SOHO

Key facts

Mission & specifications

Mission

SOHO, the Solar and Heliospheric Observatory, has kept an unblinking watch on the Sun for nearly thirty years. A joint project of the European Space Agency and NASA, it launched on 2 December 1995 and took up station at the L1 point about 1.5 million kilometres sunward of Earth, where it hangs between the two and stares continuously at our star. Its dozen instruments study the Sun from its hidden interior to its outermost atmosphere: by watching the surface ripple, they probe the churning layers inside through the technique of helioseismology; coronagraphs blot out the Sun's bright disc to reveal the faint corona and the great eruptions, called coronal mass ejections, that drive space weather and can threaten satellites and power grids. SOHO became civilisation's solar early-warning system. It is also, unexpectedly, the most successful comet discoverer in history: its coronagraph images have caught more than five thousand sungrazing comets diving toward the Sun, the vast majority spotted by amateur volunteers poring over the data online. The mission nearly ended in 1998 when a series of operator errors sent the spacecraft tumbling and silent, but a painstaking recovery brought it back to life. Built to last two years, SOHO has instead become one of the longest-lived and most beloved space observatories ever flown.

1. 1995Launch toward L1Launch

   The joint ESA/NASA observatory sets off for a vantage point between the Earth and the Sun.

2. 1996Watch on the Sun beginsMilestone

   From the L1 point, SOHO starts its continuous study of the Sun's interior, surface and corona.

3. 1998Lost and recoveredMilestone

   Operator errors send SOHO tumbling and silent; a painstaking months-long effort revives it.

4. 20245,000th comet foundMilestone

   SOHO becomes by far the most prolific comet discoverer in history, most spotted by volunteers.

Images

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