back to article What a pain in the mass! Euro craft Rosetta to poke its probe in 10-BILLION-tonne comet

The European Space Agency's Rosetta probe has made the first measurement of the mass of the comet it's chasing – and has come up with a mass of 10 billion tonnes, plus or minus 10 per cent. comet 67P/Churyumov-Gerasimenko Does my mass look big in this? Rosetta has spent the past ten years, and six billion kilometres, …

  1. Mephistro

    "...a mass of 10 billion tonnes..."

    It would be nice for Elreg to specify whether they're using 'Short Scale' or 'Long Scale' when giving figures like this, or alternatively use always the same one. (In this last case, my vote goes for Long Scale).

    This time they are using Long Scale, i.e 10 billions = 10^13.

    1. Irony Deficient

      when giving figures like this

      Mephistro, if only there were a set of standard SI prefixes which one could use to unambiguously specify an object’s mass … Oh, hang on, we’re in luck — 10 billion (long scale) tonnes could be expressed as 10 Tt (or 10 Eg) without ambiguity.

      1. Mephistro

        Re: when giving figures like this

        "...set of standard SI prefixes which one could use to unambiguously specify..."

        True, but:

        a) Readers need to perform some mental arithmetic. Some will, some won't.

        b) It's only useful for SI measures. Talking about, e.g., 'tera-dollars' or even worse, 'tera-times' sounds freakish, at least to me.

        c) The article used 'billions' without indication of which scale was being used.

        1. Irony Deficient

          Re: when giving figures like this


          a′) If a Commentard of Very Little Brain (like me), who Thinks of Things in US customary units and short-scale enumeration, can equate kilo- with thousand, mega- with million, giga- with short-scale billion, and tera- with long-scale billion — no arithmetic involved, only synonyms — then any other reader here could do so too.

          b′) It’s not unknown in the States to call a unit of one million dollars a “megabuck”. It wouldn’t be too strenuous to extend that to “gigabucks”, “terabucks”, &c. Freakishness is, of course, in the eye of the beholder.

          c′) The linked ESA blog entry noted that it was “1×10^13 kg” — i.e. 10 Pg = 10 Gt = 10 billion (short scale) tonnes.

          1. Mephistro

            Re: when giving figures like this

            "The linked ESA blog entry noted that it was “1×10^13 kg” — i.e. 10 Pg = 10 Gt = 10 billion (short scale) tonnes."

            He he, you got me there. That's a secondary effect of posting at ~5 a.m. and under the influence of insomnia.

            Anyway, I think that this somehow strengthens my point, i.e. that the use of the word billion in some contexts, without an indication of which scale is being used, is quite confusing. The use of derived S.I. units instead of the base ones (tonnes instead of kilograms) only worsens the problem.

            Anyway, this one is on me ;-).

        2. Vic

          Re: when giving figures like this

          Talking about, e.g., 'tera-dollars' ... sounds freakish

          The jokes just write themselves these days...


    2. Mike Bell

      Re: ...a mass of 10 billion tonnes...

      Nearly everyone uses a billion to refer to 10^9 these days.

      As is the case in this article.

      A lump of ice with a volume of one cubic kilometre has a mass of 10^9 tonnes. The comet is only about 10 times this size, and its density won't be wildly different from that of ice, so its mass will be about 10 x 10^9 tonnes. Or ten billion tonnes, as people say these days.

    3. diodesign (Written by Reg staff) Silver badge

      Re: Mephistro

      Billion is always 1,000 million.


      1. StevieB

        Comet chasing

        I think the journey has been longer than 6 million km,

        perhaps more like 6.4 Tm.

        edit: Wow, submitting a correction works faster than posting a comment!!!

        Now I feel a fool.

        See icon.

      2. AbelSoul

        Re: Billion is always 1,000 million.

        But is Always always Always?

      3. Mephistro

        Re: Mephistro (@ Mike Belll & Diodesign)

        "Billion is always 1,000 million."

        Not for about a third of the Earth's population, including some English speaking countries. From the relevant Wikipedia article: "However, in most of these (English speaking) countries, some limited long scale use persists and the official status of the short scale use is not clear."

        I'm only asking for more clarity in the articles. If you don't want to specify which scale is being used, then at least use exponential notation or write the whole number. Not a great deal, is it?

  2. Youngone Silver badge


    So from the ESA's nice graphic, the comet's orbit doesn't seem to go a whole lot further out than Jupiter. I had an idea that comets were from the Oort cloud or the Kuiper belt, so I've learnt something today.

    Thanks ESA.

    1. mtp

      Re: Orbit

      They are originally from the Oort cloud but any comet that comes near Jupiter will have a significant orbital perturbation which moves the outer part of the orbit to something more Jupiter like.

      The relevant word is apogee and I intended to give this as a reference but googling it shows that the word has been seriously corrupted. The original meaning makes the occasional appearances but 90% of them are adverts.

      Luckily a alternative name for this 'Aphelion' is so far uncorrupted although the wikipedia page is Earth centric.

      1. Mike Bell

        Re: Orbit

        @mtp: The clue is in the word suffix. 'gee' pertains to the Earth, as in geography, geosynchronous etc. Since the comet orbits the Sun, not the Earth, apogee is not relevant to the orbital perturbation that you mention. Whereas aphelion most certainly is.

        Apogee: furthest point from the Earth

        Aphelion: furthest point from the Sun

        Perigee: closest approach to the Earth

        Perihelion: closest approach to the Sun

        1. Ben Bonsall

          Re: Orbit

          Apogee: To apologize for a speech impediment

          Perigee: 6 letters later than Perrier

          1. A K Stiles

            Re: Orbit

            What about Apoapsis and Periapsis ? Or should I stop messing about with KSP for a while?

  3. Martin Budden Silver badge

    Duck's arse

    Will Philae land on the flattest part of the comet, i.e. the "underside" of the duck shape?

    1. kryptonaut

      Re: Duck's arse

      Could be risky, that's probably where the squeaker is.

  4. Martin Budden Silver badge


    Philae has no thrusters, so does Rosetta "throw" it at the comet?

    1. Mike Bell

      Re: Chuck-a-duck

      Philae has a cold gas thruster. But it will, more or less, throw itself at the comet.

      1. Martin Budden Silver badge

        Re: Chuck-a-duck

        Useful link, thanks.

        I see that the cold gas thruster is powered by Cologne, which should make the duck smell nice.

  5. John Brown (no body) Silver badge

    The time to worry... when the enormous duck enters Earth orbit, turns itself up on its head and starts making strange communication-like "noises" to talk to it's Earth-bound cousins.

  6. Cryo

    How can you people go on about the use of the term 'billion' and not notice the more glaring error in the article...

    "3.5 by 4 kilometres wide – or to use the Reg standards converter, it's 29 by 25 Brontosauruses."

    This conversion would place the length of a brontosaurus at around 140 meters, when they are actually only estimated to have averaged around 23 meters or so in length. Even their largest known cousins didn't reach half that long. If the Register intends for brontosauruses to become a worldwide standard for measurement, they need to more clearly define their size.

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