- Satellite imagery shows some Himalayan slopes undergo measurable ground movements before catastrophic collapse.
- Remote sensing involves huge volumes of data, even for a target area, yet these warning signs were not enough to predict when the slope would finally collapse in the case of the recent Nepal disaster.
- Experts call for better localised forecasts and rigorous monitoring that can contribute to effective early warning and early action.
On October 11, 2026, a flash flood tore through Nepal’s Budhi Gandaki river basin in Gorkha district, hurling sediment and debris downstream, destroying homes and roads, and leaving many others homeless.
The disaster struck less than seven weeks after a catastrophic collapse of rock and glacier ice on Langtang Lirung on August 26, which unleashed a destructive surge of ice and debris through the Gyirong-Rasuwa corridor along the Nepal-China border.
As Nepal tried to cope with the second disaster, a group of scientists was about to release a synthesis report. They reported that satellite signals indicated the mountain slope involved in the August Langtang Lirung collapse had moved measurably before it collapsed.
Scientists are now exploring whether such satellite observations could be translated into reliable early warnings of catastrophic slope failures and save lives.
Clairvoyance from space
ICIMOD scientists highlighted two sets of satellite observations in their synthesis report. The first, from Sentinel-1 radar, revealed that ground shifted by up to 40 cm along the satellite’s line of sight on the slopes of Langtang Lirung between July 7 and August 24, and that the slopes subsequently collapsed. Similarly, NISAR satellite radar recorded up to 30 cm of displacement in the same area between July 26 and August 19, they added.
Both satellites use radar to detect changes in Earth’s surface and can see through clouds and darkness, which matters in the monsoon-bound Himalayas. Scientists use a technique called Synthetic Aperture Radar Interferometry (InSAR) to measure ground movement by comparing radar images captured at different times.
The two satellites use radar signals of different wavelengths, so they pick up different details. The European Union’s Copernicus Sentinel-1 mission uses a constellation of two satellites equipped with C-band radar instruments with 5.5 cm wavelength signals that capture images of Earth’s surface, day and night, in virtually all weather conditions.
NASA-ISRO Synthetic Aperture Radar (NISAR) carries two radar systems: L-band from NASA, operating at a wavelength of about 25 cm, and S-band from ISRO, with a wavelength of about 10 cm. Together, they provide complementary observations of changes on Earth’s surface, including ground deformation.
“The satellite can be programmed to look specifically at a land area of interest. Later it can look again to see how much it has changed.” Chaitra Rao, NISAR project director at ISRO, Bengaluru, told Mongabay India. Such close observations matter for climate change and disaster management studies, Union Minister of State for the Department of Space Jitendra Singh noted at the time of launch.
Glacier movement
ICIMOD scientists found no direct evidence linking rapid glacier melt to the Gyirong-Rasuwa disaster; the event appears to show a broader pattern. As glaciers retreat, they expose bare rock and sediment that absorb more solar heat, potentially contributing to destabilisation of surrounding slopes.
The scientists also highlighted how loose debris can intensify such disasters. Debris flows can snowball as they sweep up material along their path. Loose material along the Gyirong-Rasuwa flow path may have added to the initial mass of ice, rock and sediment as it thundered downhill, swelling into a massive, fast-moving torrent, the report suggested.
Can satellites actually foretell disasters?
Research in the Himalayas suggests that satellites can detect signs of instability that precede some catastrophic mountain collapses. Before the 2021 Chamoli disaster in Uttarakhand, for instance, the rock mass that eventually collapsed had shown signs of gradual movement and fracturing over several years. Yet, these warning signs were not enough to predict when the slope would finally collapse.
Weather forecasting challenges add another layer to the problem, as Abhilash S., head of the department of atmospheric sciences at the Cochin University of Science and Technology, told Mongabay-India. He explained that heavy rainfall, warming temperatures, changing snow loads and thawing permafrost can influence mountain stability.
