The NASA-ISRO Synthetic Aperture Radar (NISAR) mission detected significant ground deformation on a Himalayan slope before the deadly 2026 ice-rock avalanche in the Nepal-Tibet region, highlighting how satellite radar could potentially help identify unstable mountain areas before a major collapse.
Analysis of NISAR radar observations using Synthetic Aperture Radar pixel-offset tracking found cumulative movement at the same location where the ice and rock mass later collapsed. The data indicates that the steep slope had already been experiencing substantial deformation in the weeks before the disaster.
The observed movement was concentrated in the upper section of the steep slope rather than being evenly distributed across the surrounding landscape. Researchers say this concentration is important because such localized deformation could represent a detectable signal of increasing instability.
Radar measurements showed several metres of displacement in the area that ultimately became the source of the avalanche. Three-dimensional visualisations further highlighted the concentration of movement on the unstable upper slope.
The avalanche subsequently contributed to a devastating flood disaster downstream. When large quantities of ice, rock and debris enter Himalayan river valleys, they can rapidly alter water flow and create hazards that extend well beyond the original collapse site. Communities located considerable distances from the mountain slope can therefore be exposed to the resulting floods and debris flows.
NISAR's observations demonstrate the potential of frequent radar-based monitoring in mountainous regions where terrain is difficult to access and weather conditions can interfere with conventional satellite imagery.
Synthetic Aperture Radar has a major advantage over optical satellite systems because it can collect data through clouds and does not require daylight. This makes radar particularly useful for monitoring remote Himalayan terrain, where persistent cloud cover can make optical observations difficult.
SAR pixel-offset tracking works by comparing radar images acquired at different times and measuring shifts in surface features. In the Himalayan case, this technique revealed substantial cumulative movement during the period preceding the ice-rock avalanche.
Such satellite observations could eventually become part of an additional early-warning system for high-risk mountain regions. Regular radar monitoring could help identify slopes showing unusual movement and provide information that can be combined with geological and ground-based observations.
However, deformation by itself does not establish that a landslide or avalanche is about to occur. Mountain slopes can experience movement without ultimately collapsing. Determining the level of danger requires repeated satellite observations, geological assessments and, where possible, monitoring from the ground.
The findings nevertheless underline the potential importance of systematically monitoring unstable Himalayan slopes from space. Regular access to deformation data could help authorities investigate areas showing unusual movement and assess whether additional precautions are necessary.
Because Himalayan hazards can affect areas on different sides of national borders, researchers also highlight the value of cooperation and information sharing between countries. Satellite-based monitoring could provide an important common source of information for regions exposed to cross-border natural hazards.
The NISAR observations offer a striking example of how space-based radar can reveal changes in remote mountain terrain that may otherwise be difficult to detect. With more frequent monitoring and better integration with ground-based systems, such technology could potentially provide valuable information before dangerous slope failures occur.
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