Ben Bromley

Dust launched from the moon’s surface or from a space station positioned between Earth and the sun could reduce enough solar radiation to mitigate the impacts of climate change.

On a cold winter day, the warmth of the sun is welcome. Yet as humanity emits more and more greenhouse gases, the Earth's atmosphere traps more and more of the sun's energy and steadily increases the Earth's temperature. One strategy for reversing this trend is to intercept a fraction of sunlight before it reaches our planet. For decades, scientists have considered using screens, objects or dust particles to block just enough of the sun’s radiation—between 1 or 2%—to mitigate the effects of global warming.

A University of Utah-led study explored the potential of using dust to shield sunlight. They analyzed different properties of dust particles, quantities of dust and the orbits that would be best suited for shading Earth. The authors found that launching dust from Earth to a way station at the “Lagrange Point” between Earth and the sun (L1) would be most effective but would require astronomical cost and effort. An alternative is to use moondust. The authors argue that launching lunar dust from the moon instead could be a cheap and effective way to shade the Earth.

The team of astronomers applied a technique used to study planet formation around distant stars, their usual research focus. Planet formation is a messy process that kicks up lots of astronomical dust that can form rings around the host star. These rings intercept light from the central star and re-radiate it in a way that we can detect it on Earth. One way to discover stars that are forming new planets is to look for these dusty rings.

“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 Ben Bromley, professor of physics and astronomy and lead author for the study.

"It is interesting to contemplate how moon dust—which took over four billion years to generate—might help to solve climate change, a problem that took us less than 300 years to produce,” said Scott Kenyon, co-author of the study from the Center for Astrophysics at Harvard + Smithsonian.

The paper  was published on Wednesday, Feb. 8, 2023, in the journal PLOS Climate.

A simulation from dust launched from the way station at Lagrange point 1. The shadow cast on Earth is exaggerated for clarity.

Casting a shadow

A shield’s overall effectiveness depends on its ability to sustain an orbit that casts a shadow on Earth. Sameer Khan, undergraduate student and the study’s co-author, led the initial exploration into which orbits could hold dust in position long enough to provide adequate shading. Khan’s work demonstrated the difficulty of keeping dust where you need it to be.

“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,” said Khan.

Two scenarios were promising. In the first scenario, the authors positioned a space platform at the L1 Lagrange point, the closest point between Earth and the sun where the gravitational forces are balanced. Objects at Lagrange points tend to stay along a path between the two celestial bodies, which is why the James Webb Space Telescope (JWST) is located at L2, a Lagrange point on the opposite side of the Earth.

In computer simulations, the researchers shot test particles along the L1 orbit, including the position of Earth, the sun, the moon, and other solar system planets, and tracked where the particles scattered. The authors found that when launched precisely, the dust would follow a path between Earth and the sun, effectively creating shade, at least for a while. Unlike the 13,000-pound JWST, the dust was easily blown off course by the solar winds, radiation, and gravity within the solar system. Any L1 platform would need to create an endless supply of new dust batches to blast into orbit every few days after the initial spray dissipates.

“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,” Khan said.

A simulation of dust launched from the moon’s surface as seen from Earth.

In the second scenario, the authors shot lunar dust from the surface of the moon towards the sun. They found that the inherent properties of lunar dust were just right to effectively work as a sun shield. The simulations tested how lunar dust scattered along various courses until they found excellent trajectories aimed toward L1 that served as an effective sun shield. These results are welcome news, because much less energy is needed to launch dust from the moon than from Earth. This is important because the amount of dust in a solar shield is large, comparable to the output of a big mining operation here on Earth. Furthermore, the discovery of the new sun-shielding trajectories means delivering the lunar dust to a separate platform at L1 may not be necessary.

Just a moonshot?

The authors stress that this study only explores the potential impact of this strategy, rather than evaluate whether these scenarios are logistically feasible.

“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,” said Bromley.

One of the biggest logistical challenges—replenishing dust streams every few days—also has an advantage. Eventually, the sun’s radiation disperses the dust particles throughout the solar system; the sun shield is temporary and shield particles do not fall onto Earth. The authors assure that their approach would not create a permanently cold, uninhabitable planet, as in the science fiction story, “Snowpiercer.”

“Our strategy could be an option in addressing climate change,” said Bromley, “if what we need is more time.”

by Lisa Potter, first published @ theU Lead photo by aerolite.org

Tanmoy Laskar

Mysterious bright flash is a black hole jet pointing straight at Earth.

Earlier this year, astronomers at the Palomar Observatory detected an extraordinary flash in a part of the sky where no such light had been observed the night before. From a rough calculation, the flash appeared to give off more light than 1,000 trillion suns.

The team, led by researchers at NASA, Caltech, and elsewhere, posted their discovery to an astronomy newsletter, where the signal drew the attention of astronomers around the world, including scientists at MIT and the University of Utah. Over the next few days, multiple telescopes focused in on the signal to gather more data across multiple wavelengths in the X-ray, ultraviolet, optical and radio bands, to see what could possibly produce such an enormous amount of light.

