For the second time in a row a Royal Navy test firing of an unarmed Trident missile has ended in failure.

The government says it was an “anomaly” and that it has “absolute confidence” in the nuclear deterrent, but it hasn’t explained why the missile failed.

It’s called the ultimate insurance policy, but can it still deter Russia after two very public failures?

Professor Michael Clarke explains what could have gone wrong, why the government insists we can have confidence, and whether it’s just an embarrassment or a serious worry.