So, what is the carbon footprint of a rocket launch?
The short answer is that nobody knows. Because rockets inject various substances into the upper atmosphere–both on the way up and the way down–they have the potential to chemically alter the atmosphere, and thus the planet’s greenhouse effect, in addition to whatever greenhouse gas emissions are associated with construction and launch. And nobody has studied the issue yet.
Other aspects of the carbon footprint of space travel are possible to calculate, but are still not quite straight-forward.
Reportedly, a single Space Shuttle launch released 28 tons of carbon dioxide, but I haven’t been able to find out how that was calculated. Is that the burning of rocket fuel only? Does it include all associated ground-based transportation, from moving the shuttle out to the launch pad to the morning commute of everybody who works in Mission Control? The problem with carbon footprinting is that there is seldom an unambiguous distinction between the footprint of one activity and the footprint of a different activity. What do you include? What do you exclude? Why? The best you can do is be clear about what you did include so that a fair comparison can be made with the footprints of other activities.
Some rocket launches can be described in terms that are seriously misleading; hydrogen is a popular rocket fuel, and burning hydrogen yields water as its exhaust, not carbon dioxide. While water vapor is also a greenhouse gas, the amount of water vapor in the atmosphere depends on how much the atmosphere can hold, not how much steam is released. Look at just the launch of a hydrogen-burning rocket, therefore, and we seem to see a carbon footprint of zero–yay! But hydrogen cannot be simply collected, like so much firewood–it must be produced. And the production of hydrogen takes energy, which comes from….
It comes from wherever the production plant gets its electricity, but it’s a good bet much of that energy comes from coal-fired plants. Not only is coal itself an incredibly carbon-intensive fuel, but look at how many times this energy must transform on the way to the rocket. The chemical energy in the coal is converted to heat, which boils water to make steam, which spins a turbine, which makes electricity, which converts water to hydrogen–that’s at least six transformations! As per the Second Law of Thermodynamics, each time energy is transformed some of it is lost, meaning there is a lot less energy in that hydrogen than there was in the coal. The rocket would have a much smaller carbon footprint if it could just burn coal directly (except coal is too heavy to use in rockets).
Ultimately, the carbon footprint of one rocket is less important than the carbon footprint of the entire space industry; are we talking about a few rocket launches per year, or are we talking about thousands, or hundreds of thousands of them? The difference matters.
The reason for the launch might matter, too.
Space flight is not only a source of carbon emissions and other pollutants, it is also a critical part of climatological and meteorological research. Without data from satellites, we would not be able to effectively model the changing climate, nor would our predictions of extreme weather events be as accurate. GPS satellites and communications satellites are also important in research, not to mention facilitating communication that would otherwise require actually going somewhere, with associated carbon emissions. Conceivably, the true footprints of some launches could be less than zero if those satellites lead to meaningful climate action policies or make possible reduced carbon footprints on Earth.
The Future of Spaceflight and Climate Sanity
The reason I began thinking about all of this is I’ve been wondering whether the carbon-sane future can include space flight.
Climate sanity is all about energy; the function of fossil fuel use has been to give us more energy than the biosphere can spare, and it is this over-use of energy that is destabilizing the climate and collapsing biodiversity. The enhanced greenhouse effect is the mechanism by which the climate is being destabilized, but the overdraft of energy is the ultimate cause. If there were a way to access just as much energy by some means other than fossil fuel use, the planet would be pushed to crisis by some other mechanism.
It follows, therefore, that humanity, to become sustainable, must go on an energy diet. The change need not be painful; greater efficiency and new technology could ensure that standards of living remain about the same, or even improve, for most people. But some energy-intensive practices will have to be left behind.
Will space flight be one of them?
I’m specifically thinking of those various research and communications satellites, not manned space flight or military applications, both of which we really can do without, and not brief trips to the edge of space and back down again. How much energy is necessary to launch a satellite into orbit, and will that energy be available under a sustainable energy budget?
Calculating the energy required to put up a satellite is fairly easy, if you happen to know the relevant equations and don’t have a learning disability involving math (meaning I’m out of luck, but maybe you’re not).
Here is an article discussing part of the process.
