Guest essay by Eric Worrall
Before you ask, it’s not because of all the climate satellites NASA plans to launch in the next few years.
What if Space Junk and Climate Change Become the Same Problem?
Changes to the atmosphere caused by carbon dioxide emissions could increase the amount of debris that stays in orbit.
By Jonathan O’CallaghanMay 12, 2021, 12:38 p.m. ET
It’s easy to compare the space junk problem to climate change. Human activities leave too many dead satellites and fragments of machinery discarded in Earth orbit. If left unchecked, space junk could pose significant problems for future generations — rendering access to space increasingly difficult, or at worst, impossible.
Yet the two may come to be linked. Our planet’s atmosphere naturally pulls orbiting debris downward and incinerates it in the thicker lower atmosphere, but increasing carbon dioxide levels are lowering the density of the upper atmosphere, which may diminish this effect. A study presented last month at the European Conference on Space Debris says that the problem has been underestimated, and that the amount of space junk in orbit could, in a worst-case scenario, increase 50 times by 2100.
The research is “very important work,” said John Emmert, an atmospheric scientist at the U.S. Naval Research Laboratory in Washington, D.C., who has studied atmospheric density loss. However, Dr. Emmert says more research is needed to understand the severity of the problem — with the impact of the sun’s solar cycle also known to be a major factor in atmospheric density changes.
Some scientists appear to favour solar activity as the main forcing, with low solar activity associated with periods of reduced drag experienced by near Earth satellites and space junk.
Record‐low thermospheric density during the 2008 solar minimum
We use global‐average thermospheric total mass density, derived from the drag effect on the orbits of many space objects, to study the behavior of the thermosphere during the prolonged minimum in solar activity between cycles 23 and 24. During 2007–2009 thermospheric densities at a fiducial altitude of 400 km were the lowest observed in the 43‐year database, and were anomalously low, by 10–30%, compared with climatologically expected levels. The density anomalies appear to have commenced before 2006, well before the cycle 23/24 minimum, and are larger than expected from enhanced thermospheric cooling by increasing concentrations of CO2. The height dependence of the mass density anomalies suggests that they are attributable to a combination of lower‐than‐expected exospheric temperature (−14 K) and reductions in the number density of atomic oxygen (−12%) and other species (−3%) near the base of the diffusive portion of the thermosphere.
In Earth’s thermosphere, density at a fixed geometric height is highly sensitive to variations of solar irradiance at extreme ultraviolet (EUV) wavelengths (0–120 nm). EUV and far ultraviolet (FUV) photons are the primary heat source of the thermosphere [Roble, 1995], which expands and contracts in response to temperature changes. Solar EUV irradiance increases by a factor of 2 or more from the minimum to maximum of the 11‐year solar activity cycle [Lean, 1997], driving order‐of‐magnitude increases of total mass density near 400 km [e.g., Emmert and Picone, 2010]. Cycle 23 (1996.4–2008.8, 12.4 years long) was unusually prolonged relative to prior cycles 22 (9.7 years) and 21 (10.3 years). The minimum of cycle 23/24 had the most days without sunspots since the 1933 minimum [Livingston and Penn, 2009]. In response to corresponding prolonged low levels of solar EUV irradiance during this period, the thermosphere is expected to have been unusually cool and contracted. Measurements of ion temperatures [Heelis et al., 2009] provide indirect evidence of this.
Another paper from 2018;
EUV Irradiance Inputs to Thermospheric Density Models: Open Issues and Path Forward
One of the objectives of the NASA Living With a Star Institute on “Nowcasting of Atmospheric Drag for low Earth orbit (LEO) Spacecraft” was to investigate whether and how to increase the accuracy of atmospheric drag models by improving the quality of the solar forcing inputs, namely, extreme ultraviolet (EUV) irradiance information. In this focused review, we examine the status of and issues with EUV measurements and proxies, discuss recent promising developments, and suggest a number of ways to improve the reliability, availability, and forecast accuracy of EUV measurements in the next solar cycle.
Solar variability influences human society in many ways, from long‐term climatic changes to telecommunications to the longevity of spacecraft. Of particular concern, here, are the effects of solar variability on the thermosphere (90–600 km altitude) where many spacecraft, including the International Space Station, reside. The main solar thermospheric drivers are (1) the extreme ultraviolet (EUV) radiant flux per unit area (irradiance) at wavelengths below ~200 nm and (2) intermittent solar wind inputs from coronal mass ejections (CMEs) and high‐speed streams (HSSs) (Chen et al., 2012; McGranaghan et al., 2014).
Given we appear to be experiencing a prolonged period of abnormally low solar activity, any fluctuation in the thermosphere is more likely due to change in solar EUV emissions, rather than anthropogenic CO2. The EUV component of solar emissions is far more variable than total solar irradiance, so even a small change in solar activity can have a profound impact on the energy budget of layers of the atmosphere which are especially sensitive to EUV flux.