} 2020, the Crew Dragon launched astronauts from U.S. soil for the first time since the Shuttle’s last launch in 2011. Atop a Falcon 9 rocket, Doug Hurley and Bob Behnken were sent into orbit for a rendezvous with the International Space Station (ISS) to demonstrate SpaceX’s ability to ferry astronauts to and from the space station successfully. It was the final major step required to get certification by NASA’s Commercial Crew Programme for more manned missions into space. A historic step in American space travel. With its success, commercial trips into Earth outer orbit are now on the horizon opening up a whole new travel industry.

But what additional contribution is this new outer-worldly enterprise going to have on an already carbon costly sector? As we know, transportation makes up 14% of global greenhouse gas (GHG) emissions, an already significant portion. Trips into space is only going to add to that percentage. In this article we calculate the carbon spent on the Crew Dragon mission to send astronauts into space to get a feel as to what affect commercial space travel could have on the globe’s carbon budget.

To calculate Crew Dragon’s climate change contribution it takes a bit of maths. I’m not going to claim my calculations are entirely akin to reality, this is literally rocket science after all. But, by taking a few key pieces of data, we can hopefully get a rough idea of how much GHG Elon Musk’s latest entrepreneurial venture has produced. From the results we’ll hopefully be able to get a rough idea whether opening space to the public is environmentally a good idea.

**The mathsy bit**

To send its passengers into Earth’s orbit, the Falcon 9 rocket launched in two stages. The first stage used 9 Merlin 1D Engines for propulsion fuelled by Rocket Propellant 1 (RP-1) (a highly refined kerosene) and liquid oxygen (LOX) to get the Shuttle from sea-level to the upper atmosphere. The second stage used just 1 Merlin 1D Engine fuelled again by RP-1 and LOX and initiated once the first stage was complete (see link for Falcon 9 specs).

The total mass of RP-1 fuel needed for the first stage equalled 119,100kg and for the second stage 27,850kg. In order to determine the amount of carbon produced as a result of burning this fuel, a GHG emissions factor must be used to convert kg RP-1 to kg CO_{2}e. However, a direct GHG emissions factor for RP-1 is not publicly available, so aviation turbine fuel (or Jet A1 fuel), reported to be very similar to RP-1, is used as an alternative.

According to the UK government’s 2020 conversion factors, for every 1kg of aviation turbine fuel burned it produces 3.18141kg of CO_{2}e. But it is not simply a case of multiplying the total RP-1 of Falcon 9 with the GHG emissions factor. Radiative forcing must be considered.

Radiative forcing is the measure of the capacity of GHGs to affect the energy balance of the Earth. As energy comes into the Earth’s system as light it is either absorbed or reflected (to put it simply). Absorbed energy heats the planet, reflected energy reduces heating potential. GHGs are good absorbents of energy, and so add to the greenhouse effect more significantly than other gases (hence their name). Radiative forcing is a direct measure of the amount that the Earth’s energy budget is out of balance. Therefore, the more GHGs in the atmosphere the greater the unbalance of radiative forcing towards increased warming.

Radiative forcing is significant in the case of space travel because emissions such as CO_{2}, black carbon soot and water vapour (all byproducts of launching into space) produced at higher altitudes have a greater radiative forcing affect. As a result, an additional radiative forcing factor must be applied to accommodate.

There are significant uncertainties with the magnitude of the radiative forcing affects. However, a recent study that systematically reviewed state of the art approaches to the accounting for radiative forcing of aircraft emissions recommend a radiative forcing index factor of 2 for emissions in the atmosphere or 5.2 for the higher atmosphere. These are the factors applied to calculations in this article.

As the Falcon 9 launched in two stages, the Shuttle continuously increased in altitude, fuel was burned at different heights within the atmosphere. Therefore, for our calculation the radiative forcing factor of 2 is applied to the first stage and the factor of 5.2 applied to the second stage.

So, by taking the mass of RP-1 and applying both the emissions and radiative forcing factors the following equation can be used to calculate the total CO_{2}e of the Crew Dragon launch:

Total CO_{2}e (kg) = RP-1 Mass (kg) x (GHG emissions factor x radiative forcing factor)

**Results**

First Stage: 119,100kg x (3.18141 x 2) = 757,811.862 kgCO_{2}e

Second Stage: 27,850kg x (3.18141 x 5.2) = 460,731.796 kgCO_{2}e

Total = 757,811.862 kgCO_{2}e + 460,731.796 kgCO_{2}e = **1,218,543.658 kgCO _{2}e = 1,218.5 tCO_{2}e**

By our calculations the Crew Dragon mission to the ISS contributed **1,218.5 tCO _{2}e** into the atmosphere.

To put that into perspective, 1,218.5 tCO_{2}e is roughly equivalent to driving around 4,365,662 miles in a medium sized car (such as a VW Golf). That distance would cover approximately 612 UK drivers’ average annual mileage. In terms of air travel, the total carbon expenditure of the Crew Dragon mission is equivalent to a single passenger travelling roughly 6,248,942km on a long hall flight, which is roughly the same as 367 people travelling from London to Sydney.

If we compare this one mission to the globe’s annual transportation emissions (14% of 36.15 billion tCO_{2}), it’s percentage contribution is so minimal it’s hardly worth a mention (<0.000001%). But, if space travel becomes more frequent and we see commercial missions on a regular basis this will significantly rise. As a one-off however, the impact is negligible.

It is worth mentioning that there are a number of other contributors to SpaceX’s carbon footprint not considered within our calculations. There have been a great number other unmanned missions and tests to allow commercial space travel to get this far. There would have been great production costs building the Falcon 9 rocket and everything needed to allow it to launch safely and successfully. But this is the case with all travel, albeit not on quite the same scale. For example, cars need to be built and tested, planes need runways, and ships need docks, the list goes on.

So as travel goes, in terms of carbon cost, commercial space travel isn’t yet contributing to climate change on the scale of its travel counterparts: aviation and road vehicles. The infrequent nature of each mission means carbon costs have been relatively low so far. However, the carbon contribution per passenger is performing poorly. Even if all 7 seats aboard Crew Dragon had been filled it would have transported a fraction of the amount of passengers a long hall flight could achieve expending the same amount of carbon, and aviation is already considered one of the most wasteful industry emitters.

But, as commercial space travel finds it’s feet and the technology improves, capacity will definitely increase as Musk tries to maximises profits. The reusability of Falcon 9 rockets is already a massive step in the right direction of sustainability. So, it will only be a matter of time before space becomes the next top holiday destination.

## Chris Hazell

MSc Sustainability & Consultancy