Placing objects at defined locations on planetary surfaces

• Grant Hutchison's Contributions for Celestia

A couple of times recently, people have raised the problem of placing an object at or near the surface of a planet or satellite, and having it stay where it's put, as if it were resting on the surface, or hovering above it.

A synchronous orbit only works at the equator - otherwise the object ends up sliding around in a large figure-of-eight. So I've produced a workaround that allows you to do this.

The invisible object of Celestia v1.3 is the obvious choice as a place-holder when placing objects on planetary surfaces. Even though the placeholder object is embedded within the planet, the use of the "invisible" class removes all the concerns about getting radius and albedo right, in order for the orbiting body to be visible.

1. Create a dummy object that hovers at an appropriate distance from the centre of the parent body, displaced north or south along the rotation axis.
2. Place your object in orbit around this dummy, with an orbital radius that puts it on the planetary surface, and a rotation period that's synchronous with the planet's rotation. If you do this right, you now have your object sitting at the right latitude.
3. Adjust the MeanLongitude of your object's orbit so that it sits at the correct planetary longitude.
4. Tweak your object's Orientation and RotationOffset so that it is correctly orientated relative to the surface of the planet.
5. Admire.

As proof-of-concept, I've placed the 2001 Moonbus model (the only model I've got with landing legs) at the Apollo 17 landing site (the farthest from the Moon's equator). I've annotated the code so that you can see where all the numbers come from. (The hardest bit is adjusting the model orientation - the technique will vary from model to model, depending on how they are originally orientated on the X,Y,Z axes.)

Here's the code, making use of the new "invisible" class of object provided by Celestia 1.3.0:

---

 
#To place a lander on the Moon at 20.18809N, 329.22525W and keep it there 

#Desired radius = radius of Moon + desired altitude 
#in this case, 1737.54 (lunar radius +10m) 

"#" "Sol/Earth/Moon"         #A dummy body to provide a suitable orbit centre 
{ 
    Class "invisible"

    EllipticalOrbit { 
        Period        1e12          #Effectively stationary 
        SemiMajorAxis   599.630455  #Desired radius * sin(Latitude) 
        AscendingNode    43         #Moon's RotationOffset 
        Inclination      90         #Polar orbit 
        MeanAnomaly      90         #Fixed on the lunar rotation axis 
    } 
} 
    


"Lander" "Sol/Earth/Moon/#" 
{ 
    Class "spacecraft" 
    Mesh "Moonbus.3ds" 
    Radius 0.020 

    EllipticalOrbit { 
        Period        27.321661         #Lunar rotation period 
        SemiMajorAxis   1630.793846     #Desired radius * cos(Latitude) 

                                        #Moon's RotationOffset =    +43 
        MeanLongitude   -286.22525      #Subtract west longitude = -329.22525 
                                        #                           ========= 
                                        #                          -286.22525 
    } 

                                    #Latitude in x rotation 
    Orientation [20.18809 1 0 0]    #(This parameter will vary depending on the 
                                    # orientation of the specific model used) 

                                #Re-orientation of model =      -90 
                                #Moon's RotationOffset   =      +43 
    RotationOffset -16.22525    #Subtract west longitude =     -329.22525 
                                #                               ========= 
                                #                              -376.22525 

    Albedo 0.80 
} 

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Here's another example, placing a spacecraft near the surface of Eros:

---

 
#Desired radius = 5.5 
#Latitude = -31.3 
#Longitude = 134.4 

"#" "Sol/Eros"         #A dummy body to provide a suitable orbit centre 
{ 
    Class "invisible"

    EllipticalOrbit { 
        Period            1e12     #Effectively stationary 
        SemiMajorAxis    -2.857    #Desired radius * sin(Latitude) 
        AscendingNode   158.165    #Eros RotationOffset 
        Inclination      90        #Polar orbit 
        MeanAnomaly      90        #Fixed on Eros' rotation axis 
    } 
} 

"NEAR" "Sol/Eros/#" 
{ 
    Class "spacecraft" 
    Mesh "galileo.3ds" 
    Radius 0.01 

    EllipticalOrbit { 
        Period           0.21958333 #Eros' rotation period (in days) 
        SemiMajorAxis    4.699      #Desired radius * cos(Latitude) 
  
                                #Eros' RotationOffset =            +158.165 
        MeanLongitude  203.765  #Subtract west longitude =         -134.4 
                                #Add 180 to reorientate on 3ds =   +180 
                                #                                   ======= 
                                #                                   203.765 
    } 


    Albedo 0.5 
} 

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• Grant Hutchison's Contributions for Celestia
• Selden's List of Celestia Resources

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The content of this Web page was provided by Grant Hutchison.

This Web page is maintained by Selden Ball at Wilson Lab.
Please send any comments or corrections to seb@lepp.cornell.edu