Next week, on 23 September, Rosetta will depart on a three-week excursion that will take it up to 1500 km from the nucleus of Comet 67P/Churyumov-Gerasimenko, much farther than it has been since arriving at the comet in August 2014.
The main science goal driving this course of action is to study the coma of 67P/C-G on a broader scale while the comet’s activity is still high in the post-perihelion phase. While almost all instruments on Rosetta will be operating during the excursion, this exploration of the coma at large will be especially interesting to study the plasma environment of the comet with the Rosetta Plasma Consortium (RPC) instruments.
In particular, scientists are aiming at detecting the bow shock, a boundary between the comet’s magnetosphere and the ambient solar wind. The existence of a bow shock in a comet’s environment around its activity peak was predicted in 1967 by Ludwig Biermann, and confirmed in the past decades by observations at comets 21P/Giacobini–Zinner, 1P/Halley, 26P/Grigg–Skjellerup and 19P/Borrelly.
“Previous measurements that were performed during fly-bys only provided limited data points about the bow shocks of a handful of comets. Rosetta, instead, will take data over several days, monitoring the evolution of the plasma environment of 67P/C-G shortly after its perihelion,” says Claire Vallat, a Rosetta Science Ground Segment scientist at ESA’s European Space Astronomy Centre (ESAC).
Along the new trajectory, Rosetta will move away from the nucleus up to 1500 km in the direction of the Sun, where the bow shock is expected to be found. This maximum distance will be reached by the end of September, with the spacecraft returning to closer distances by mid-October.
“While it may appear odd to depart from the nucleus at this time, these measurements are also key to understanding the comet’s behaviour at large and must be performed not too long after perihelion, so that the comet is still appreciably active,” adds Claire.
Departure on the 1500-km excursion will be kicked off by an early morning thruster burn set for 01:40 GMT (03:40 CEST) on 23 September. The burn commands for a 2.34 m/s impulse push will be uploaded in advance and Rosetta will be pushed onto a slow escape path.
After the burn is complete, Rosetta will move out from its current orbit, approximately 450 km from the nucleus, aiming at the farthest point on the excursion with a phase angle of 50 deg, and arriving at 1500 km from the comet on 30 September. On that date, the spacecraft will be arriving on the morning side of 67P/C-G, over the comet’s southern hemisphere, at -60 degrees latitude.
“Once we’re far from the comet, we won’t be able to identify landmarks for navigation anymore as we’ll be too far out. Navigation will be based on the determination of the comet centre in NavCam images,” says Spacecraft Operations Manager Sylvain Lodiot at ESA’s European Space Operations Centre (ESOC).
Having reached the farthest point on this stretch, Rosetta will conduct a return burn, which will bring it back to about 500 km above the comet by 7 October. While the spacecraft is on the excursion, the cometary environment will continue to evolve, but the mission operations team won’t have a firm, up-to-date characterisation of the level of activity, so will make the return approach cautiously.
“We won’t stay at 500 km, but we’ll only get closer step by step, as we understand what’s then happening at the comet and regain knowledge of its activity,” says Lodiot.
As a comet gets closer to the Sun, frozen molecules – including water, carbon monoxide and carbon dioxide – both on and below the nucleus surface sublimate. As the outflowing gases leave the nucleus, they carry dust particles along and, together, they produce the comet’s coma.
The molecules in the coma are originally neutral, but can be stripped off of one or more of their electrons, thus becoming ionised, due to a variety of physical processes in the comet’s environment. The resulting molecular ions, such as H2O+ and O+, build up the comet’s magnetosphere and start interacting with the solar wind – a stream of charged particles and ions flowing from the Sun throughout the solar system.
The cometary ions, which move very slowly with respect to the high-speed flow of the solar wind, are “picked up” by the solar wind, adding more and more mass to its flow. As a consequence, the solar wind feels the presence of an obstacle, represented by the active comet, and decelerates gradually, until eventually a discontinuity arises with a sharp difference of the magnetic field values between the two plasma environments: the bow shock.
During fly-bys of previously visited comets, bow shocks were detected at distances of several thousands of km from the nucleus. In 1986, ESA’s Giotto mission measured a bow shock around one million km away from the nucleus of comet 1P/Halley; later, in 1992, it detected another bow shock during its fly-by of comet 26P/Grigg–Skjellerup, about 20,000 km from the nucleus.
“The location of the bow shock depends on the comet’s activity,” explains Hans Nilsson from the Swedish Institute for Space Physics, who is the Principal Investigator of the Ion Composition Analyser – one of the RPC instruments.
“Comet 1P/Halley was much more active than 67P/C-G, and the bow shock was much further away than what we expect to find with Rosetta. On the other hand, 26P/Grigg-Skjellerup was a relatively low-activity comet, and its gas production rate at the time of the Giotto encounter was similar to that of 67P/C-G at the time of perihelion.”
While Rosetta will not venture this far from the nucleus, the timing of the far excursion – six to eight weeks after perihelion – was planned in such a way that the bow shock will be closer to the nucleus.
“Hybrid plasma simulations indicate that a bow shock should have formed by now, and that we should see it around a thousand km from the nucleus,” explains Christoph Koenders, an RPC scientist from the Institute for Geophysics and Extraterrestrial Physics at the Technische Universität Braunschweig, Germany.
“The exact location of the boundary depends on the solar wind velocity and density, on the comet’s gas production rate and on the interplanetary magnetic field, and small variations in these parameters might shift it considerably. However, we are confident that we will detect the bow shock at some point during the excursion.”
During the far excursion starting on 23 September, RPC scientists are planning to sample the magnetosphere of 67P/C-G at a range of distances from the nucleus that have not been probed yet, measuring the properties of ions and electrons and the magnetic field in the plasma environment. Besides the bow shock, they expect to detect several other transition regions, such as a cometopause and a cometary magnetosheath, as well as some other possible boundaries, which will all show a unique signature in each of the measurements.
While the temporal resolution of the data will be similar to that obtained during previous fly-bys of other comets, the spatial resolution will be improved by several orders of magnitude thanks to Rosetta’s much lower velocity with respect to the comet. In addition, there will be a chance to study temporal variations of the comet’s plasma environment, as the spacecraft will spend significant time in each region of the comet magnetosphere.
“Shocks are a ubiquitous phenomena in astrophysics and studying them in situ is a great way to get at the physics,” says Matt Taylor, Rosetta project scientist at ESA.
“For example, ESA’s Cluster mission explored the remarkably thin bow shock of our own planet a few years ago, revealing that it is an ideal site for particle acceleration. Now, Rosetta will allow us to study a bow shock of a very different celestial body in great detail. Since the conditions at this comet are just on the limit for the bow shock to form, we will have a chance to investigate in great detail how these boundaries arise.”
Scientists are looking forward to using these data to learn about the formation of shock waves and other boundaries in the plasma environment of a comet, and to examine how these affect the transfer of energy and momentum from the solar wind to the comet’s atmosphere. Comet 67P/C-G provides a new environment that allows to study the interaction of the solar wind in a context that is much different from that of a planet.