ESA’s Proba-2 micro-satellite joined a flotilla of spacecraft observing deep-frozen Comet Lovejoy’s plunge through the million degree corona enshrouding the Sun, providing a close-up extreme ultraviolet view of the comet passing just 120 000 km from the Sun’s surface – and then, surprisingly, surviving.
Lovejoy had not been predicted to endure its swing by the Sun, but is now headed back out to the colder outer reaches of the Solar System, and should be visible from Earth’s northern hemisphere in mid-January.
Proba-2’s SWAP imager took part in a coordinated effort to track Comet Lovejoy as it came closest to the Sun on 16 December, working along with the ESA/NASA SOHO solar watchdog, Japan’s Hinode mission, NASA’s twin STEREO spacecraft and its Solar Dynamics Observatory.
SWAP showed the comet as a bright streak in the solar corona, with interactions between the comet tail causing brief coronal brightening and wiggles in the comet’s tale. This was only the second time ever that a comet has been observed through an extreme-ultraviolet (EUV) solar telescope. The instrument’s observations – interrupted briefly as Proba-2 crossed behind Earth – show the comet going behind the Sun and then emerging back into view from the other side.
Comets are drawn to the gravitational pull of the Sun like moths to a flame – SOHO has identified thousands of Sun-grazing comets over the last 16 years. But up until now, what happens when a comet draws closest to the Sun has been a mystery.
Comets in the Sun’s neighbourhood usually seen with ‘coronagraph’ telescopes that block out the bright solar disc to observe the faint solar corona they are tuned for. This makes detailed images of comets nearing the Sun very hard to obtain.
It turns out however that EUV imagers that detect the extreme ultraviolet corona from the solar disk can also show the comet. Since the dust and other material making up a comet’s tail do not radiate at EUV wavelengths, this came as a surprise.
As comets are so dim compared to the radiance of the Sun and its corona, the Proba-2 team made a careful calculation of the comet’s path to know where to look, and performed careful processing to make the comet stand out from the bright coronal plasma.
A comet’s tail is formed by the outgoing solar wind, leading it to always point away from the Sun. The Proba-2 images show this tail wobbling, possibly due to localised gusts of solar wind blowing the tail at different speeds near the Sun.
About Comet Lovejoy
Comet Lovejoy is one of a family of comets called Kreutz sun-grazer comets. These are all part of a cloud of debris left over from one large comet that previously broke apart. These comets share an elliptical orbit around the Sun, part of which takes them close enough that many smaller comets don’t survive (unlike Comet Lovejoy).
About Proba-2’s SWAP
Proba-2 is a technology demonstration mission that also hosts scientific instruments, including the Sun Watcher with Active Pixels and Image Processing, SWAP, operated by the Royal Observatory of Belgium. SWAP observes the solar corona, whose outer layers are almost invisible to the naked eye because they radiate in ultraviolet and EUV. SWAP converts EUV to a visible picture, acquiring a new image around once per minute.