Now, the U and MIT astronomers and collaborators have determined a likely source for the signal. Tanmoy Laskar, Assistant Professor in the Department of Physics and Astronomy at the U, was co-author of a study that appeared on Nov. 30 in Nature Astronomy. The scientists report that the signal, named AT 2022cmc, likely comes from a relativistic jet of matter launched by a supermassive black hole at close to the speed of light. They believe the jet is the product of a black hole that suddenly began devouring a nearby star, releasing a huge amount of energy in the process.

Astronomers have observed other such “tidal disruption events,” or TDEs, in which a passing star is torn apart by a black hole’s tidal forces. AT 2022cmc is brighter than any TDE discovered to date. The source is also the farthest TDE ever detected, at some 8.5 billion lights years away—more than halfway across the universe.

Palomar Observatory

How could such a distant event appear so bright in our sky? The team said the black hole’s jet may be pointing directly toward Earth, making the signal appear brighter than if the jet were pointing in any other direction. The effect is called “Doppler boosting.”

AT 2022cmc is the fourth Doppler-boosted TDE ever detected and the first such event that has been observed since 2011. It is also the first TDE discovered using an optical sky survey.

“One of the tell-tale signatures of the presence of such a jet is powerful radio emission from a small volume of space,” said Laskar. A preliminary report alerted the team that this event might have detectable radio emissions. “So, we followed it up with the Karl G. Jansky Very Large Array in New Mexico, and boom, there it was! Bright radio emission signaling a compact, Doppler-boosted jet.”

As more powerful telescopes start up in the coming years, they will reveal more TDEs, which can shed light on how supermassive black holes grow and shape the galaxies around them.

“We know there is one supermassive black hole per galaxy, and they formed very quickly in the universe’s first million years,” said co-author Matteo Lucchini, a postdoc in MIT’s Kavli Institute for Astrophysics and Space Research. “That tells us they feed very fast, though we don’t know how that feeding process works. So, sources like a TDE can actually be a really good probe for how that process happens.”

Feeding frenzy

Following AT 2022cmc’s initial discovery, the team focused in on the signal using the Neutron star Interior Composition ExploreR (NICER), an X-ray telescope that operates aboard the International Space Station.

“Things looked pretty normal the first three days,” recalled the study’s lead author Dheeraj “DJ” Pasham, who is an Einstein Fellow at MIT. “Then we looked at it with an X-ray telescope, and what we found was, the source was too bright.”

Typically, such bright flashes in the sky are gamma-ray bursts—extreme jets of X-ray emissions that spew from the collapse of massive stars.

“Both GRBs and TDEs are events that have superfast jets pointed at Earth,” said Laskar. “One of the key ways to distinguish between them is in the X-rays. Jetted TDEs seem to also have strongly variable X-ray emission.” Indeed, the team found that X-ray emissions from AT 2022cmc swung widely by a factor of 500 over a few weeks.

The team then gathered observations from other X-ray, radio, optical and UV telescopes and tracked the signal’s activity over the next few weeks. Another remarkable property they observed was the signal’s extreme luminosity in the X-ray band.

“This particular event was 100 times more powerful than the most powerful gamma-ray burst afterglow,” Pasham said. “It was something extraordinary.”

They suspected that such extreme X-ray activity must be powered by an extreme accretion episode—an event that generates a huge churning disk, such as from a tidal disruption event, in which a shredded star creates a whirlpool of debris as it falls into a black hole.

The team found that AT 2022cmc’s X-ray luminosity was comparable to, though brighter than, three previously detected jetted TDEs. These bright events happened to generate jets of matter pointing straight toward Earth. The researchers wondered: If AT 2022cmc’s luminosity is the result of a similar Earth-targeting jet, how fast must the jet be moving to generate such a bright signal? To answer this, Lucchini modeled the signal’s data, assuming the event involved a jet headed straight toward Earth.

“We found that the jet speed is 99.99% the speed of light,” Lucchini said.

To produce such an intense jet, the black hole must be in an extremely active phase—what Pasham described as a “hyper-feeding frenzy.”

“It’s probably swallowing the star at the rate of half the mass of the sun per year,” Pasham estimated. “A lot of this tidal disruption happens early on, and we were able to catch this event right at the beginning, within one week of the black hole starting to feed on the star.”

“We expect many more of these TDEs in the future,” Lucchini added. “Then we might be able to say, finally, how exactly black holes launch these extremely powerful jets.”

“When the next TDE is discovered, we will again be ready to catch its light from X-rays to radio waves,” Laskar said. “By combining such data with physical models, we hope to build a full picture of how supermassive black holes at the centers of galaxies grow, evolve, and shape their environments over cosmic time.”

by Lisa Potter | Adapted from a release by Jennifer Chu, MIT News Office
first published in @theu