They call them 'chase planes'. They're a fleet of fighter aircraft of the type usually operated by the US Air Force, Navy and Marine Corps. But these fast jets have a different mission - helping turn pilots into astronauts, and keeping a watchful eye on Nasa's spacecraft and experimental aircraft.
The video you see here, released on 27 May, shows the pilot of an F-15D Eagle fighter manoeuvring behind a KC-135 tanker; pilots are trained to use these 'aerial petrol stations' to top up their fuel tanks on long-range missions.
The procedure requires skill; the fighter pilot has to bring his aircraft behind the much slower tanker, and position the jet so that the tanker's boom - the flexible hose which the fuel travels down - slots into the F-15's fuel tank. In the KC-135, a boom operator lies prone at the back of the plane, making adjustments to the probe if needed.
So why does Nasa need these fast jets, the kind normally only fielded by air forces with deep pockets? Chase planes have been part of the Nasa inventory since the early days; in the 1960s, front-line fighters such as the rocket-shaped F-104 Starfighter were modified to test high-speed and high-altitude flying.
The chase planes are often used to monitor other Nasa vehicles in flight; the pilots are an extra pair of expert eyes during test flights. They also serve as camera platforms. Nasa has operated a fleet of T-38 Talon trainers (the same kind used to train USAF pilots) since the 1960s; T-38s used to shadow the Space Shuttle when the spaceplane landed.
The fleet currently includes F-15s and the US Navy's F/A-18, both of which are used to record test flights, but also to keep future astronauts' flying skills maintained.
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