• The F / A buttons toggles between Floating and Absolute Modes. The button's caption changes to A when in the floating mode, meaning that pressing it puts you in Absolute mode. When in absolute mode, the button's caption is F, meaning that pressing it puts you in Floating mode. In absolute mode, the object displayed in the Focus Object window is locked to the center of the screen. In Floating mode, the Focus object's barycenter is locked to the center of the screen. The barycenter is defined as the center of mass of the focus object and all objects whose reference object is the focus object.

  In this illustration, the red and yellow objects are of equal mass. They orbit their common barycenter in an elliptical orbit.
  

• The T button toggles between Trails and No Trails. This determines whether the objects on the screen trace a path. You may also toggle between Trails / No Trails with the T button on your keyboard.

  

• The P button toggles between Plot and Don't Plot modes. In the don't plot mode, the simulation appears to be paused. But it is still crunching numbers in the background. The simulation runs faster if it doesn't have to display any output. This is helpful for long-term simulation where you may want to leave the simulation running overnight or for a few days.

• The L button toggles between Labels and No Labels. You may also toggle between the Labels / No Labels mode with the L button on your keyboard. With Labels turned on, every object on the screen has its name displayed next to it.

  

• The button with the crosshairs allows you to choose a custom focus point, rather than choosing an object from the Focus Object window.

  Clicking this button turns the cursor into crosshairs . Click on the screen on the point you'd like the screen to be centered. Clicking this button again returns the cursor to the default arrow and disallows custom centering.

  

The Full Screen option hides the title bar and menu. Press ESC on the keyboard to return to normal view. You may also enter Full Screen mode by pressing F11 or ESC on the keyboard.

Preferences

Time

Autopilot

Choose which caption you would like (1-4), change the font if you wish, set the position on the screen and press OK.

Extra

NASA's Jet Propulsion Laboratory has a web-based service called Horizons. This service lets you access the positions and velocities of thousands of solar system objects. It contains every planet, all moons except moons recently-discovered by the Cassini spacecraft. It contains thousands of asteroids, comets, trans-Neptunian objects (TNO's), and many spacecraft.

You can access this data and import it into your Gravity Simulator simulations. The simulation fullsystem.gsim, which is included in your install package contains nearly 200 solar system objects. Check back often for updated versions of this simulation on the web site (www.gravitysimulator.com).

You may also import objects yourself:

• Decide on a simulation date.
• Create a simulation beginning on this date. You can either start from an existing simulation such as fullsystem.gsim and propogate it forward or backwards to your desired date (use a small timestep!) or you can start from scratch with File > New, and Time > Set Time. Make sure your simulation is paused so it remains at your simulations desired start date. Also, it is recommended that you re-name the only object in the simulation from Object1 to Sun. You might want to make it yellow too.
• Create objects. Use the menu Objects > Create Objects to create as many objects as you wish. Their orbital elements and other properties are not imporant at this moment. Just create them so they exist. Make sure your semi-major axis exceeds the radius of the Sun, or they'll instantly merge with the Sun. Just plug any number such as 100 AU into the Semi-major box.
• Query Horizons for the data. Send a modified version of the following e-mail to horizons@ssd.jpl.nasa.gov . Put the word job in the subject line.

  !$$SOF
  EMAIL_ADDR=''
  START_TIME = '2005-Jan-01 00:00:00'
  STOP_TIME = '2005-Jan-01 00:00:01'
  TABLE_TYPE = 'Vector'
  REF_PLANE = 'Ecliptic'
  CENTER = '@399'
  COMMAND='301'
  !$$EOF
  

  The start time is the time and date of your simulation. Start time is exactly 1 second later.

