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    Study: Climate change will reduce the number of satellites that can safely orbit in space

    MIT aerospace engineers have found that greenhouse gas emissions are changing the environment of near-Earth space in ways that, over time, will reduce the number of satellites that can sustainably operate there.In a study appearing today in Nature Sustainability, the researchers report that carbon dioxide and other greenhouse gases can cause the upper atmosphere to shrink. An atmospheric layer of special interest is the thermosphere, where the International Space Station and most satellites orbit today. When the thermosphere contracts, the decreasing density reduces atmospheric drag — a force that pulls old satellites and other debris down to altitudes where they will encounter air molecules and burn up.Less drag therefore means extended lifetimes for space junk, which will litter sought-after regions for decades and increase the potential for collisions in orbit.The team carried out simulations of how carbon emissions affect the upper atmosphere and orbital dynamics, in order to estimate the “satellite carrying capacity” of low Earth orbit. These simulations predict that by the year 2100, the carrying capacity of the most popular regions could be reduced by 50-66 percent due to the effects of greenhouse gases.“Our behavior with greenhouse gases here on Earth over the past 100 years is having an effect on how we operate satellites over the next 100 years,” says study author Richard Linares, associate professor in MIT’s Department of Aeronautics and Astronautics (AeroAstro).“The upper atmosphere is in a fragile state as climate change disrupts the status quo,” adds lead author William Parker, a graduate student in AeroAstro. “At the same time, there’s been a massive increase in the number of satellites launched, especially for delivering broadband internet from space. If we don’t manage this activity carefully and work to reduce our emissions, space could become too crowded, leading to more collisions and debris.”The study includes co-author Matthew Brown of the University of Birmingham.Sky fallThe thermosphere naturally contracts and expands every 11 years in response to the sun’s regular activity cycle. When the sun’s activity is low, the Earth receives less radiation, and its outermost atmosphere temporarily cools and contracts before expanding again during solar maximum.In the 1990s, scientists wondered what response the thermosphere might have to greenhouse gases. Their preliminary modeling showed that, while the gases trap heat in the lower atmosphere, where we experience global warming and weather, the same gases radiate heat at much higher altitudes, effectively cooling the thermosphere. With this cooling, the researchers predicted that the thermosphere should shrink, reducing atmospheric density at high altitudes.In the last decade, scientists have been able to measure changes in drag on satellites, which has provided some evidence that the thermosphere is contracting in response to something more than the sun’s natural, 11-year cycle.“The sky is quite literally falling — just at a rate that’s on the scale of decades,” Parker says. “And we can see this by how the drag on our satellites is changing.”The MIT team wondered how that response will affect the number of satellites that can safely operate in Earth’s orbit. Today, there are over 10,000 satellites drifting through low Earth orbit, which describes the region of space up to 1,200 miles (2,000 kilometers), from Earth’s surface. These satellites deliver essential services, including internet, communications, navigation, weather forecasting, and banking. The satellite population has ballooned in recent years, requiring operators to perform regular collision-avoidance maneuvers to keep safe. Any collisions that do occur can generate debris that remains in orbit for decades or centuries, increasing the chance for follow-on collisions with satellites, both old and new.“More satellites have been launched in the last five years than in the preceding 60 years combined,” Parker says. “One of key things we’re trying to understand is whether the path we’re on today is sustainable.”Crowded shellsIn their new study, the researchers simulated different greenhouse gas emissions scenarios over the next century to investigate impacts on atmospheric density and drag. For each “shell,” or altitude range of interest, they then modeled the orbital dynamics and the risk of satellite collisions based on the number of objects within the shell. They used this approach to identify each shell’s “carrying capacity” — a term that is typically used in studies of ecology to describe the number of individuals that an ecosystem can support.“We’re taking that carrying capacity idea and translating it to this space sustainability problem, to understand how many satellites low Earth orbit can sustain,” Parker explains.The team compared several scenarios: one in which greenhouse gas concentrations remain at their level from the year 2000 and others where emissions change according to the Intergovernmental Panel on Climate Change (IPCC) Shared Socioeconomic Pathways (SSPs). They found that scenarios with continuing increases in emissions would lead to a significantly reduced carrying capacity throughout low Earth orbit.In particular, the team estimates that by the end of this century, the number of satellites safely accommodated within the altitudes of 200 and 1,000 kilometers could be reduced by 50 to 66 percent compared with a scenario in which emissions remain at year-2000 levels. If satellite capacity is exceeded, even in a local region, the researchers predict that the region will experience a “runaway instability,” or a cascade of collisions that would create so much debris that satellites could no longer safely operate there.Their predictions forecast out to the year 2100, but the team says that certain shells in the atmosphere today are already crowding up with satellites, particularly from recent “megaconstellations” such as SpaceX’s Starlink, which comprises fleets of thousands of small internet satellites.“The megaconstellation is a new trend, and we’re showing that because of climate change, we’re going to have a reduced capacity in orbit,” Linares says. “And in local regions, we’re close to approaching this capacity value today.”“We rely on the atmosphere to clean up our debris. If the atmosphere is changing, then the debris environment will change too,” Parker adds. “We show the long-term outlook on orbital debris is critically dependent on curbing our greenhouse gas emissions.”This research is supported, in part, by the U.S. National Science Foundation, the U.S. Air Force, and the U.K. Natural Environment Research Council. More

