The US Department of Defense (DoD) has announced its intention to produce a new high-yield nuclear gravity bomb, pending Congressional authorization and funding.

The DoD said on Friday that they sought to create a modern variant of the B61 nuclear gravity bomb, to be termed the B61-13. The bomb will have a similar yield to the B61-7, a Cold War-era bomb that can produce explosions of up to 360 kilotons. For context, the bomb that America dropped on Hiroshima at the end of World War II had a 16-kiloton yield, meaning that it produced an explosion equivalent to 16,000 tonnes of TNT. 

The new bombs will slowly phase out older weapons in the US's aging nuclear stockpile, and will be deliverable by modern aircraft, which the DoD says will provide the President with "additional options against certain harder and large-area military targets".

"Today's announcement is reflective of a changing security environment and growing threats from potential adversaries," Assistant Secretary of Defense for Space Policy John Plumb said in a statement. "The United States has a responsibility to continue to assess and field the capabilities we need to credibly deter and, if necessary, respond to strategic attacks, and assure our allies."

While the name "gravity bomb" conjures up sci-fi images of gravity being warped by a nuclear explosion, its alternative names – including the "free-fall bomb" and "dumb bomb" – explain its nature a little better. Simply put, gravity bombs are bombs that are dropped from aircraft to their target, continuing on their ballistic trajectory without any further course corrections from guidance systems. Since guidance systems weren't developed until after World War II, all bombs before then could be referred to as gravity bombs.

The US signed up to the Treaty on the Non-Proliferation of Nuclear Weapons in 1968, which aims to "prevent the spread of nuclear weapons and weapons technology, to promote cooperation in the peaceful uses of nuclear energy and to further the goal of achieving nuclear disarmament and general and complete disarmament". Since signing, the US's stockpile has slowly reduced, though it and Russia have 89 percent of the world's nuclear stockpile between them.

"The B61-13 represents a reasonable step to manage the challenges of a highly dynamic security environment," Plumb added. "While it provides us with additional flexibility, production of the B61-13 will not increase the overall number of weapons in our nuclear stockpile."

[H/T: New Atlas]