  COMMAND is the desired object, and CENTER is the object your desire object orbits. In the above example, Horizons is being queried for the Moon's position relative to the Earth on January 1, 2005 at 00:00:00. 399 is the numerical code for the Earth. 301 is the numerical code for the Moon. Horizons numbering codes for moons and planets works like this:

  • Mercury = 199
  • Venus = 299
  • Earth = 399
  • Mars = 499
  • Jupiter = 599
  • Saturn = 699
  • Uranus = 799
  • Neptune = 899
  • Pluto = 999
  • Earth's first (only) moon = 301
  • Mars' first moon (Phobos) = 401
  • Mars' second moon (Deimos) = 402
  • Jupiter's first moon (Io) = 501
  etc... You may substitute an object's name for its id number. Sometimes this works, sometimes it doesn't. But if it doesn't work, you'll get an e-mail telling you why it didn't work, and offering suggestions. So if you want the data for Sedna, but you don't know its ID number, just try typing Sedna. You can visit http://ssd.jpl.nasa.gov/sat_eph.html for a list of all moons, http://ssd.jpl.nasa.gov/data/ELEMENTS.NUMBR , http://ssd.jpl.nasa.gov/data/ELEMENTS.UNNUM , AND http://ssd.jpl.nasa.gov/data/ELEMENTS.COMET for a complete list of comets and asteroids.

  Spacecraft are available too. They have negative id numbers:

  • Pioneer 10 spacecraft = -23
  • Pioneer 11 spacecraft = -24
  • Lunar Prospector (LP) spacecraft = -25
  • Stardust spacecraft = -29
  • Deep Space 1 Spacecraft DS1 DS-1 = -30
  • Voyager 1 spacecraft = -31
  • Voyager 2 spacecraft = -32
  • Mars Express spacecraft MEX = -41
  • Genesis spacecraft = -47
  • Mars Odyssey spacecraft = -53
  • Ulysses spacecraft = -55
  • Infared Space Observatory ISO = -64
  • Deep Impact spacecraft = -70
  • Galileo spacecraft = -77
  • Giotto spacecraft flyby = -78
  • Spitzer Space Telescope = -79
  • Cassini spacecraft = -82
  • NEAR spacecraft = -93
  • Hayabusa spacecraft Muses-c = -130
  • Huygens spacecraft (Cassini mission) = -150
  • Chandra Observatory = -151
  • Nozomi spacecraft = -178
  • INTEGRAL spacecraft = -198
  • CONTOUR-A (SPACECRAFT FRAGMENT) = -204
  • CONTOUR-B (SPACECRAFT FRAGMENT) = -205
  • CONTOUR-C (SPACECRAFT FRAGMENT) = -206
  • Rosetta = -226
  • Opportunity spacecraft MER = -253
  • Spirit spacecraft MER = -254
  • Galileo Probe = -344
  • NEOCP J002E3 = -998

• After sending the e-mail, Horizons will reply automatically, usually instantly. Scan through their e-mail for the following data:

  *******************************************************************************
  $$SOE
  2453371.500000000 = A.D. 2005-Jan-01 00:00:00.0000 (CT)
  -3.727791908064512E+05 1.414748077722513E+05 2.677783108907881E+04
  -3.131725599039820E-01 -9.278602623572122E-01 -5.829082121527118E-02
  1.332990315365641E+00 3.996204431336606E+05 -4.025204624447116E-02
  $$EOE
  *******************************************************************************

  Notice the italicized numbers. (They're not italicized in the e-mail). These are the X,Y,Z positions and X,y,Z velocities of your object. Highlight all 6 of these numbers and copy them. Be careful not to including any leading spaces before the first number. Also be sure to include the negative sign in the first number.

• Open the Edit Objects window using menu Objects > Edit Objects
• Select one of the items you created. Make sure that the reference object for this object is the same object which you used as your Center object in the e-mail to Horizons. Give it a new name if you wish. Highlight all the data in the Position X box and paste the data you copied from the Horizons reply e-mail in this box. It will automatically parse this data for you and divide it up into the remaining 5 position and velocity boxes.
• Make any other changes you wish to make at this time, such as color and mass. The e-mail from Horizons will also give you the object's mass under the "Physical Properties" section.
• Press Apply and repeat for the next object.
• Repeat until you are finished.
• Save your simulation.
• Unpause the simulation and enjoy!