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    Liquid on Mars was not necessarily all water

    Dry river channels and lake beds on Mars point to the long-ago presence of a liquid on the planet’s surface, and the minerals observed from orbit and from landers seem to many to prove that the liquid was ordinary water. Not so fast, the authors of a new Perspectives article in Nature Geoscience suggest. Water is only one of two possible liquids under what are thought to be the conditions present on ancient Mars. The other is liquid carbon dioxide (CO2), and it may actually have been easier for CO2 in the atmosphere to condense into a liquid under those conditions than for water ice to melt. While others have suggested that liquid CO2 (LCO2) might be the source of some of the river channels seen on Mars, the mineral evidence has seemed to point uniquely to water. However, the new paper cites recent studies of carbon sequestration, the process of burying liquefied CO2 recovered from Earth’s atmosphere deep in underground caverns, which show that similar mineral alteration can occur in liquid CO2 as in water, sometimes even more rapidly.The new paper is led by Michael Hecht, principal investigator of the MOXIE instrument aboard the NASA Mars Rover Perseverance. Hecht, a research scientist at MIT’s Haystack Observatory and a former associate director, says, “Understanding how sufficient liquid water was able to flow on early Mars to explain the morphology and mineralogy we see today is probably the greatest unsettled question of Mars science. There is likely no one right answer, and we are merely suggesting another possible piece of the puzzle.”In the paper, the authors discuss the compatibility of their proposal with current knowledge of Martian atmospheric content and implications for Mars surface mineralogy. They also explore the latest carbon sequestration research and conclude that “LCO2–mineral reactions are consistent with the predominant Mars alteration products: carbonates, phyllosilicates, and sulfates.” The argument for the probable existence of liquid CO2 on the Martian surface is not an all-or-nothing scenario; either liquid CO2, liquid water, or a combination may have brought about such geomorphological and mineralogical evidence for a liquid Mars.Three plausible cases for liquid CO2 on the Martian surface are proposed and discussed: stable surface liquid, basal melting under CO2 ice, and subsurface reservoirs. The likelihood of each depends on the actual inventory of CO2 at the time, as well as the temperature conditions on the surface.The authors acknowledge that the tested sequestration conditions, where the liquid CO2 is above room temperature at pressures of tens of atmospheres, are very different from the cold, relatively low-pressure conditions that might have produced liquid CO2 on early Mars. They call for further laboratory investigations under more realistic conditions to test whether the same chemical reactions occur.Hecht explains, “It’s difficult to say how likely it is that this speculation about early Mars is actually true. What we can say, and we are saying, is that the likelihood is high enough that the possibility should not be ignored.”  More