Thermobaric weapons, sometimes called vacuum bombs, use powdered metal to create a rapidly expanding fireball. Under international law, they must only be used against military targets, not civilians.
Update on 10 March: According to the UK Ministry of Defence, Russia has confirmed it has used the TOS-1A weapon system, which uses thermobaric rockets, in Ukraine.
The Russian invasion of Ukraine seems to be escalating, with the reported use of highly destructive thermobaric weapons. Although this hasn’t been confirmed, Oskana Markarova, Ukraine’s ambassador to the US, told reporters on 28 February that Russia is using the weapons and a number of images posted online have shown specialist thermobaric launcher vehicles in the Russian invasion force.
The use of these weapons in populated areas would be a violation of international humanitarian law according to Marc Garlasco, an adviser to Netherlands-based peace organisation PAX.
Normal high explosives are molecules that break apart on detonation, releasing energy. In contrast, thermobaric explosives typically involve powdered metal that burns in air, producing an intense rapidly expanding fireball with a shockwave at its leading edge as the explosive reaction continues outwards.
Thermobarics are sometimes called vacuum bombs because after the initial explosion, a low-pressure zone sucks back towards the explosion site. This push-pull effect can be highly damaging to structures, but the main reason thermobaric explosions are so destructive is the prolonged outwards pressure wave.
This is far more dangerous than the brief pulse produced by high explosives and causes distinct injuries, typically in the lungs where the pressure wave can destroy delicate air sacs or cause a massive embolism, leaving victims dead with no signs of external damage.
A prolonged thermobaric blast can go also around corners, making it effective against bunkers, trenches and tunnels, which would provide shelter from a normal blast.
Images posted by Ukrainian civilians seem to show that Russia has deployed the TOS-1 Buratino, a tracked vehicle equipped with 24 rockets with thermobaric warheads. One salvo from a single TOS-1 will cover an area measuring 200 by 300 metres.
“Russia has a record of using thermobarics against cities and towns in Syria, causing widespread civilian harm,” says Garlasco, who has studied the effects of explosive weapons on civilian areas. For example, Russian attacks reportedly using unguided thermobaric rockets in Eastern Ghouta killed dozens of civilians in 2018, he says.
Garlasco says that international humanitarian law means that weapons must be used against distinct, military targets, and must be used proportionally so that the any harm is in accordance with the military gain.
“It is hard to understand how the TOS-1 can be used lawfully in a city,” says Garlasco. He calls the vehicle “a war crime on tracks”.
Garlasco notes that Western armies have used smaller, individual thermobaric weapons in Iraq and elsewhere, but these were deployed against specific military targets such as caves and buildings in urban combat.
“The TOS-1 is an indirect fire rocket with an area effect that has the potential to cause widespread death and destruction in towns and cities. Such weapons should never be used in areas with concentrations of civilians due to the likelihood of civilian harm,” says Garlasco.
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