Astro is a project originally created while I was in graduate school. Orbit tools is a java class which has functions that are useful in creating astrodynamic java applications and applets. It was used in a program I created while working at the Space Environment Center. A separate class, planetary elements, calculates the orbital elements for a planet at a given time. Three applets based on these two classes are on this site the Julian Date Converter, Planetary Elements Applets and Direct Trajectory Calculator.
Simple applet to convert from a Julian Date to normal time and normal time to Julian Date. Note: normal time is Greenwhich Mean Time (GMT)
Gets the orbital elements at the Julian Date inputed. It also converts the orbital elements into position and velcoity vectors relative to the sun. (Coordinate system is mean elciptic of J2000)
You put in the planets you want to travel between, how long you want to take and what dates to look at and it gives which day is best to leave. It also gives position and & velcoty vectors for leaving the departure planet and arriving at the target planet and the heliocentric orbital elements of the transfer orbit when leaving the departure planet. This information can be input into mission design programs as the starting point. It is based on what is commonly called the zero-patched-conic method (aiming from the center of one planet to the other ignoring everything except the Sun's gravity). HEV is the hyperbolic excess velocity (aka V infinity).