quote: Op dinsdag 20 december 2016 11:49 schreef Wildekamp het volgende:Mwah.. het was toch beter geweest als we middels Philae DE manier hadden gevonden om op een dergelijk lichaam te landen. Helaas moeten we nu dus naar andere mogelijkheden kijken.
quote: Op dinsdag 20 december 2016 21:20 schreef qajariaq het volgende:Philae langdurig laten meten op het oppervlak had toch wel de kers op de taart geweest neem ik aan.
quote: Op donderdag 22 december 2016 04:15 schreef Outpost76 het volgende:Ik ben nieuw in dit topic en het spijt me als ik dingen ophaal die al besproken zijn....Wat als....Rosetta door ons "onderzoek" over jaren en jaren neerstort op de aarde en daarmee de aarde beschadigd.....?Iedereen maakt zich druk om nu.... Stelletje ego's
quote: Op donderdag 22 december 2016 08:54 schreef rubbereend het volgende:[..]hij zit niet in een baan die hem op aarde brengt
quote: Op vrijdag 30 december 2016 01:14 schreef Outpost76 het volgende:[..]Dat is wat de mensheid nu berekend heeft. Maar wat als hij door onze tussen komst een andere komeet oid tegenkomt? Daardoor van baan veranderd met alle gevolgen van dien? Of die andere komeet oid daardoor veranderd van baan met alle gevolgen van dien...Is er ooit zo ver door gedacht?
quote: Op vrijdag 30 december 2016 01:43 schreef Quyxz_ het volgende:[..]Wat is dat nou weer voor een raar vraagstuk?Het zou net zo goed kunnen dat een dergelijke gebeurtenis door Rosetta's invloed juist is voorkomen.
quote:21 March 2017Growing fractures, collapsing cliffs, rolling boulders and moving material burying some features on the comet’s surface while exhuming others are among the remarkable changes documented during Rosetta’s mission. A study published in Science today summarises the types of surface changes observed during Rosetta’s two years at Comet 67P/Churyumov-Gerasimenko. Notable differences are seen before and after the comet’s most active period – perihelion – as it reached its closest point to the Sun along its orbit. “Monitoring the comet continuously as it traversed the inner Solar System gave us an unprecedented insight not only into how comets change when they travel close to the Sun, but also how fast these changes take place,” says Ramy El-Maarry, study leader The changes, which were either unique transient phenomena or taking place over longer periods, are linked to different geological processes: in situ weathering and erosion, sublimation of water-ice, and mechanical stresses arising from the comet’s spin. In situ weathering occurs all over the comet, where consolidated materials are weakened – such as by heating and cooling cycles on daily or seasonal timescales – causing their fragmentation. Combined with heating of subsurface ices that lead to outflows of gas, this can ultimately result in the sudden collapse of cliff walls, the evidence of which is apparent in several locations on the comet. A completely different process is thought to be responsible for the 500 m-long fracture spotted in August 2014 that runs through the comet’s neck in the Anuket region, and which was found to have extended by about 30 m by December 2014. This is linked to the comet’s increasing spin rate in the lead up to perihelion. Furthermore, in images taken in June 2016, a new 150–300 m-long fracture was identified parallel to the original fracture. Close to the fractures, a 4 m-wide boulder moved by about 15 m, as determined by comparing images taken in March 2015 and June 2016. It is not clear if the fracture extension and movement of the boulder are related to each other or caused by different processes. A substantially larger boulder, some 30 m wide and weighing 12 800 tonnes, was found to have moved an impressive 140 m in the Khonsu region, on the larger of the two comet lobes. It is thought that the boulder moved during the perihelion period, as several outburst events were detected close to its original position. The movement could have been triggered in one of two ways: either the material on which it was sitting eroded away, allowing it to roll downslope, or a forceful outburst could have directly lifted it to the new location.Erosion caused by the sublimation of material, and deposition of dust falling from outbursts, are also thought to be responsible for sculpting the landscape in different ways, either uncovering previously hidden surfaces or depositing material elsewhere. For example, scarps in several smooth plains have been observed to retreat by tens of metres and at a rate of up to a few metres per day around perihelion. "Scarp retreats were observed before on Comet Tempel 1, inferred by comparing images taken during flybys of the comet by NASA's Deep Impact in 2005, and Stardust-NExT in 2011," says Ramy. "What we were able to do with Rosetta was to monitor similar changes continuously, and at a higher resolution. "Our observations additionally tell us that scarp retreat seems to be a common process on comets, specifically in smooth-looking deposits." Furthermore, in the smooth plains of Imhotep, previously hidden circular features, along with small boulders, have been exposed by the removal of material. In one location, a depth of about three metres had been removed, most likely through the sublimation of underlying ices. Changes were also noted in the comet's smooth neck region near the distinctive ripples that were likened to Earth's sand dunes when they were first identified. Close monitoring of the ripple formations showed this location to also display expanding circular features in the soft material that reached diameters of 100 m in less than three months. They subsequently faded away to give rise to new sets of ripples. Scientists speculate that the repeated development of these unique features at the same spot must be linked to the curved structure of the neck region directing the flow of sublimating gas in a particular way. Another type of change is the development of honeycomb-like features noticed in the dusty terrains of the Ma'at region on the comet's small lobe in the northern hemisphere, marked by an increase in surface roughness in the six months leading up to perihelion. Similar to other seasonal changes, these features faded substantially after perihelion, presumably as a result of resurfacing by the deposition of new particles ejected from the southern hemisphere during this active period. The scientists also note that although many small-scale localised changes have occurred, there were no major shape-changing events that significantly altered the comet's overall appearance. Ground-based observations over the last few decades suggest similar levels of activity during each perihelion, so they think that the major landforms seen during Rosetta's mission were sculpted during a different orbital configuration. "One possibility could be that earlier perihelion passages were much more active, perhaps when the comet had a larger inventory of more volatile materials in the past," speculates Ramy. "This documentation of changes over time was a key goal of Rosetta's mission, and shows the surface of comets as geologically active, on both seasonal and short transient timescales," says Matt Taylor, ESA's Rosetta Project Scientist.
