• Inside Rosetta's Comet

    From baalke@1:2320/100 to sci.space.news on Fri Feb 5 21:43:37 2016
    From Newsgroup: sci.space.news


    http://sci.esa.int/rosetta/57307-inside-rosetta-s-comet/

    Inside Rosetta's comet
    European Space Agency
    04 February 2016

    There are no large caverns inside Comet 67P/Churyumov-Gerasimenko.
    ESA's Rosetta mission has made measurements that clearly demonstrate
    this, solving a long-standing mystery.

    Comets are the icy remnants left over from the formation of the
    planets 4.6 billion years ago. A total of eight comets have now
    been visited by spacecraft and, thanks to these missions, we have
    built up a picture of the basic properties of these cosmic time
    capsules. While some questions have been answered, others have
    been raised.

    Comets are known to be a mixture of dust and ice, and if fully
    compact, they would be heavier than water. However, previous
    measurements have shown that some of them have extremely low
    densities, much lower than that of water ice. The low density
    implies that comets must be highly porous.

    But is the porosity because of huge empty caves in the comet's
    interior or it is a more homogeneous low-density structure?

    In a new study, published in this week's issue of the journal
    Nature, a team led by Martin P|ntzold, from Rheinische Institut
    f|+r Umweltforschung an der Universit|nt zu K||ln, Germany, have
    shown that Comet 67P/Churyumov-Gerasimenko is also a low-density
    object, but they have also been able to rule out a cavernous interior.

    This result is consistent with earlier results from Rosetta's
    CONSERT radar experiment showing that the double-lobed comet's
    'head' is fairly homogenous on spatial scales of a few tens of metres.

    The most reasonable explanation then is that the comet's porosity
    must be an intrinsic property of dust particles mixed with the ice
    that make up the interior. In fact, earlier spacecraft
    measurements had shown that comet dust is typically not a
    compacted solid, but rather a 'fluffy' aggregate, giving the dust
    particles high porosity and low density, and Rosetta's COSIMA and
    GIADA instruments have shown that the same kinds of dust grains
    are also found at 67P/Churyumov-Gerasimenko <http://blogs.esa.int/rosetta/2015/04/09/giada-investigates-comets-fluffy-dust-grains/>.

    P|ntzold's team made their discovery by using the Radio Science
    Experiment (RSI) to study the way the Rosetta orbiter is pulled by
    the gravity of the comet, which is generated by its mass.

    The effect of the gravity on the movement of Rosetta is measured
    by changes in the frequency of the spacecraft's signals when they
    are received at Earth. It is a manifestation of the Doppler
    effect, produced whenever there is movement between a source and
    an observer, and is the same effect that causes emergency vehicle
    sirens to change pitch as they pass by.

    In this case, Rosetta was being pulled by the gravity of the
    comet, which changed the frequency of the radio link to Earth.
    ESA's 35-metre antenna at the New Norcia ground station in
    Australia is used to communicate with Rosetta during routine
    operations. The variations in the signals it received were
    analysed to give a picture of the gravity field across the comet.
    Large internal caverns would have been noticeable by a tell-tale
    drop in acceleration.

    ESA's Rosetta mission is the first to perform this difficult
    measurement for a comet.

    "Newton's law of gravity tells us that the Rosetta spacecraft is
    basically pulled by everything," says Martin P|ntzold, the
    principal investigator of the RSI experiment.

    "In practical terms, this means that we had to remove the
    influence of the Sun, all the planets rCo from giant Jupiter to the
    dwarf planets rCo as well as large asteroids in the inner asteroid
    belt, on Rosetta's motion, to leave just the influence of the
    comet. Thankfully, these effects are well understood and this is a
    standard procedure nowadays for spacecraft operations."

    Next, the pressure of the solar radiation and the comet's escaping
    gas tail has to be subtracted. Both of these 'blow' the spacecraft
    off course. In this case, Rosetta's ROSINA instrument is extremely
    helpful as it measures the gas that is streaming past the
    spacecraft. This allowed P|ntzold and his colleagues to calculate
    and remove those effects too.

    Whatever motion is left is due to the comet's mass. For Comet 67P/Churyumov-Gerasimenko, this gives a mass slightly less than 10
    billion tonnes. Images from the OSIRIS camera have been used to
    develop models of the comet's shape and these give the volume as
    around 18.7 km^3 , meaning that the density is 533 kg/m^3 .

    Extracting the details of the interior was only possible through a
    piece of cosmic good luck.

    Given the lack of knowledge of the comet's activity, a cautious
    approach trajectory had been designed to ensure the spacecraft's
    safety <http://blogs.esa.int/rosetta/2014/08/01/how-rosetta-arrives-at-a-comet/>.
    Even in the best scenario, this would bring Rosetta no closer than
    10 km.

    Unfortunately, prior to 2014, the RSI team predicted that they
    needed to get closer than 10 km to measure the internal
    distribution of the comet. This was based on ground-based
    observations that suggested the comet was round in shape. At 10 km
    and above, only the total mass would be measurable.

    Then the comet's strange shape was revealed as Rosetta drew
    nearer. Luckily for RSI, the double-lobed structure meant that the
    differences in the gravity field would be much more pronounced,
    and therefore easier to measure from far away.

    "We were already seeing variations in the gravity field from 30
    km away," says P|ntzold.

    When Rosetta did achieve a 10 km orbit <http://blogs.esa.int/rosetta/2014/10/08/go-for-10-km/>, RSI was
    able to gather detailed measurements. This is what has given them
    such high confidence in their results, and it could get even better.

    In September, Rosetta will be guided to a controlled impact on the
    surface of the comet. The manoeuvre will provide a unique
    challenge for the flight dynamics specialists at ESA's European
    Space Operations Centre (ESOC) in Darmstadt, Germany. As Rosetta
    gets nearer and nearer the complex gravity field of the comet will
    make navigating harder and harder. But for RSI, its measurements
    will increase in precision. This could allow the team to check for
    caverns just a few hundred metres across.


    Notes for Editors

    "A homogeneous nucleus for comet 67P/ChuryumovrCoGerasimenko from
    its gravity field <http://dx.doi.org/10.1038/nature16535>,/" by M.
    P|ntzold et al. is published in the journal Nature,
    doi:10.1038/nature16535


    For more information, please contact:

    Martin P|ntzold
    Rheinische Institut f|+r Umweltforschung an der Universit|nt zu
    K||ln, Germany
    Email: [email protected]

    Matt Taylor
    ESA Rosetta Project Scientist
    Email: [email protected]

    Markus Bauer
    ESA Science and Robotic Exploration Communication Officer
    Tel: +31 71 565 6799
    Mob: +31 61 594 3 954
    Email: [email protected]

    SEEN-BY: 154/30 2320/100 0 1 227/0