Nasa scientists have turned off one of the instruments on the near 50-year-old Voyager 2 space probe as its electrical power supply continues to decline.

Voyager 2 was launched in 1977 and has travelled more than 20 billion kilometres from Earth. It continues to use four science instruments to study the region outside the solar system’s heliosphere, the protective bubble of particles and magnetic fields created by the Sun.

While mission engineers have been trying to avoid turning off any of its key instruments for as long as possible, they recently made the decision to shut down the plasma science instrument that is designed to measure the flow of charged atoms in the space surrounding the craft. It has only managed to collect limited data in recent years because of its orientation relative to the direction that plasma is flowing in interstellar space.

The instrument consists of four ‘cups’ – three point in the direction of the Sun and observe the solar wind while inside the heliosphere...