MADRID — A glitch in an avionics box on Europe’s fourth ATV unmanned cargo vehicle discovered during testing will force a replacement and retesting of the box, and that will force the 20,000-kilogram vehicle to miss its planned April launch date to the international space station, the ATV-4 mission manager said Feb. 22.

In a briefing with journalists, Alberto Novelli said the station’s traffic management has openings in May or June, each lasting several days, and that the ATV-4 should be able to make one of these launch windows.

But Novelli said the final tests of the box were still continuing as of Feb. 22 and that a firm launch date would await completion of the tests. An internal NASA launch schedule shows a no-earlier-than date of May 7 for the mission.

ATV-4, dubbed Albert Einstein, arrived at Europe’s Guiana Space Center in French Guiana, on the northeast coast of South America, in September. Novelli said the launch campaign for ATV-4 is slightly longer than the previous vehicle because, for the first time, the ATV will be carrying a full complement of water.

Novelli said it was not immediately clear what caused the defect in the avionics box, which had undergone testing in Europe and at the spaceport.

Peter B. de Selding was the Paris Bureau Chief for SpaceNews.