A fire broke out at the base of a Japanese H-2B rocket Tuesday during final preparations for launch of an HTV cargo ship to the International Space Station (ISS). Torrents of water were sprayed on the pad and the fire eventually was extinguished, but the launching was postponed pending inspections and work to figure out what repairs might be needed.

The cause of the blaze was not immediately known, but a spokeswoman for the rocket's builder, Mitsubishi Heavy Industries, said no injuries were reported. The HTV cargo ship, carrying replacement batteries for ISS' solar power system and 2.5 tons of crew supplies and equipment, suffered no apparent damage.

But open flame near a rocket loaded with nearly 400,000 pounds of liquid hydrogen and oxygen propellants, along with four solid-fuel strap-on boosters, is potentially catastrophic — and engineers will need to carry out a detailed inspection before the rocket and its ground support equipment can be readied for another launch attempt.

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Liftoff from the picturesque Tanegashima Space Center, just south of Japan's southern Kyushu island, was targeted for 5:33 p.m. EDT (6:33 a.m. Wednesday local time).

A live YouTube feed showing the rocket on the launch pad appeared normal until around 2:05 p.m. EDT (3:05 a.m. local time) when bright orange flame suddenly erupted at or near the base of the rocket. A few moments later, jets of water began soaking the aft end of the booster and its mobile launch stand.

Two framegrabs from a live YouTube feed show fire at the base of a Japanese H-2B rocket and, later, jets of water soaking the pad to extinguish the flames. Nicovideo Video Solutions

Hydrogen gas can burn invisibly, and the water continued to flow well after the visible flames were extinguished. More than 90 minutes after the incident began, a Japanese loudspeaker announcement said "we are trying to extinguish a fire."

Going into Tuesday's launch campaign, the H-2B rocket boasted a flawless record in seven previous flights of HTV space station cargo ships. The space station currently is well supplied and a delay getting the HTV-8 cargo to the lab complex is not expected to cause any hardship for the crew.

But the launch delay Tuesday, depending on how long the rocket is grounded, threatens to throw a wrench into an already tight schedule aboard the ISS.

The station crew was gearing up for two and possibly three spacewalks to install the six new batteries being delivered by the HTV, the third of four sets needed to replace older, less efficient batteries in the lab's solar power system.

The battery replacement spacewalks were expected to begin late this month, sandwiched in between the arrival of three fresh crew members aboard a Soyuz ferry ship September 25, the departure of three others October 3 and the first of five and possibly six spacewalks in the November timeframe to repair an ailing high-energy physics experiment known as the Alpha Magnetic Spectrometer.

Northrop Grumman plans to launch a Cygnus cargo ship October 21 followed by a SpaceX resupply mission in early December. Against that backdrop, Boeing is readying its CST-100 Starliner crew ferry ship for a long-awaited unpiloted test flight to the station last this fall or early winter.