# Mars will get a close-encounter with a comet this weekend

Posted 5 years ago
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 This Sunday, October 19th, Comet C/2013 A1 (Siding Spring) will make a very close flyby of Mars. Many of the orbiters and rovers on Mars will hopefully be making some amazing photos of the comet as it passes by. When originally discovered last year, it looked like the comet might impact Mars, but as the orbit was refined, it was found that it would pass within a couple hundred thousand miles of Mars. Close enough that some of the orbiters around Mars are being repositioned to avoid possible impacts from cometary debris during closest approach.You can query Wolfram|Alpha to see an orbit diagram. Another alternative is to use the Wolfram Language to construct your own 3D graphic. Graphics3D[{ColorData[1][1], PlanetData[PlanetData[], "OrbitPath"], Orange, CometData["CometC2013A1SidingSpring", "OrbitPath"], Red, PointSize[.02], Point[QuantityMagnitude[ EntityValue["Planet", EntityProperty["Planet", "HelioCoordinates", {"Date" -> DateObject[{2014, 10, 19}]}]], "AstronomicalUnit"]], Point[QuantityMagnitude[ CometData["CometC2013A1SidingSpring", EntityProperty["Comet", "HelioCoordinates", {"Date" -> DateObject[{2014, 10, 19}]}]], "AstronomicalUnit"]]}, PlotRange -> 2, Axes -> True] The orbits of the inner planets are shown in blue and the comet's orbit is shown in orange. Its hard to tell visually because they are so close, but the comet and Mars are nearly on top of each other and look like one point here.
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Posted 5 years ago
 When I execute this I get this error message: Coordinate QuantityMagnitude[Missing["UnknownQualifierSyntax"], "AstronomicalUnit"] should be a triple of numbers, or a Scaled form. and a pink box'd result without the planets in the graphic:The problematic portion of the code is QuantityMagnitude[ EntityValue["Planet", EntityProperty["Planet", "HelioCoordinates", "Date" -> DateObject[{2014, 10, 19}]]], "AstronomicalUnit"] which, for me, returns {QuantityMagnitude[Missing["UnknownQualifierSyntax"], "AstronomicalUnit"], QuantityMagnitude[Missing["UnknownQualifierSyntax"], "AstronomicalUnit"], QuantityMagnitude[Missing["UnknownQualifierSyntax"], "AstronomicalUnit"], QuantityMagnitude[Missing["UnknownQualifierSyntax"], "AstronomicalUnit"], QuantityMagnitude[Missing["UnknownQualifierSyntax"], "AstronomicalUnit"], QuantityMagnitude[Missing["UnknownQualifierSyntax"], "AstronomicalUnit"], QuantityMagnitude[Missing["UnknownQualifierSyntax"], "AstronomicalUnit"], QuantityMagnitude[Missing["UnknownQualifierSyntax"], "AstronomicalUnit"]} 
Posted 5 years ago
 I get the same error. OS X, Mathematica 10.0.1.0
Posted 5 years ago
 Looks like I had a missing set of curly brackets around the Qualifier specification in the first EntityValue call. The released versions don't like that. I have edited the code in the original post above. Hopefully this should now work.
Posted 5 years ago
 Works fine now... I guessed you were using a pre-release version....
Posted 5 years ago
 I have a lot of trial and error to discover how to get an EntityValue and use it. A lot of the documentation gets the entities from Wolfram|Alpha (or the equivalent key combination), and there seems to be some art to using the resulting structures. Any chance of a tutorial sometime?The code works now, of course, and I will be reading the entrails.