By now I’m sure you have all heard about Rosetta, and the long awaited arrival at its destination, and you’ve all seen the stunning images of comet 67P/Churyumov-Gerasimenko that are being returned to Earth, giving an unprecedented hires insight to a piece of our ancient history. Rosetta’s arrival to its comet is one of those rare happy occasions, when Space achievements are widely reported in general press. Tons of scientific, and not-so-scientific ;-), reports have already been posted all over the internet and dozens of arguably more qualified than me people have explained most of what is to be explained.
I was there at ESA’s ESOC facilities in Darmstadt, Gremany to witness the moment when at 09:30 UTC, the ground based stations received the signal confirming the successful execution of the first of a series of maneuvers that will eventually bring the spacecraft into a 50 klm orbit around the comet. You can view an animation of Rosetta’s maneuvers planned for the coming weeks here. Although clearly the arrival at the comet was not the biggest achievement in this project, I chose to be there because it felt to me as a big moment. A moment to pause and reflect. Reflect on what was done, what it took to do it, and how this fits into the history of the world, into the story of us.
Exploring a comet on location (in situ, as the scientists call it) was a dream conceived three decades ago, inspired by the one of the most fundamental human needs, the need to understand, to explain and thus to explore. Composed from the primordial ingredients of the early Solar System, comets have been roaming interplanetary space since its birth unchanged “by the elements” or by time. They are hoped to contain the answers to our questions. Here is a video created by NASA JPL explaining more on why we think comets are important.
Since the beginning, the Rosetta project had to endure scientific skepticism, political turmoil, financial uncertainties, and unprecedented technological challenges. Yet it succeeded. It survived NASA’s withdrawal from the project, a last minute change in target-comet selection, and a most-feared 2-year-long hibernation during its 10 year and 6.4 billion kilometer journey into the cold inhospitable Solar System. Constructed on what is now considered obsolete technology, by engineers who have long since retired, Rosetta proved to be a marvel of engineering and a triumph of knowledge transfer and process continuity between generations of team members. It became the little space-engine that could, and in the words of ESA Director General Jean-Jacques Dordain, somewhere between Jupiter and Mars, Rosetta’s drivers proved to be the best drivers in the world.
While the bulk of the engineer’s work is now completed, the mission is far from an end. As eloquently put by ESA representatives, this is not the beginning of the end, but the end of the beginning. The scientists’ work is only now starting. I was amazed to hear how must science has already been done on the comet. Comet 67P-Churyumov-Gerasimenko, which is not even the comet originally targeted for, is an unexpected jewel. It’s rubber-duck-like shape indicates that it might actually consist of two pieces (the head and the body) fused together, providing to be a buy-one-get-one-free bonus for research.
Named after astronomers Klim Churyumov and Svetlana Gerasimenko, who discovered it in 1969, comet 67P is one of numerous short period comets which have orbital periods of less than 20 years and a low orbital inclination. Since their orbits are controlled by Jupiter’s gravity, they are also called Jupiter Family comets. These comets are believed to originate from the Kuiper Belt, a large reservoir of small icy bodies located just beyond Neptune. As a result of collisions or gravitational perturbations, some of these icy objects are ejected from the Kuiper Belt and fall towards the Sun.
67P has been observed from Earth on seven approaches to the Sun – 1969 (discovery), 1976, 1982, 1989, 1996, 2002 and 2009. As most of these comets, 67P is pitch-black and not reflective, so very difficult to observe. As you are admiring the spectacular photographs, keep in mind that they are false-color images composed for better comprehension of the very subtle color variations observed on the surface. ESA is releasing new images from Rosetta’s navigational camera on a daily basis.
Rosetta scientists have already been able to determine that the comet’s rotation is stable with little or no wobbling, on a tilted rotational axis running down between the head and the body of comet. The spacecraft began feeling the comet’s gravity two weeks ago. Not being a spherical object, 67P’s gravity is not pulling exactly surface-down on most locations, which will prove a challenge for the second stage of the experiment, a hard-docking type of landing on the surface, which is planned for November this year. In the next weeks the comet’s mass and gravity will be precisely calculated by measuring the bending of the spacecraft’s triangular trajectory.
Although still at a distance of more than 140 million klm from the Sun, 67P is already active, venting water gas at a rate of 0.3kg/sec which is the equivalent of a soda can of water per second. Water spectra from the tail of the comet show a Doppler redshift, indicating the direction and speed at which it is moving away from the Earth. They also reveal very little gas activity on the dark side, i.e. the side facing away from the Sun.
The next challenge is choosing a suitable place to land! The computer graphic below was created from Rosetta OSIRIS and NavCam images and shows the first estimations of possible landing sites. Yellow and orange indicate what are so far considered the best landing areas, while the green circles and ellipses mark provisional landing sites. The red areas are parts of the comet’s northern hemisphere in constant sun: the Comet 67P/Churyumov–Gerasimenko summer. The blue areas are now in constant darkness, either due to the weird shape of the comet and/or the side being in the Comet 67P/Churyumov–Gerasimenko winter.
It was originally envisaged that the nucleus of the comet would be rock hard, and so in order to avoid re-bouncing on impact, the lander Philae is designed to immediately transform all kinetic energy in its legs to electric energy, while its harpoons immediately drill in the comet’s surface to secure the lander in position. Now that 67P is within reach, its surface is thought to be more soft than originally expected. The max temperature measured indicates a surface that is porous and has little water. Dust particles from the comet have also started to be analysed, revealing a density equal to or perhaps even higher of that predicted by the models.
But landing on the comet is not the primary objective of Rosetta. And it also is not a sure thing, especially now that 67P has revealed itself to be a more complex structure than ever dreamed of. Rosetta will be orbiting the comet for the next two years as it gets closer and closer to the Sun. Rosetta’s instruments will be measuring the comet’s transformation getting all the data required from the orbiter. Any measurements obtained from the lander, if it manages to successfully dock, will be an added bonus.
Following the tales of this little spacecraft will be a fabulous journey. In an era when space exploration looks rather stagnant, Rosetta is giving us back the hope that the best is yet to come.
Credit for all images on this post belongs to ESA and the Rosetta instrument teams.