Alongside nuclear war or a massive impact from an asteroid, anthropogenic climate change is one of the greatest existential threats facing humanity today. With the rise in greenhouse gas emissions through the 20th century, Earth’s atmosphere continues to absorb more of the Sun’s energy. This has led to rising temperatures, rising sea levels, and increased drought, famine, wildfires, and other ecological consequences. According to the Intergovernmental Panel on Climate Change (IPCC), global temperatures will increase by an average of 1.5 to 2 °C (2.7 to 3.6 °F) by 2050.
For some parts of the world, the temperature increases will be manageable with the right adaptation and mitigation strategies. For others, especially in the equatorial regions (where most of Earth’s population lives), the temperature increases will be severe and will make life untenable for millions of people. For decades, scientists have considered using a sunshield to block a fraction of the Sun’s energy (1 to 2%) before it reaches Earth’s atmosphere. According to a new study by a team led by the University of Utah, lunar dust could be used to shield Earth from sunlight.
The study was conducted by Ben Bromley and Sameer H. Khan, a professor of theoretical and computational astrophysics and a computer science student at the University of Utah (respectively), and Scott J. Kenyon, a theoretical astrophysicist with the Harvard & Smithsonian Center for Astrophysics (CfA). Their paper, which was published on February 8th in the journal PLOS Climate, describes the properties of different types of dust particles, the quantities required, and the orbits that would be best suited for shading Earth.
The sunshield concept calls for a solar shade stationed at the L1 Lagrange Point between the Earth and the Sun. This would ensure the sunshield remains in a stable orbit between the Earth and Sun, providing constant protection. For the sake of their research, the Utah-led team applied a technique used to study planet formation around distant stars. According to the Nebular Hypothesis, this consists of protoplanetary rings of dust and gas that orbit young stars and eventually accrete (due to angular momentum) to create planets.
These rings intercept light from the star and radiate it as heat, which astronomers study using infrared telescopes. “That was the seed of the idea; if we took a small amount of material and put it on a special orbit between the Earth and the Sun and broke it up, we could block out a lot of sunlight with a little amount of mass,” said Bromley in a University of Utah (@theU) press release. The team then applied this theory to lunar dust and found that its inherent properties were just right to effectively work as a sunshield.
Similarly, they ran computer simulations to test how dust particles scattered until they found the optimal trajectories that would place it in orbit at L1, where it would shield Earth from solar radiation. Sameer Khan led the initial exploration into which orbits could hold dust in position long enough to provide adequate shading. Said Khan:
“Because we know the positions and masses of the major celestial bodies in our solar system, we can simply use the laws of gravity to track the position of a simulated sunshield over time for several different orbits. It was rather difficult to get the shield to stay at L1 long enough to cast a meaningful shadow. This shouldn’t come as a surprise, though, since L1 is an unstable equilibrium point. Even the slightest deviation in the sunshield’s orbit can cause it to rapidly drift out of place, so our simulations had to be extremely precise.”
In the end, they found that two possible scenarios were the most promising. The first consisted of dust being launched from Earth to a platform at the Earth-Sun L1 Lagrange Point, where it would be released in multiple directions – including the position of Earth, the Sun, the Moon, and other solar system planets. They found that if the dust were launched precisely, it would follow a path between the Earth and the Sun and create a solar shade. However, the dust would be easily blown off course by solar wind, radiation, and the gravity of the many bodies in the Solar System.
A bootprint in the lunar regolith, taken during the Apollo 11 mission in 1969. Credit: NASA.
The team concluded that the platform would need to create an endless supply of dust and launch it every few days to maintain the sunshield. The second scenario consisted of lunar dust shot from the Moon’s surface toward the Sun. While it would need to be done regularly, the second scenario proved to be the more cost-effective option. Due to the Moon’s lower gravity, a rocket needs to achieve an escape velocity of only 2.38 km/s (1.74 mi/s) to break free of the Moon’s gravity, whereas Earth’s escape velocity is 11.2 km/s (6.96 mi/s).
This was welcome news since the amount of dust needed to make a solar shield is comparable to the output of a major mining operation here on Earth. Furthermore, they found that a separate platform at L1 may not be necessary for the second scenario. Said Kenyon:
“It is amazing to contemplate how moon dust—which took over four billion years to generate—might help slow the rise in Earth’s temperature, a problem that took us less than 300 years to produce. It is astounding that the Sun, Earth, and Moon are in just the right configuration to enable this kind of climate mitigation strategy.”
The authors also stress that their study explores the potential impact of these strategies and does not evaluate whether or not they are feasible. In addition, the authors insist that none of these scenarios would create a runaway cooling effect that would cause an artificial ice age on Earth. Since the Sun’s radiation naturally disperses dust through the Solar System, the sunshield would fade away without regular replenishment. As Bromley stated:
“We aren’t experts in climate change, or the rocket science needed to move mass from one place to the other. We’re just exploring different kinds of dust on a variety of orbits to see how effective this approach might be. We do not want to miss a game changer for such a critical problem.”