Orbits about collinear libration points are unstable, requiring stationkeeping maneuvers to maintain. Several different methods for calculating libration-point orbit stationkeeping maneuvers have been proposed. A tight control technique was used by the third International Sun-Earth Explorer (ISEE-3), the first libration-point mission. An easily-implemented "orbital energy balancing" loose control strategy was developed later and has been used successfully by the Solar Heliospheric Observatory (SOHO) and Advanced Composition Explorer (ACE). For SOHO, this loose control has resulted in station-keeping AV costs of just over 2 m/sec per year with maneuvers performed about four times a year, an improvement of almost a factor of four over ISEE-3's tight control. Part of the gain probably results from better trajectory determinations from improved radiometric tracking data for SOHO. All three missions, especially SOHO, have had operational mishaps that have resulted in temporary expenditures of fuel that were much larger than expected, but in spite of these, it appears that the spacecraft can be kept in their designed halo orbits for periods longer than planned before launch.

Stationkeeping Techniques for Libration-Point Satellites

Survey of Station-Keeping Techniques for Libration Point Orbits

Low-energy ballistic lunar transfers

The technology of high ISP propulsion systems with long lifetime and low thrust is improving, and opens up numerous possibilities for future missions. The use of continuous thrust can be applied in all directions including prependicular to the flight direction to force the spacecraft out of a natural orbit (or A orbit) into a displaced orbit (a non-Keplerian or B orbit): such orbits could have a diverse range of potential applications. Using the equations of motion we generate a catalogue of these B orbits corresponding to displaced orbits about the Sun, Mercury, Venus, Earth, the Moon, Mars, Phobos and Deimos, the dwarf planet Ceres, and Saturn. For each system and a given thrust, contours both in and perpendicular to the plane of the ecliptic are produced in the rotating frame, in addition to an equithrust surface. Together these illustrate the possible domain of B orbits for low thrust values between 0 and 300mN. Further, the required thrust vector orientation for the B orbit is obtained and illustrated. The sub-category of solar sail enabled missions is also considered. Such a catalogue of B orbits enables an efficient method of indentifying regions of possible displaced orbits for potential use in future missions.

Edward A. Euler

Papers

Optimal Station-Keeping at Collinear Points

Optimal station-keeping at collinear points