Next week, on 23 September, Rosetta will skip on a three-week outing that will take it adult to 1500 km from a iota of Comet 67P/Churyumov-Gerasimenko, most over than it has been given nearing during a comet in Aug 2014.
The categorical scholarship idea pushing this march of movement is to inspect a coma of 67P/C-G on a broader scale while a comet’s activity is still high in a post-perihelion phase. While roughly all instruments on Rosetta will be handling during a excursion, this scrutiny of a coma during vast will be generally engaging to inspect a plasma sourroundings of a comet with a Rosetta Plasma Consortium (RPC) instruments.
In particular, scientists are aiming during detecting a crawl shock, a operation between a comet’s magnetosphere and a ambient solar wind. The existence of a crawl startle in a comet’s sourroundings around a activity rise was likely in 1967 by Ludwig Biermann, and reliable in a past decades by observations during comets 21P/Giacobini–Zinner, 1P/Halley, 26P/Grigg–Skjellerup and 19P/Borrelly.
“Previous measurements that were achieved during fly-bys usually supposing singular information points about a crawl shocks of a handful of comets. Rosetta, instead, will take information over several days, monitoring a expansion of a plasma sourroundings of 67P/C-G shortly after a perihelion,” says Claire Vallat, a Rosetta Science Ground Segment scientist during ESA’s European Space Astronomy Centre (ESAC).
Along a new trajectory, Rosetta will pierce divided from a iota adult to 1500 km in a instruction of a Sun, where a crawl startle is approaching to be found. This extent widen will be reached by a finish of September, with a booster returning to closer distances by mid-October.
“While it competence seem peculiar to skip from a iota during this time, these measurements are also pivotal to bargain a comet’s poise during vast and contingency be achieved not too prolonged after perihelion, so that a comet is still appreciably active,” adds Claire.
Departure on a 1500-km outing will be kicked off by an early morning thruster bake set for 01:40 GMT (03:40 CEST) on 23 September. The bake commands for a 2.34 m/s incentive pull will be uploaded in allege and Rosetta will be pushed onto a delayed shun path.
After a bake is complete, Rosetta will pierce out from a tide orbit, approximately 450 km from a nucleus, aiming during a farthest prove on a outing with a proviso angle of 50 deg, and nearing during 1500 km from a comet on 30 September. On that date, a booster will be nearing on a morning side of 67P/C-G, over a comet’s southern hemisphere, during -60 degrees latitude.
“Once we’re distant from a comet, we won’t be means to brand landmarks for navigation anymore as we’ll be too distant out. Navigation will be shaped on a integrity of a comet centre in NavCam images,” says Spacecraft Operations Manager Sylvain Lodiot during ESA’s European Space Operations Centre (ESOC).
Having reached a farthest prove on this stretch, Rosetta will control a lapse burn, that will pierce it behind to about 500 km above a comet by 7 October. While a booster is on a excursion, a cometary sourroundings will continue to evolve, though a goal operations group won’t have a firm, present characterisation of a turn of activity, so will make a lapse proceed cautiously.
“We won’t stay during 500 km, though we’ll usually get closer step by step, as we know what’s afterwards function during a comet and recover believe of a activity,” says Lodiot.
As a comet gets closer to a Sun, solidified molecules – including water, CO monoxide and CO dioxide – both on and next a iota aspect sublimate. As a outflowing gases leave a nucleus, they lift dirt particles along and, together, they furnish a comet’s coma.
The molecules in a coma are creatively neutral, though can be nude off of one or some-more of their electrons, so apropos ionised, due to a accumulation of earthy processes in a comet’s environment. The ensuing molecular ions, such as H2O+ and O+, build adult a comet’s magnetosphere and start interacting with a solar breeze – a tide of charged particles and ions issuing from a Sun via a solar system.
The cometary ions, that pierce really solemnly with honour to a high-speed upsurge of a solar wind, are “picked up” by a solar wind, adding some-more and some-more mass to a flow. As a consequence, a solar breeze feels a participation of an obstacle, represented by a active comet, and decelerates gradually, until eventually a hiatus arises with a pointy disproportion of a captivating margin values between a dual plasma environments: a crawl shock.
During fly-bys of formerly visited comets, crawl shocks were rescued during distances of several thousands of km from a nucleus. In 1986, ESA’s Giotto goal totalled a crawl startle around one million km divided from a iota of comet 1P/Halley; later, in 1992, it rescued another crawl startle during a fly-by of comet 26P/Grigg–Skjellerup, about 20,000 km from a nucleus.
“The plcae of a crawl startle depends on a comet’s activity,” explains Hans Nilsson from a Swedish Institute for Space Physics, who is a Principal Investigator of a Ion Composition Analyser – one of a RPC instruments.
“Comet 1P/Halley was most some-more active than 67P/C-G, and a crawl startle was most serve divided than what we design to find with Rosetta. On a other hand, 26P/Grigg-Skjellerup was a comparatively low-activity comet, and a gas prolongation rate during a time of a Giotto confront was identical to that of 67P/C-G during a time of perihelion.”
While Rosetta will not try this distant from a nucleus, a timing of a distant outing – 6 to 8 weeks after perihelion – was designed in such a approach that a crawl startle will be closer to a nucleus.
“Hybrid plasma simulations prove that a crawl startle should have shaped by now, and that we should see it around a thousand km from a nucleus,” explains Christoph Koenders, an RPC scientist from a Institute for Geophysics and Extraterrestrial Physics during a Technische Universität Braunschweig, Germany.
“The accurate plcae of a operation depends on a solar breeze quickness and density, on a comet’s gas prolongation rate and on a interplanetary captivating field, and tiny variations in these parameters competence change it considerably. However, we are assured that we will detect a crawl startle during some prove during a excursion.”
During a distant outing starting on 23 September, RPC scientists are formulation to representation a magnetosphere of 67P/C-G during a operation of distances from a iota that have not been probed yet, measuring a properties of ions and electrons and a captivating margin in a plasma environment. Besides a crawl shock, they design to detect several other transition regions, such as a cometopause and a cometary magnetosheath, as good as some other probable boundaries, that will all uncover a singular signature in any of a measurements.
While a temporal fortitude of a information will be identical to that performed during prior fly-bys of other comets, a spatial fortitude will be softened by several orders of bulk interjection to Rosetta’s most reduce quickness with honour to a comet. In addition, there will be a possibility to inspect temporal variations of a comet’s plasma environment, as a booster will spend poignant time in any segment of a comet magnetosphere.
“Shocks are a entire phenomena in astrophysics and inspect them in situ is a good approach to get during a physics,” says Matt Taylor, Rosetta plan scientist during ESA.
“For example, ESA’s Cluster goal explored a remarkably skinny crawl startle of a possess world a few years ago, divulgence that it is an ideal site for molecule acceleration. Now, Rosetta will concede us to inspect a crawl startle of a really opposite astronomical physique in good detail. Since a conditions during this comet are only on a extent for a crawl startle to form, we will have a possibility to inspect in good fact how these bounds arise.”
Scientists are looking brazen to regulating these information to learn about a arrangement of startle waves and other bounds in a plasma sourroundings of a comet, and to inspect how these impact a send of appetite and movement from a solar breeze to a comet’s atmosphere. Comet 67P/C-G provides a new sourroundings that allows to inspect a communication of a solar breeze in a context that is most opposite from that of a planet.
Source: Rosetta blog