After a prolonged silence stretching the best part of 2 years, the UK MoD has published new information about Project AETHER, a concept development initiative supported by the Ministry of Defence’s procurement body, Defence Equipment & Support (DE&S), to explore the use of “pseudo-satellite” platforms flying at extremely high altitude to collect intelligence. The aim is to build a ultra-persistent communication and intelligence, surveillance, and reconnaissance (ISR) collection capability operating in the stratosphere.

The UK MoD has now announced that trials took place in South Dakota, USA, with the involvement of UK company Voltitude in partnership with Landguard Systems (UK) and Aerostar (US). This new experiment was centered on the use of uncrewed, high-altitude balloons which were operated as a constellation for the1st time, in the process ensuring “near-continuous” coverage of a target zone for “nearly a month”, far exceeding the endurance of the single balloons.  

The balloons employed each traveled over 2,000 nautical miles at altitudes between 60,000 and 80,000 feet.

The MoD had last shared information about AETHER’s status in 2023, when the “2nd phase” of the project was formally launched with test flights of solutions put forward by 3 industry partners: Voltitude, Sierra Nevada and Airbus, with the first 2 firms offering solutions based on balloons and Airbus putting forward the ZEPHYR pseudo-satellite, solar-powered drone (originally developed by the MoD semi-in house via Qinetiq before Airbus took over).

Near the end of 2023 it was reported that Sierra Nevada Corporation (SNC) and Airbus had been picked for month-long flight tests. Voltitude clearly also re-entered the frame at some point during the project.

In its press release, the MoD confirms it is still evaluation not only balloons, but also “vehicles capable of carrying significantly higher payloads for mission durations of 6-12 months”.

The UK government revealed earlier this year that it had procured 3 “uncrewed ISR aircraft” but refused to disclose further information so it’s impossible to say if those were ZEPHYRs. The MoD had procured a number of earlier models of the ZEPHYR in the previous decade. Back in 2016 it had also funded multiple projects for innovative, ultra-lightweight sensor and communication relay payloads for the type including wide-area IR, LIDAR for 3D imagery collection and even a thin towed radar array by the University of Birmingham, known as SIMITAR, able to spot targets through dense foliage.  

Back in May, AALTO HAPS Ltd., the Airbus subsidiary based in Farnborough, UK that develops ZEPHYR, set a world-record for global aviation with the drone completing 67 days, 6 hours and 52 minutes of continuous flight in the stratosphere. The mission, launched from Kenya, did however end with the drone lost in the Indian Ocean on 28 April 2025.

BAE Systems also entered the HAPS market with its PHASA-35 solar-powered stratospheric drone which completed flights in 2023 and 2024. The project, developed by PRISMATIC, now sits within the Falcon Works advanced projects office of BAE. With a 35 meters wingspan and 150 kg of weight (including 15 of payload), PHASA has overtaken ZEPHYR in terms of sizes (25 meters and 75 kg) although there had been proposals already years ago for a ZEPHYR “T” twin tailed with 140 kg mass and 20 kg of payload.

It wouldn’t surprise if PHASA-35 was at some point involved in AETHER trials.

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