quote:21 March 2017Rosetta scientists have made the first compelling link between an outburst of dust and gas and the collapse of a prominent cliff, which also exposed the pristine, icy interior of the comet. Sudden and short-lived outbursts were observed frequently during Rosetta's two-year mission at Comet 67P/Churyumov-Gerasimenko. Although their exact trigger has been much debated, the outbursts seem to point back to the collapse of weak, eroded surfaces, with the sudden exposure and heating of volatile material likely playing a role. In a study published today in Nature Astronomy, scientists make the first definitive link between an outburst and a crumbling cliff face, which is helping us to understand the driving forces behind such events. The first close images of the comet taken in September 2014 revealed a 70 m-long, 1 m-wide fracture on the prominent cliff-edge subsequently named Aswan, in the Seth region of the comet, on its large lobe. Over the course of the following year as the comet drew ever closer to the Sun along its orbit, the rate at which its buried ices turned to vapour and dragged dust out into space increased along the way. Sporadic and brief, high-speed releases of dust and gas punctuated this background activity with outbursts. One such outburst was captured by Rosetta's navigation camera on 10 July 2015, which could be traced back to a portion of the comet's surface that encompassed the Seth region. The next time the Aswan cliff was observed, five days later, a bright and sharp edge was spotted where the previously identified fracture had been, along with many new metre-sized boulders at the foot of the 134 m-high cliff. "The last time we saw the fracture intact was on 4 July, and in the absence of any other outburst events recorded in the following ten-day period, this is the most compelling evidence that we have that the observed outburst was directly linked to the collapse of the cliff," says Maurizio Pajola, the study leader. The event also provided a unique opportunity to study how the pristine water-ice otherwise buried tens of metres inside the comet evolved as the exposed material turned to vapour over the following months. Indeed, after the event, the exposed cliff face was calculated to be at least six times brighter than the overall average surface of the comet nucleus. By 26 December 2015 the brightness had faded by half, suggesting much of the water-ice had already vapourised by that time. And by 6 August 2016, most of the new cliff face had faded back to the average, with only one large, brighter block remaining. In addition, the team had a clear 'before and after' look at how the crumbling material settled at the foot of the cliff. By counting the number of new boulders seen after its collapse, the team estimated that 99% of the fallen debris was distributed at the bottom of the cliff, while 1% was lost to space. This corresponds to around 10,000 tonnes of removed cliff material, with at least 100 tonnes that did not make it to the ground, consistent with estimates made for the volume of dust in the observed plume. Furthermore, the size range of the new debris, between 3 m and 10 m, is consistent with the distributions observed at the foot of several other cliffs identified on the comet. "We see a similar trend at the foot of other cliffs that we have not been so fortunate to have before and after images, so this is an important validation of cliff collapse as a producer of these debris fields," says Maurizio. But what actually led to the cliff suddenly collapsing at this particular moment? An earlier study suggested that both rapid daily changes in heating or longer-term seasonal changes can create thermal stresses that lead to fracturing and subsequent exposure of volatile materials, triggering a rapid outburst that can cause the weakened cliff to collapse. Even though the Aswan cliff region had been experiencing large temperature changes in the months before the collapse, interestingly, the collapse occurred at local night, ruling out a sudden extreme temperature change as the immediate trigger. Instead, both daily and seasonal temperature variations may have propagated fractures deeper into the subsurface than previously considered, predisposing it to the subsequent collapse. "If the fractures permeated volatile-rich layers, heat could have been transferred to these deeper layers, causing a loss of deeper ice," explains Maurizio. "The gas released by the vapourising material could further widen the fractures, leading to a cumulative effect that eventually led to the cliff collapse. "Thanks to this particular event at Aswan, we think that the cumulative effect led by strong thermal gradients could be one of the most important weakening factors of the cliff structure." "Rosetta's images already suggested that cliff collapses are important in shaping cometary surfaces, but this particular event has provided the missing 'before-after' link between such a collapse, the debris seen at the foot of the cliff, and the associated dust plume, supporting a general mechanism where comet outbursts can indeed be generated by collapsing material," says Matt Taylor, ESA's Rosetta project scientist.