The short answer is that it takes over 31 million joules of energy to put one kilogram of something into orbit, except that all those joules need to come from somewhere, and the fuel necessary to deliver that much energy is going to weigh more than a kilogram, meaning you’ll need more energy to lift the fuel….It sounds as though any rocket must be infinitely large in order to lift itself and its ever-increasingly large fuel supply, and obviously that’s not true, but rockets do have to be much more powerful than casual thought might suggest–and if the payload is bigger, the rocket must be much bigger, in order to account for the extra fuel, and the extra fuel necessary to carry the extra fuel, etc.
Calculating the total energy need for a launch thus requires an extra layer of math, but the process still sounds fairly straight forward, provided you know what you’re using for fuel (so you know how much weight you’ve got per joule), have the right equations, and, once again, are not me.
Then you’d have to figure out whether that much energy is available under the new energy budget, and that involves….
A Simpler Way to Figure It
So, how does a writer who cares about science but can’t do math figure out whether the climate-sane future has rockets in it?
I don’t know how to work the various equations, but I have seen an object launched into orbit aboard the Antares rocket. The Antares, as configured for launch to the ISS, is almost 43 meters long, or about 140 feet, and about 12 feet across. Total burn time, counting both stages, is about six minutes. The first stage burns 64,740kg, or 142727 pounds, of a highly-refined form of kerosene. The second stage burns 12,815kg, or 28252 pounds, of polyurethane. It can deliver a payload of up to 5,400kg, or 11,905 pounds. That’s the equivalent of about three average-sized cars.
A gallon of kerosene weighs 6.8 pounds, meaning we’re looking at well over 20,000 gallons of kerosene. That’s a lot, but offhand it doesn’t sound like more than could exist in a post-petroleum future–and yes, aviation-grade kerosene can be made from plant-derived oils. I’m not sure how much sense it makes to attempt to calculate gallons of polyurethane, but it’s going to be much smaller than the kerosene figure. And polyurethane, too, can be made from plant-derived oils.
Some time ago I attempted to calculate the per-gallon prices of various fuels in a post-petroleum world. Because I can’t begin to anticipate market forces in such an economy, I did not calculate the prices in money but rather by comparison to food. The two main biofuels, ethanol and biodiesel, can both be made out of edible substances: corn and soybeans or Canola seed, respectively. So, for every gallon of ethanol, how much corn must be removed from the human food-stream? For every gallon of biodiesel, how much soy or Canola seed must be lost? I didn’t mean that fuel must always be made from food, only that the comparison provides an intuitively accessible way to understand both the economic and ecological cost of fuel production in terms that are going to be relevant no matter what type of economy we end up using.
I have not attempted similar calculations for either kerosene or polyurethane–we’re interested in ballpark figures at present, in getting to the right scale, so for our purposes, the biodiesel figures are probably close enough.
If so, then the first stage of an Antares rocket alone burns the economic/ecological equivalent of enough food for at least 164 people–possibly as many as 657, depending on what kind of oil you start with–to eat for a year. And that’s not counting the energy involved in refining and chemically converting the oil into fuel, building the rocket, building the satellites, transporting materials or finished parts, running all necessary computers and communications equipment, and covering the loss of the occasional rocket the blows up during launch (as I saw an Antares do). And don’t forget all the people involved with all of this, who need to eat and so forth.
It’s a lot, but it’s not so much that the United States could not collect the resources for a launch every year or three, which is really all that should be necessary for satellites for science, GPS, and communication.
And this is for launch of the Antares, a launch vehicle certainly capable of delivering a good-sized payload to low-Earth orbit, but it’s certainly not the only way to get the job done. A smaller payload–or a launch vehicle made of lighter materials–would need dramatically less fuel, thanks to the issue we explored earlier of needing fuel to lift the fuel. Launching a rocket from the upper atmosphere (a balloon lifts it up to the launch “site”) reduces drag on the rocket and again reduces fuel dramatically.
Lunar colonies and space tourism are probably still out, but those applications of space flight that yield the biggest benefits to us here on Earth sound doable.
I had initially assumed that the carbon-neutral future would have to do without spaceflight. Now it looks like I was wrong.