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Applications of LAOs

LEOSynch

LEOsynch orbits

LEOSynch orbits precess so that satellites cover the same longitude line on the Earth's surface every orbit.

The orbit of a charged spacecraft in a magnetic field precesses about the magnetic pole in a direction determined by the polarity of the charge. A constant charge (with no modulation necessary) results in a constant precession about the magnetic dipole axis.

LEOSynch orbit animation (11 MB AVI)

This LAO has very useful applications. A near-polar satellite with constant charge precesses around the Earth in a way that enables it to cover the same point on the ground each orbit. That is, the orbital plane precesses at 360 degrees per day. Thus, it is a geosynchronous (though not geostationary) LEO satellite. For this reason, we call this a LEOSynch orbit. Potential applications include high-resolution planetary observation of events that change on a scale of a day.

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Planetary Flyby and Escape

jovian capture

Jupiter capture, precession, and escape LAO trajectories

In a traditional flyby, a spacecraft takes on a little of the planet's orbital angular momentum about the Sun as it passes. In the LAO analogue, the spacecraft steals a little of the planet's spin angular momentum about its mass center (not about the Sun). As with all LAOs, this is a form of propellantless propulsion.

As a charged spacecraft orbits a planet, the electric field associated with the rotating planetary magnetic field does mechanical work to change the orbit's semimajor axis. The spacecraft can use this effect to enter or depart from a geostationary transfer orbit without propellant. Jupiter's very high magnetic field and high spin rate make it ideal for this application. Executing a Jupiter capture maneuver with an LAO would save propellant and would offer a means of making the maneuver plan more forgiving of navigation errors.

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J2 Cancellation

The Lorentz force can cancel the reduction in orbital energy by atmospheric drag on a per-orbit basis. While the effect may require more power than an equivalent electric propulsion system, we emphasize that the LAO approach requires no propellant. Consequently, the space system's lifetime is not limited by the propellant load, and more payload can be incorporated in place of this saved mass.

Some inclination of the orbit above the magnetic equator allows the same precession effect to act like J2 precession (the gravitational perturbation due to planetary oblateness). For the right amount of charge, the J2 perturbation can be canceled, or even reversed.

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Propellentless Rendezvous

The Lorentz force can work against gravity in a way that makes spacecraft behave as if the Earth is more or less massive than it really is. This effect means that a circular LAO may exhibit an orbital speed that differs from that of a Keplerian orbit with the same radius. One obvious application of this principle is that an LAO-capable spacecraft can rendezvous with another without the use of propellant.

The basic principle can be extended to elliptical orbits. The NASA Vision for Space Exploration includes many such maneuvers, including in-orbit servicing, assembly, and transfer of passengers and cargo among various vehicles. The clear relevance of this single application is a powerful argument for pursuing LAO technology. There are likely more applications than we have identified to date.

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Radial Formation Flight

radial formation

A formation of radially separated LAO-capable spacecraft traveling with the same orbit velocity.

Formation-flying satellites with traditional propulsion can achieve only certain very limited formation shapes, since spacecraft at different orbital altitudes travel at different speeds. However, using the Lorentz force opens up far more possibilities. LAO-capable spacecraft can hold a formation throughout the entire orbit by using the Lorentz force to adjust their mean motions. That is, spacecraft in Lorentz-augmented orbits can fly with the same orbital period at varying radii. These formations can also achieve surprising shapes, some that are clearly impossible with Keplerian orbits and practical propellant usage.

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Sun- and Moon-Synchronous Missions

The J2 gravitational perturbation is currently used to establish Sun-synchronous orbits-orbits that precesses 360 degrees per year so that the spacecraft is always in the Sun. Since the Lorentz effect can cause spacecraft to precess about a planet, it can be used the same way. However, only a very special range of orbital inclinations, semimajor axes, and eccentricities can be used for traditional Sun-synchronous orbits. In contrast, a Sun-synchronous LAO can be of virtually any inclination and semimajor axis, and any eccentricity. The result is a capability for high-power satellites that need no batteries because they would never enter eclipse.

A related orbit is the Moon-synchronous one. This orbit would be inclined so that its plane is normal to the Earth-Moon line, but it would precess at about 36 degrees per day. The result is a spacecraft in low-Earth orbit that is always in view of the Moon. Such a satellite could serve as a communications relay for manned activity on the Moon, a power relay for a lunar power-generation station that beams energy back to the earth, for lunar geodesy, or for lunar science.

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Joviastat

One particularly interesting LAO is a stationary planetary observer orbit. The satellite is not stationary in the planet's reference, as with a geosynchronous orbit. Rather, it hovers over the planet at a fixed altitude but otherwise stationary in an inertial frame. The required charge-to-mass ratio is independent of altitude. For Earth, this ratio is probably out of reach. However, at Jupiter, q/m = 0.00041 C/kg, a value that may be achievable within the current state of the art. The viability of the concept at Jupiter inspires the name Joviastat. The fact that a Joviastat does not exactly orbit the planet, but hovers above as the planet spins underneath, suggests that it may have application to formations and space-elevator concepts.

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Team Members

  • Justin Atchison
  • Brett Streetman

Undergraduates

  • Parker Imrie
  • Richard Koontz
  • Phillipe Tosi

Funding

  • NASA Institute for Advanced Concepts