Science: Baby Bombs

The atom bombs of World War II were bulky monsters weighing close to five tons. The B295 that atom-bombed Japan carried only one on each mission. But atom bombs have been getting smaller and handier. The Army’s description last week of its atomic gun gives an idea of how small a modern atom bomb can be.

The gun’s bore is 280 millimeters (11 in.). Since the vital parts of an atom bomb must be roughly spherical, the atomic explosive packed into the gun’s shell is not likely to be much larger than a sphere eleven inches in diameter (a regulation basketball is 8.5 in.).

If it were all uranium—the heaviest metal—such a sphere would weigh about 460 Ibs., but only a small part (one guess: 24 Ibs.) can be uranium or plutonium. There must be a chemical explosive to start the nuclear reaction, and there may be some empty space.

Encasing the early bombs was a large mass of “tamper,” i.e., a heavy metal such as lead or tungsten, whose inertia held the bomb together while the nuclear explosion was getting under way. If the tamper were eliminated, which is possible, the bomb would weigh not much more than an eleven-inch sphere of TNT (about 40 lbs.).

Bombs not intended for cramming into a shell are probably heavier and bulkier for the sake of explosive efficiency, but even bombs to be dropped from planes may be made small and light. A B-36 on an intercontinental mission could carry not one but 50 or more such lightweight bombs.