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How well was the gravitational field of Philae's landing spot known before the landing? I am referring to the absolute value of the pull, the direction of pull, and the tidal forces.

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Rosetta had earlier this year been doing a triangular pattern of flybys to see how much the comet's mass would deviate it from a straight-line path.

According to this article they had a mass estimate with 10% uncertainty, which tells the overall gravitation to the same uncertainty if 67P is considered as a point mass. They may have refined the mass estimate since then, without Wikipedia keeping up. ;)

With a reasonably good model of the 3-D geometry of the comet body, and assuming relatively homogenous makeup, they could certainly make an estimate of the gravitational force on Philae as it approached the landing site. Both the model and the homogeneity would be subject to some uncertainty as well, though, so -- and this is just me guessing here -- there might be more like 15%-20% uncertainty in the estimate.

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  • $\begingroup$ Thank you Russell. I'm not so sure that a mass estimate with an uncertainty of 10% translates to a gravity estimate of 10%. At 10+ KM out Rosetta could treat the comet as a point source, but the tidal effects at the surface would be huge. So far as we know, the comet is not homogenous and it is far from being a sphere. Even the moon and the Yucatan peninsula have measurably lumpy gravity. That said, +1 for the link! $\endgroup$ – dotancohen Nov 13 '14 at 19:22
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    $\begingroup$ @dotancohen - Tidal effects from a tiny body are tiny. You are conflating tidal effects with non-spherical effects. They are very, very different. It's the non-spherical nature of gravitation that is problematic. $\endgroup$ – David Hammen Nov 13 '14 at 19:26
  • $\begingroup$ I was thinking of delta-g as a function of distance from the surface. Id est the gravity at 1000 meters from the surface may be significantly weaker than the gravity at the surface, percentage-wise (and thus station-keepingwise and navigation-wise). $\endgroup$ – dotancohen Nov 13 '14 at 19:29
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  • $\begingroup$ Thank you. I cannot access the Harvard paywall, but I could download the PDF from the second link. In that paper, Werner et al state that the methods presented cannot be used to model surface gravity. That said, it is a wonderful paper and the Rosetta mission is a unique chance to see this theoretical work be applied. Thank you for the link. $\endgroup$ – dotancohen Nov 13 '14 at 20:07

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