Photograph: ESA
In the 1990s, European scientists decided to develop a more ambitious mission that would match the orbit of an approaching comet and follow it as it approaches the sun. They also suggested that a smaller spaceship would try to land on the comet. The main two-ton spacecraft was called the Rosetta, which carried a dozen scientific instruments, and its 100-kilogram lander with 9 other instruments was called the Philae.
The Rosetta mission was launched in 2004 and delays in launching the rocket caused it to miss its original target comet, so an alternative destination was chosen, Comet Churyumov-Gerasimenko (named by the two discoverers but commonly referred to as the 67P). The orbital period of this comet is 6.45 years. which makes it a Jupiter family comet.
Since the European Space Agency did not have access to the plutonium nuclear power sources used by NASA for space missions, Rosetta had to run on solar power, which required particularly large solar-panels. However, this was not enough to keep the spacecraft running as it coincided with the orbits of 67P near the comet’s aphelion. The only solution was to shut down all of the spacecraft’s systems and let it coast toward the sun for several years, out of contact with the controllers on earth, until the solar energy was stronger. The success of the mission depended on an automatic timer that turned the power back on when it approached the sun, luckily this strategy worked.
In August 2014, Rosetta began a gradual approach to the comet’s nucleus, which is a strangely deformed object about three miles in diameter that is quite different from the smooth appearance of Halley’s nucleus (but equally dark). Their rotation time is 12 hours. On November 12, 2014, the Philae lander fell, slowly descending for 7 hours, then lightly hit the ground.
It bounced and rolled, coming to rest-under an overhang where there wasn’t enough sunlight to charge the batteries. After operating for a few hours and sending data back to orbiter, Philae was silent. However, the main Rosetta spacecraft continued to operate as the comet’s activity increased with gas steamers jetting from the surface. When the comet approached perihelion in September 2015, the spacecraft withdrew to ensure its safety.
Rosetta’s images (and data from other instruments) far exceed anything that astronomers have seen on a comet. The best image resolution was almost 100 times higher than the best Halley images. On this scale, the comet appears surprisingly rough, with sharp angles, deep holes, and overhangs.
The double-lobed shape of the 67P nucleus was tentatively attributed to the collision & merger of two independent cometary nuclei. The spacecraft confirmed that the comet’s dark surface was covered with carbon-rich organic compounds mixed with sulfides and iron-nickel grains. 67P has an average density of only 0.5g / cm3 (remember that the water in these units has a density of 1 g / cm3). This low density indicates that the comet is quite porous; that is, there is a lot of empty space between its materials.
We already knew that ice evaporation from comets was sporadic and limited to small jets, but this was taken to extremes with Comet 67P. At any point in time, more than 99% of the surface is inactive. Active vents are only a few meters away. wide, with the material being limited to narrow jets that last only a few minutes. The level of activity is heavily dependent on solar heating & increased by a factor of 10 between July and August 2015. The isotope analysis of the deuterium in the water ejected by the comet shows that it differs from the water found on Earth. Therefore, comets like 67P apparently did not contribute to the origin of our oceans or the water in our bodies, as some scientists thought.
