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    Spider Star

    The human colony on the planet Argo has long explored and exploited the technology left behind by an extinct alien race. But then an archaeology team accidentally activates a terrible weapon...
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  • Praise for Star Dragon

    Spider Star

    "Seldom does a storytelling talent come along as potent and fully mature as Mike Brotherton. His complex characters take you on a voyage that is both fiercely credible and astonishingly imaginative. This is Science Fiction."
    -- David Brin

    "Star Dragon is terrific fare, offering readers a fusion of hard science and grand adventure."
    -- Locus Magazine

    "Star Dragon is steeped in cosmology, the physics of interstellar travel, exobiology, artificial intelligence, bioscience. Brotherton, author of many scientific articles in refereed journals, has written a dramatic, provocative, utterly convincing hard science sf novel that includes an ironic twist that fans will love."
    -- Booklist starred review

    "Readers hungry for the thought-provoking extrapolation and rigorous technical detail of old-fashioned hard SF are sure to enjoy astronomer Brotherton's first novel."
    -- Publishers Weekly

    "Mike Brotherton, himself a trained astrophysicist, combines the technical acuity and ingenuity of Robert Forward with the ironic, postmodern stance and style of M. John Harrison. In this, his debut novel, those twin talents unite to produce a work that is involving on any number of levels. It's just about all you could ask for in a hardcore SF adventure."
    -- Paul di Fillippo, SCI-FI.COM

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What’s Wrong Here? Wolfram Alpha and Roche Limits

June 25th, 2013

Blog reader Iljitsch van Beijnum, inspired by images an an artist made of planets as seen from Earth if they were as far away as our moon, sent me an email about a calculation from Wolfram Alpha:

It occurred to me that we would then be within Jupiter’s Roche limit. So I went to Wolfram Alpha to investigate. But strangely, it reports 709000 km for Jupiter and 880000 for Saturn. And 441200 km for Earth:


That can’t be right, we wouldn’t have a moon…

I love checking these things out quantitatively, and for checking to see if the answers make sense!  That’s thinking like a scientist and a good hard science fiction writer!  If you haven’t come across the term “roche limit” before, it’s the location where tidal forces from a primary body (like a central star, or a planet with moons around it) match the gravitational force from the orbiting body’s own gravity.  Inside that limit, rings would be expected, outside it moons.  It’s a little more complicated than that, but not a lot more complicated.  More more information see my reply:

I saw those pictures a few months back and enjoyed them very much.

You’re correct, Wolfram Alpha is wrong.  Wikipedia has the right expressions you need to do the calculations and some examples:


I don’t use Wolfram Alpha.  I’m not sure where it’s getting this number from, but it’s totally wrong.  Is there a way to see its sources?  I hate black box stuff — it’s hard to trust, especially when it isn’t giving the right answer.  I wonder if it’s answering a different question?

Well, I poked around for a few minutes to satisfy myself Wolfram Alpha is giving wrong answers.  I don’t see how to figure out where it’s getting its information.  Anyone reading who knows the answer?



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