ICIMOD synthesis suggests that rock-and-ice collapses do not always have identifiable short-term weather triggers. They may result from the gradual destabilisation of mountain slopes, even without extreme rainfall or abrupt temperature changes.
The Himalayas’ geological instability, changing climate, glacier retreat, extreme rainfall, avalanches, glacial lake outburst floods (GLOFs), landslides and unregulated development interact to shape future disaster risks.
“Real-time monitoring systems should be developed to track glacier movement, lake levels, rainfall, river levels and ground displacement in high-altitude areas,” Abhilash said. “Technologies such as satellites, radar, automatic weather stations, river gauges and GNSS should be integrated with local monitoring systems.”
Challenges ahead
Not every dangerous slope accelerates visibly before failure, and detected movement does not necessarily mean collapse is imminent. “Repeated satellite observations can detect surface displacement, fractures, and other signs of slope instability. They can also cover inaccessible mountain terrain. These observations help identify slopes that need closer investigation and monitoring,” Prashant Baral, cryosphere analyst at ICIMOD, told Mongabay India.
ICIMOD’s preliminary findings for Langtang Lirung also report deformation before failure. This does not necessarily mean that those observations could have supported an operational warning.
“Satellite monitoring has limitations,” Baral explained. “Clouds affect optical imagery. Steep terrain, changes in snow and ice, and viewing geometry can limit radar measurements. Gaps between satellite acquisitions and delays in processing can also affect timely detection.”
That means satellite observations need to be combined with expert assessment, targeted ground monitoring, and clear procedures for protective action. “Additionally, it is important to understand that a slope cannot be considered safe simply because no movement has been detected,” Baral cautioned.
While scientists at the National Remote Sensing Centre (NRSC), part of ISRO, gather, process and disseminate remote-sensing satellite and aerial data products, their real-world use in disaster detection also faces practical challenges, Rao pointed out. She explained, “The volume of observations is enormous, but analysing data for identified disaster-prone areas is more manageable.”
What improvements are most urgently needed?
Scientists say they face two connected challenges: identifying unstable slopes before they fail, and anticipating how a collapse high in the mountains could become a destructive avalanche, debris flow or flash flood far downstream.
“Climate is often said to have no political boundaries. The same is true of disasters. The Hindu Kush-Himalayan region, stretching more than 3,500 kilometres from Afghanistan to Myanmar, connects several countries,” Abhilash said.
“Changes occurring in a glacier or river basin in the high mountains can affect the lives and infrastructure of people in countries downstream. Therefore, countries such as India, Nepal, Bhutan, China and Bangladesh must share timely information on weather, the cryosphere, glaciers, glacial lakes, river flows, seismic activity and disasters. International observation and information-sharing systems coordinated by the World Meteorological Organization (WMO), as well as regional cooperation mechanisms such as ICIMOD, need to be strengthened.”
“Potentially dangerous slopes need regular screening,” Baral said. “Priority locations need continuous monitoring. Seismic instruments, ground-based sensors, cameras and river monitoring can help detect collapses and track downstream flows. The appropriate combination will depend on the site. Hazard assessments should identify which settlements, roads and hydropower facilities could be affected. They should also estimate how quickly the flows could reach these locations.”
“Warning responsibilities must be clear. Information must be shared quickly across borders,” Baral added. Systems need reliable communications, backup power, and regular maintenance. Communities and workers need identified evacuation routes and regular drills. Alert thresholds and response procedures must be tested under operational conditions.”
“What the Himalayas need, therefore, is an integrated regional system that goes beyond disaster prevention to focus on anticipating risks, monitoring hazards, sharing information, issuing timely warnings and safely evacuating vulnerable populations,” Abhilash concluded.
Banner image: A rescue worker near excavators at the core disaster-hit area at Gyirong Port following the August 26 mudslide on September 6, 2026. (Tingshu Wang/Pool photo via AP)
