speed of sound

"If a projectile is moving through a medium faster than the speed of sound, the medium's molecular structure doesn't have time to respond," Stojkovic says.

They included the "Oreshnik" - a new type of missile which flies at 10 times the speed of sound and - according to Putin - cannot be intercepted.

Putin claimed it travelled at 10 times the speed of sound - and that there are "no ways of counteracting this weapon".

In Friday's address Putin said the Oreshnik hypersonic missiles flew at 10 times the speed of sound and ordered them to be put into production.

"As the shock moves through pockets of cold gas, it travels at hypersonic speeds -- several times the speed of sound in the intergalactic medium of Stephan's Quintet* -- powerful enough to rip apart electrons from atoms, leaving behind a glowing trail of charged gas, as seen with WEAVE," Dr Arnaudova said.