W HEN, IN EARLY NOVEMBER, INS Arihant surfaced off India’s east coast, its submariners breathed in the claggy air of the Bay of Bengal for the first time in almost three weeks. They may also have breathed a sigh of relief. Although the Indian authorities are understandably cagey about the details, the sub is believed to have carried as many as a dozen nuclear missiles through the Indian Ocean, in all probability around the southern tip of India and into the shallower waters of the Arabian Sea close to Pakistan.

The Arihant’s inaugural voyage was a triumphal step forward in India’s long, often tortuous quest to deploy atomic weapons at sea. Until now India has relied on aircraft armed with nuclear bombs, which might struggle to break through air defences, or land-based missiles, which are at risk of being spotted by gimlet-eyed satellites. Hiding missiles in the ocean solves these problems, giving India more confidence that its forces could survive a nuclear attack from China or Pakistan, and hit back.

But managing such weapons is not easy. One difficulty is ensuring that a submarine can receive orders without giving away its location. India has been building low-frequency radio stations, which use large antennas to propel signals underwater, for this purpose. Yet these are also vulnerable to attack, which is why some nuclear-armed states use airborne transmitters as well.

A second hitch is that the K -15 missiles aboard the Arihant can only fly a puny 750km, which means that the submarine would have to park itself dangerously close to China’s coastline to have a hope of striking big cities. Longer-range missiles, which could be fired from the safety of Indian waters, are in the works. But bigger missiles, and more of them, necessitate a bigger hull. That, in turn, requires that the nuclear-powered subs be fitted with bigger reactors—a fiendish technical challenge.

A third problem is keeping the Arihant safe. Nuclear submarines can only do their job if they can slip silently out of port and into the oceans. They are typically chaperoned by leaner attack submarines. But admirals complain that the navy, whose share of the defence budget has dwindled to 15%, has just 13 of these. The delivery of new French attack subs has been delayed.

Meanwhile India’s nuclear arsenal is swelling. A recent report by the Bulletin of the Atomic Scientists, a research organisation, estimates that it has 130-140 nuclear warheads, with enough fissile material for 60-70 more. The stockpile, though smaller than Pakistan’s and half the size of China’s, has roughly doubled since 2010. Many of the new warheads will go to sea. A second nuclear submarine, the Arighant, is nearing completion, and a third is in the works. Others are expected to follow. Indian sailors should enjoy the fresh air while they can.