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A tool developed for the preliminary design of low-thrust trajectories is described. The trajectory is discretized into segments and a nonlinear programming method is used for optimization. The tool is easy to use, has robust convergence, and can handle many intermediate encounters. In addition, the tool has a wide variety of features, including several options for objective function and different low-thrust propulsion models (e.g., solar electric propulsion, nuclear electric propulsion, and solar sail). High-thrust, impulsive trajectories can also be optimized.

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V, leveraging can significantly reduce the launch energy requirements for interplanetary missions. The objective of this study is to analytically examine this technique and to expand its application. AV-EGA (AV Earth Gravity Assist) trajectories are used as the first example of V, leveraging. The equations are solved using an iterative algorithm, and the trade-off between V, and turn angle at the Earth gravity assist is examined. Simplifying assumptions reduce the equations to a single function. The AV-EGA concept can be extended to other planets, such as Venus, which is used as a second example.

Analytic and numeric techniques are used to assess trajectory options for the Pluto Express sciencecraft for a launch early in the next decade. The constraints placed on the Pluto Express trajectory for this study are severe-total ight time to Pluto of 12 years or less using a Delta-class launch vehicle. In addition, no ybys of Earth are permitted. Suitable trajectories are found with launch windows before, near, and after the date of the baseline launch. All of these trajectories take advantage of a gravity assist with Jupiter, and all use two or three gravity assists with Venus before arriving at Jupiter. In two cases, a Mars gravity assist is used in conjunction with three Venus gravity assists. Several asteroid yby opportunities are presented for the baseline mission and for a backup trajectory, which launch in March 2001 and July 2002, respectively. For example, a yby of the asteroid Seraphina (which has a radius of 32 km) can be accommodated in the baseline mission for an additional deterministic delta-velocity of 0.12 km/s, well within the capability of the system. Nomenclature C 3 = V 2 1 , km 2 /s 2 g = standard gravitationalaccelerationon the Earth, km/s 2 I sp = speci c impulse, s m f = total injected dry mass ( nal mass), kg m i = total injected wet mass (initial mass), kg m p = propellant mass, kg m s=c = spacecraft mass (excluding propellant tanks), kg R J = radius of Jupiter r po = radius of circular parking orbit, km V V = velocity of Venus with respect to the sun, km/s V 1 = hyperbolic excess velocity vector, km/s 1V = magnitude of a change in velocity, km/s or m/s 1V NAV = navigation 1V , km/s or m/s 1V PL = deterministic postlaunch 1V , km/s 1V TPL = total postlaunch 1V , km/s ¹ E = gravitational parameter of the Earth, km 3 /s 2