Massive sun 'umbrella' attached to asteroid could help fight global warming, scientist says

An illustration of a blue, triangular "umbrella" attached to an asteroid in space.
An artist's illustration of the theoretical sun "umbrella." (Image credit: Brooks Bays/UH Institute for Astronomy)

To help combat the effects of global warming, scientists are toying with an innovative idea to shield our planet from the sun with a spaceborne "umbrella" of sorts.

"In Hawaii, many use an umbrella to block the sunlight as they walk about during the day," István Szapudi, an astronomer at the University of Hawaii Institute of Astronomy, said in a statement. "I was thinking, could we do the same for Earth and thereby mitigate the impending catastrophe of climate change?" 

The reason carbon dioxide and other greenhouse gases contribute to global warming is that they trap sunlight around our planet that should be released back into space, ultimately leading to rising temperatures. But it's the sun, and not greenhouse gases, that creates the heat to begin with. That opens up the idea of building Earth a shade.

So, Szapudi drew up an "umbrella" of his own. It would rest at the L1 Lagrange point between the sun and Earth, hypothetically joining sun- or solar-wind-observing probes such as the Solar and Heliospheric Observatory (SOHO) and Advanced Composition Explorer (ACE) that dwell there today. In theory, a large-enough solar shield could effectively block around 1.7 percent of solar radiation at L1, enough to prevent a catastrophic rise in Earth's temperatures. 

Related: NASA searches for climate solutions as global temperatures reach record highs

However, any sort of solar shade is bound to face a stark engineering challenge: At L1, they'd be subject to both the sun's and Earth’s gravities while experiencing a constant torrent of solar radiation. A viable shade would thus need to be massive — weighing millions of tons — and made of a material sturdy enough to stay in place and stay intact. Simply, we don’t have a practical way of launching that much stuff into orbit.

But to get around that issue, Szapudi proposed, much of the material itself can come from space — from a captured asteroid or even lunar dust. That matter could theoretically serve as a counterweight, tethered to a much smaller shield weighing only around 35,000 tons. Right now, even such a smaller shield would be far too heavy for a rocket to lift, but with advances in materials, Szapudi’s study suggests we could manage the feat in several decades.

Szapudi's apparatus falls under the, well, umbrella of solar geoengineering: the controversial idea of alleviating global warming by physically reducing the amount of sunlight that reaches Earth's surface. Other solar geoengineering ideas include pumping aerosols into the atmosphere and editing clouds to reflect more sunlight away into space.

The study was published on July 31 in the journal Proceedings of the Natural Academy of Sciences

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Rahul Rao
Contributing Writer

Rahul Rao is a graduate of New York University's SHERP and a freelance science writer, regularly covering physics, space, and infrastructure. His work has appeared in Gizmodo, Popular Science, Inverse, IEEE Spectrum, and Continuum. He enjoys riding trains for fun, and he has seen every surviving episode of Doctor Who. He holds a masters degree in science writing from New York University's Science, Health and Environmental Reporting Program (SHERP) and earned a bachelors degree from Vanderbilt University, where he studied English and physics. 

  • Homer10
    Bad idea ! The blocking mirror attached to the asteroid will cause the asteroid to be pushed more (a lot more), by the Sun. This will change the orbit of the asteroid a little bit by little bit until it finally gets dislodged from it's nice safe place at a La Graunge point. Then only God knows where this nearby asteroid goes. Chances are it will head towards Earth.
  • Ken Fabian
    As a thought experiment these kinds of ideas a learning and teaching function I suppose but they aren't really global warming solutions. In-space resource utilization even at small demonstration scale is not yet a thing, let alone large scale manufacturing that uses asteroid resources. Launch capabilities of sufficient scale and low cost are not yet a thing either. They aren't even close to viability. Addressing global warming can't wait on that changing.

    Homer10, I would expect the hypothetical positioning to account for light and solar wind pressure, ie a bit sunward of where "stationary" would be without the attached mirror, with the mass making it move less, but anything expected to stay in an orbiting position long term has to have the ability to move.
  • Unclear Engineer
    To deal with the solar light and "wind" pressure, the reflective surface would need to be aimed to slow the orbital velocity in an orbit closer to the Sun than L1, so that the net effect is that it orbits at the same period as Earth and L1 to stay in line. It's distance from the Sun is controlled by being closer, which would make it feel greater gravitation from the Sun, being balanced by the outward pressure of the solar light and particle emissions. And, being closer would make it orbit faster (shorter period), so the sail orientation needs to be at an angle that slows the forward speed enough to stay in line with Earth and L1. See .
  • RikF
    How much would the temperature sink, and where on earth?
  • Unclear Engineer
    The "where on Earth" is the important question. A solar-blocking satellite would not make a dark shadow on Earth even if it blocks 1.7% of the solar radiation hitting Earth. It would be something like the transit of Venus, the last one occurring June 5-6, 2012. Didn't notice that, right? With this "umbrella", you would still be able to see 98.3% of the Sun's surface when standing in its "shadow" on Earth.

    My question whether that shadow reaches the polar regions if it is centered on the equator. I could spend the time doing the geometry, but I am lazy enough to ask here before spending the effort.
  • Helio
    Only a small change may be needed to offset warming. The IPCC studies and models show a range of warming based on a several scenarios. These range from about 2 watts to 4.5 watts (per sq. meter).

    The suffix in the RCP (Representation Concentration Pathways) models are the wattage increases. Thus RCP2.6 is 2.6 watts/sq. meter; RCP4.5 is 4.5 w/sq. meter; etc.

    Their earlier extreme model used 8.5 watts/sq. meter, but this has been deemed unlikely.
    Retrograde rotation of objects reflecting sunlight will slightly offset movement as well (Yarkovsky Effect), though it may be too little for these larger objects, perhaps.