With the cost
of launching a rocket into space falling, the number of rocket launches is,
well, taking off. Last year, governments and companies across the world
successfully launched 133 rockets into orbit, breaking a record that
stood for 45 years.
But there's a catch. Breaking free from Earth's gravity requires a rocket to release a tremendous amount of energy in a short period of time. As a rocket leaves Earth, it produces hot exhaust that changes the physics and chemistry of the atmosphere as it passes through. In a paper published Tuesday in the peer-reviewed journal Physics of Fluids, a pair of physicists simulated the launch of a SpaceX Falcon 9 rocket blasting into space. They found several reasons to be concerned.
The carbon footprint isn't the
problem
Rockets aren't
responsible for putting that much carbon dioxide into the atmosphere. A
typical launch burns roughly the same amount of fuel as a day-long commercial
flight but produces seven times as much CO2 — between 200 and 300
tons — as the airliner. That's far more carbon than the average person
will generate in their lifetime, but it's a rounding error compared to
the 900 million tons of CO2 the aviation industry was spewing
annually before the pandemic.
But that's not
the whole story. “We don't care about a rocket's carbon footprint. That's
irrelevant," says researcher Martin Ross. For him, it's the
particles contained in rocket exhaust — chiefly alumina and black carbon — that
really matter. “These particles scatter and absorb sunlight. They change the
temperature and circulation of the stratosphere,” Ross says.
Unfortunately,
scientists only have a faint understanding of the total environmental impact of
a rocket launch. “The current level of data about rocket emissions does
not provide researchers with enough information to fully assess the impact of
launches on the global environment,” Ross says.
The effect of carbon emissions
high in the atmosphere is uncertain
The
researchers behind the new study are bringing the problem into sharper focus by
modeling the exhaust from the nine nozzles of a Falcon 9 rocket as it
launches into space. These simulations incorporate data about the rocket
and its propellant (RP-1) with equations that describe how gases behave under
various conditions. Thanks to some serious computing power, the researchers
were able to predict how exhaust behaves after exiting the nozzles, at
increments of roughly 0.6 miles (1 km) in altitude.
The
researchers analyzed the launch by comparing the volume of exhaust released
during one kilometer of upward travel through a certain band of the atmosphere
(e.g. between 2 km and 2.99 km) with the properties of the atmosphere at that
specific altitude. They had to adopt this somewhat confusing method because the
physical and chemical makeup of the atmosphere is different at different
altitudes.
They found
that the amount of total exhaust is "negligible" compared to the air
around it, even at high altitudes. That's a surprise because the atmosphere is
much less dense at higher altitudes. According to their calculations, the
amount of exhaust released by a Falcon 9 as it travels between 70 km and 70.99
km (roughly 43 miles) is just one-fourteenth the amount of mass found in one
cubic kilometer (roughly .25 mi3) of air at that altitude. (This is conveyed by
the blue line in the chart below.)
What isn't negligible is the amount of CO2 that a Falcon 9 introduces into higher levels of the atmosphere as it passes through (represented by the dotted red line in the figure above). Once it passes an altitude of 27 miles (43.5 km), a rocket starts emitting more than one cubic kilometer's worth of CO2 for each kilometer it climbs. By the time it reaches 43.5 miles (70 km), a Falcon 9 releases more than 25 times the amount of CO2 found in a cubic kilometer of air at that altitude.
And rocket exhaust contains
more than carbon
It's more than
CO2. "Perhaps even more crucially, the [amount of] carbon monoxide (CO)
and water (H2O) [in rocket exhaust] are of a similar order as carbon
dioxide," the authors write. That's a concern because there's hardly any
carbon monoxide or water high in the atmosphere. "Therefore, these
compounds’ emissions at high altitudes introduce an even more significant
contribution/rise to the existing, if any, trace amounts already present."
Water vapor
immediately freezes at that altitude, but researchers have no idea where those
ice crystals end up. Carbon monoxide reacts with hydroxide (O2) to form even
more CO2. The researchers also discovered that dangerous exhaust emissions
called thermal nitrogen oxides (NOx) can stick around for a long time in
hot rivers before dispersing throughout the atmosphere, especially at lower
altitudes.
The future is uncertain, but
researchers and regulators are paying attention
With just more
than 100 launches per year, some say that pollution from rockets isn't an
issue. "One of the arguments that people have used in the past was to say
that we don't really need to pay attention to rockets or to the space industry,
or the space industry is small, and it's always going to be small," Ross
says.
He doesn't
agree. "I think the developments that we're seeing the past few years show
that … space is entering this very rapid growth phase like aviation saw in the
'20s and '30s."
The authors
behind the new study feel the same way. "We believe that the problem of
atmospheric pollution caused by rocket launches is vital and needs to be
addressed appropriately as commercial space flights, in particular, are
expected to increase in the future," they write.
The problem of
pollution from rockets slowly coming into clearer focus, and it's being taken
seriously in high places. Later this year, the World Meteorological
Organization and the UN Environmental Program will release new a report that
summarizes how rocket emissions deplete ozone. With any luck, this attention
will cause atmospheric pollution to become a key factor in the design of
future rockets.
Comments
Post a Comment