Transfer Calculations
  Hohman Transfer
  Free Return Trajectory
  Spiral
  Summary of Numerical Data

Launch Windows
  Calculations
  Possible Launch Dates
  Graphical Comparison

Sources

Ion Propulsion was chosen by the DARTS design team and the Navigation team as the method of propulsion for the main equipment package because it appears to be an efficient method of transfer. The originally planned method was to use a spiral transfer in which the Ion Propulsion package (IPP) modules could spiral out of Earth's orbit. It would then enter another trajectory around the sun, completing one half of a loop before it encounters Mars' gravity well. Once it intersects Mars' orbit it would begin a spiral down to low Mars orbit (LMO).

During a recent review of the DART and Navigation team conclusions, an MIT professor indicated that this method could pose threats to the ion engines because of the Van Allen Radiation Belts. Neither the design team, nor the navigation team, had read any literature during the term that indicated that Van Allen radiation belts might pose a problem for the mission. The Earth is surrounded by Van Allen Radiation Belts, belts of charged particles that circulate along the Earth's lines of magnetic force and which are caused by interactions between solar wind and earth's electromagnetic field. The radiation from the Van Allen Belts may interfere with the ion engines, introducing unpredictability into its flight pattern. These belts also contain high-energy ions that can penetrate spacecraft shielding over long periods of time. However, we cannot determine if this effect will have a significant impact on the mission since the calculation of the time it takes for the IPP to leave Earth's gravitational influence is beyond the scope of this course. It must be noted however, that satellites in Earth's orbit often experience no ill effects due to Van Allen radiation. The magnitude of the damage is a complex function of time of exposure, intensity of the belt at that time, and position of the spacecraft in the belts. It may be possible to orient the IPP's orbit such that it encounters weak or no ion density. Unfortunately, the scale of this problem is such that it cannot be easily or rapidly solved.

Because of this information, the DARTS design team and the Navigation team have come up with a current possible solution. Each IPP module will be propelled to Mars using a chemical engine to escape the Earth's gravitational influence. Once out of Earth's gravitational influence, the modules will establish orbit around the sun with a natural orbit velocity of 30 km/s. Then the modules will decrease their velocities by 4.64 km/s using ion propulsion. By means of a spiral transfer trajectory (delta v = 8 km/s), the system modules will slowly descend into LMO. The DARTS design team and the Navigation team brainstormed other possible solutions to explore. The most obvious solution is to research newer, but unproven, low power, high efficiency methods of transfer. Another possible solution would be to use computer models of the radiation belts and of the spiral orbit to determine whether the trajectory would bring the spacecraft near high ion density regions of the belts. If this is not the case, then radiation exposure is not a problem. Also, computer models could be used to calculate the exact time that the spacecraft would be exposed to high-energy radiation. On a short enough time scale, it may be that the effect of the radiation on the payload is negligible. In this case, it may be possible to track the exact location of the vehicle on its trajectory and to use small thrusters to compensate for perturbations as they occur. Another proposed solution is to boost the IPP above the highest ion density areas using a chemical rocket. The spacecraft would then follow a spiral trajectory until it was completely free of the Earth's gravitational influence.

We cannot continue to recommend an ion-propelled exit from Earth until we determine the exact magnitude of interference caused by the Van Allen Radiation Belts. In conclusion, we now recommend that all of the previous solutions should be researched thoroughly and a safe and efficient method of Earth exit should be chosen.