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TUTORIAL - PLANETS
TUTORIAL - MOONS
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The window above shows an interactive simulation of our solar system. To get started, click or tap anywhere within the BLUE title screen. This JavaScript simulation is mobile-friendly and will also work on your iPad or Android device.

The simulation visualizes the current position of all eight planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune) as well as the Galilean Moons (Io, Europa, Ganymede, Callisto). By default the distance between the planets and the moons is not to scale, in order to fit everything on the screen. This can be adjusted by toggling the Scale Button button. The planets are shown to scale, except for the sun.

If you are curious to learn more about Celestial Mechanics and how this JS Solar System simulation was built, please check out the step-by-step tutorial on creating your own "orrery" using simple JavaScript HERE. Next to that, our second tutorial explains how to build a JS Saturnian System simulation: TUTORIAL - MOONS.

MARS MISSIONS

PATHFINDER, OPPORTUNITY, CURIOSITY, INSIGHT & MARS 2020

One of the interesting things about the simulation is the ability to look at the transfer orbit of space probes from various NASA Mars missions. As you can see, the transfer orbit is an elliptical orbit around the sun, just like that of the planets. Note that the orbit has been carefully selected, allowing the space probe to encounter Mars. Due to its elliptical orbit, the space probe will slow down as is gets further away from the sun. The approximated speed of the probe is displayed in PURPLE on the bottom left of the simulation (in KM/S). By the time the probe reaches the target planet, it is going slower than Mars and will need to generate an extra boost to catch up [1] [7].
The most propellant-efficient way to get to Mars is a so-called "Hohmann transfer orbit" [2] which takes the probe exactly 180° around the sun [3]. It uses two burn maneuvers, one to move the probe onto the transfer orbit and one to move the probe off it [4] [8]. While in the transfer orbit around the sun, the probe "coasts" and doesn't use any propellant [5] [8]. A Hohmann transfer orbit from Earth to Mars takes about 8.5 months [1]. From the NASA Mars missions you can see that the "Total Sweep Angle" never quite gets to 180°. This means that more propellant was used. At the same time this also means that the one-way transit time is less than 8.5 months [6]. The NASA CURIOSITY mission comes closest with 177°. A Lambert solver is used to calculate the scenarios with the least changes in velocity (ΔV) required to leave Earth and arrive at Mars. For human spaceflight to Mars, you will want to lower mission duration at the cost of additional propellant use [7].
For additional information on the rocket that was used for the mission you can use the Info Button button within the simulation.

NOTE: The transfer orbits shown here, are estimated based on the positions of Earth and Mars at time of launch and landing. They are not based on any actual flight data.

REFERENCES
[1] Flight to Mars: Calculations, NASA (2004)
[2] Hohmann Transfers, University of Georgia (no date)
[3] Maneuvering In Space, Federal Aviation Administration (no date)
[4] Impulsive Orbital Maneuvers, Peet, M. (no date)
[5] Hohmann Transfer Orbit & Total Travel Time, Iowa State University (no date)
[6] ORBITAL MECHANICS, Braeunig, R.A. (1997, 2005, 2007, 2008, 2011, 2012, 2013)
[7] NASA Ames Research Center Trajectory Browser, NASA (2016)
[8] Orbital Maneuvers, Abdelkhalik, O.O. (2009)