Sen—Rosetta scientists have been wowed by an impressive display of fireworks from Comet 67P/Churyumov-Gerasimenko as it approaches perihelion, its closest point to the Sun.
Activity had been steadily growing as the comet was warmed by the sunlight on the innermost stage of its 6.5-year elliptical circuit of the Sun. Images in recent months have shown it fizzing more and more as it released gas and dust from within.
But on July 29, the most powerful outburst yet seen was captured by the Rosetta spacecraft’s OSIRIS narrow-angle camera, in images just released. It erupted from a region of the neck, dubbed Anuket, between the comet’s two lobes. For just a few minutes, a jet shot out into space. It was so powerful that it managed to push back the solar wind, a stream of electrically charged particles from the Sun that were wrapped around the comet’s nucleus.
Details of the dramatic outburst were revealed by the Rosetta team as they prepared for the climax of the comet’s active phase. Carsten Güttler, OSIRIS team member at the Max Planck Institute for Solar System Research in Göttingen, Germany, said in a statement: “This is the brightest jet we’ve seen so far. Usually, the jets are quite faint compared to the nucleus and we need to stretch the contrast of the images to make them visible—but this one is brighter than the nucleus.”
The pressure sensor on Rosetta’s ROSINA instrument deteted changes in the structure of the coma, or atmosphere, and boosts in the levels of carbon dioxide, methane and hydrogen sulphide. And around 14 hours after the outburst, the GIADA instrument was recording dust hits at a rate of 30 a day, compared to just 1-3 a day in early July.
Comet 67P reaches perihelion on Aug. 13, which by coincidence is the date of the peak of the Perseid meteor shower, dust left by another comet. Though some reports have talked of 67P swinging around the Sun, its closest approach is actually still beyond the orbit of the Earth, at a distance from the Sun of 186 million km (116 million miles. That is, however, considerably closer than when the comet is furthest from the Sun, at a distance of more than 850 million km (528 million miles).
The huge difference in distance raises the temperature of the comet, causing it to release the gas and dust. Rosetta’s chief scientist Matt Taylor told Sen today: “Things are getting interesting, we are seeing much more activity, we are observing outbursts of material, and even feeling the effects at the spacecraft, so actually experiencing trajectory changes due to the impact of the gas in the coma.
“From what we are seeing, there are no massive surprises, things are looking as we expected in terms of overall activity, but the devil is in the detail. We are seeing these short-lived events, such as was blogged today. This recent one was strong enough to push the solar wind away. This was an awesome event for me personally.” (Dr Taylor is a space plasma specialist).
“At this stage of the orbit, we expect the outgassing comet to push away the incident solar wind, but only to distances of 10 km or so. This bubble of gas and dust that was ejected by the comet was so fierce it formed a short-lived cavity in the solar wind at least 184 km from the comet.”
Dr Taylor said the boost in activity was affecting his team’s flight plans for Rosetta, and reducing the chances of restoring contact with the mission’s lander, Philae, on the comet’s surface. He told us: “We are passing thought a period of maximum activity and this basically means we will have to have a pretty large stand-off distance from the comet to enable navigation (around 300-340 km at moment). At this distance it will be difficult to get a signal from the lander.
“In addition we are carrying out science observations all around the comet. If we imagine the orbit of the Rosetta as the circle of a clock face for simplicity, we only can contact from about 9—12. However, we are taking observations all of the way round, in particular we are interested in the 3-9 region, the southern hemisphere, which has only been receiving sunlight in the last few months (since May). So we are only in regions where it is possible to get a signal from Philae for 25 per cent of the time (when we are doing these full circles).
“Anyhow, the main thing is, its tougher than usual getting in contact during the most active part of the comet’s orbit, so we will have better chances after perihelion, once the activity dies down and we can get closer to the comet again. As we have learned from Rosetta, there are always surprises.”
Dr Taylor added wryly: “We just have to hope the comet doesn’t split up—or that one of these outbursts doesn’t hurl Philae off the surface of the comet :) .”
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