Increased solar activity during the peak of the Sun’s ~11-year cycle causes space junk (and satellites) in low Earth orbit (LEO) to lose altitude and re-enter Earth’s atmosphere faster.

The Sun emits more extreme ultraviolet (EUV) radiation during solar maximum (when sunspot numbers are high).

This heats and expands Earth’s thermosphere (the upper atmosphere layer where LEO objects orbit, roughly 160–2,000 km altitude).

The expanded atmosphere increases density at orbital altitudes, creating greater atmospheric drag on objects.

Drag slows the objects down, lowering their orbits and accelerating decay until they burn up on re-entry.

This effect is well-known in principle, but a May 6, 2026, peer-reviewed study from India’s Vikram Sarabhai Space Centre (ISRO) provided strong new quantification using 36+ years of data on 17 long-lived debris pieces across solar cycles 22–24.

Decay isn’t linear. Below a certain level, altitude loss is gradual. Once sunspot numbers reach roughly 67–75% (about two-thirds) of the cycle’s peak, there’s a sharp “transition boundary” where decay rates increase noticeably. This ties directly to rising thermospheric density.

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Characterizing solar cycle influence on long-term orbital deterioration of low-earth orbiting space debris

“Characterizing solar cycle influence on long-term orbital deterioration of low-earth orbiting space debris” is the title of a peer-reviewed paper published on May 6, 2026, in Frontiers in Astronomy and Space Sciences.

Led by Ayisha M. Ashruf (corresponding author) with co-authors Ankush Bhaskar, C. Vineeth, and T.K. Pant from the Space Physics Laboratory, Vikram Sarabhai Space Centre (ISRO), Thiruvananthapuram, India.

The researchers analyzed 36+ years of Two-Line Element (TLE) orbital data for 17 long-lived space debris objects in Low Earth Orbit (LEO, primarily 600–800 km altitudes). These objects, launched in the 1960s, provided a consistent dataset across Solar Cycles 22, 23, and 24.

They correlated orbital decay rates with solar activity proxies:

• Sunspot Number (SSN)
• F10.7 cm radio flux index
• Supporting EUV flux measurements (e.g., SOHO/SEM)

Key Findings

Non-linear threshold effect: Orbital decay rates increase sharply when SSN reaches ~67–75% of the cycle’s peak value. This marks a “transition boundary” where enhanced solar EUV radiation heats and expands the thermosphere, significantly raising atmospheric density and drag at LEO altitudes.

Cycle-to-cycle trends: Peak decay rates declined progressively from the stronger Solar Cycle 22 to the weaker Cycle 24, mirroring the long-term decrease in solar activity.

Modeling and validation: Ballistic coefficients from earlier cycles + NRLMSIS 2.0 atmospheric density model successfully predicted Cycle 24 decay (with scaling factors) for most objects. Lower-inclination objects matched well; two high-inclination (~99°) objects deviated, suggesting model limitations at high latitudes/polar regions.

Geomagnetic activity: Indices like AE and Dst showed weak correlation with long-term decay, indicating solar EUV-driven thermospheric changes dominate over geomagnetic effects (Joule heating, particle precipitation) on these timescales.

Implications

Improved predictions: The identified SSN/F10.7 threshold enables better forecasting of re-entry times and periods of elevated risk during solar maximum.

Space traffic management: Helps operators plan fuel use for active satellites, refine conjunction assessments, and leverage natural “cleanup” during high solar activity.

Model refinement: Highlights needs for better atmospheric density models, especially for polar orbits, as mega-constellations grow in LEO.

This is the first study to provide such long-term, multi-cycle observational quantification using passive debris (which don’t perform station-keeping maneuvers, making them ideal tracers of drag). An earlier arXiv preprint (2024) was titled “Deciphering Solar Cycle Influence…”.

Published: Frontiers

DOI: 10.3389/fspas.2026.1797886

Authors: Ayisha M. Ashruf, Ankush Bhaskar, C. Vineeth, and T.K. Pant

Abstract

The rapid increase in space debris poses a major threat to sustainable space operations and underscores the importance of understanding long-term drivers of orbital decay. Because debris objects do not perform station-keeping maneuvers, their orbital evolution directly reflects variations in thermospheric density, unlike that of operational satellites. This makes space debris an effective natural testbed for examining the long-term influence of solar activity on atmospheric drag. This study analyzes the impact of solar activity on the decay of 17 LEO debris objects across solar cycles 22, 23, and 24 using Two-Line Element (TLE) data. TLE-derived decay profiles, combined with sunspot numbers (SSN) and the F10.7 index, reveal a threshold: decay rates rise sharply when SSN exceeds ∼67%–75% of its cycle peak, corresponding to increased Extreme Ultraviolet (EUV) fluxes, thermospheric density and atmospheric drag. Peak decay rates declined progressively from cycle 22 to 24, reflecting reduced solar activity. Decay profiles for cycle 24 – predicted using ballistic coefficients from earlier cycles and MSIS 2.0 atmospheric densities – match observations well after applying a scaling factor. However, two high-inclination objects showed significant deviations, suggesting possible MSIS limitations at high latitudes, while lower-inclination objects aligned closely. Moreover, geomagnetic activity indices such as AE and Dst show little correlation with long-term orbital decay rates, suggesting a comparatively weaker role at the timescales examined, for Joule heating and particle precipitation than for solar EUV forcing in driving sustained orbital decay. Overall, the findings support solar EUV-driven thermospheric variability as a primary factor influencing long-term orbital decay and emphasize the need to refine atmospheric models, particularly for polar regions, to improve reentry predictions and satellite mission planning.