The invention relates to an unmanned aircraft for telecommunicative or other scientific purposes, which s stationed in determined height, in particular in the stratosphere. The aircraft comprises a gas-filled balloon (11) carrying a platform (10) and means for maintaining the platform position relative to the ground. The balloon (11) carrying theplatform (10) is disposed in the interior of an external balloon (12) of aerodynamical shape, particularly in the stratosphere. At least one low or high pressure insulation chamber (20; 20'; 20'') filled with a medium is arranged between them and encircles the inner balloon (11). The medium used in the insulation chamber is a gas having a low thermal conductivity. The negative effects of the temperature differences are largely compensated so that the inner balloon can be produced from a lighter and cheaper material, thereby increasing durably its longevity.