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Part 5: Potential Applications

Since objects in space are not "grounded", they are free to collect charge and reach a floating potential relative to the space plasma potential - referred to as the "spacecraft floating potential". Charge collection can take place in a variety of ways, including photoionization of spacecraft surfaces by solar radiation and by interactions between the spacecraft and the surrounding space plasma. Spacecraft designers are concerned about charge collection because it can seriously affect the operation of the spacecraft. Spacecraft charging can disrupt the operation of electrically biased instruments, attract contaminants to sensitive instrument surfaces, and ultimately cause arc discharges and sputtering that deteriorate spacecraft surfaces or even cause electronics components to fail.

For example, the International Space Station (ISS) is expected to be prone to significant charging due to its high voltage (160 volt) DC power system and its orbit in a plasma-rich region of space. The -90 to -120 volts to which the Space Station charges under normal conditions in daylight local times is considered mild compared to charging experienced by satellites in other orbits (e.g. in geosynchronous orbits in the earth's shadow). However, even such a relatively low charge is considered problematic for the ISS. Such low voltages, due to the properties of the Space Station surface (metal coated with a thin, dielectric, insulating material) could produce constant arc discharges that would deteriorate surfaces of the station unless charge collection is mitigated. Due to the capacitor-like property of the ISS surface, charging to -90 to -120 volts could even pose a shock hazard to astronauts during extravehicular activity. The Plasma Contactor Unit has been designed to harmlessly dissipate charge from the ISS, but a search is still underway to determine how best to measure the Space Station's floating potential. I believe that the SCM is the best device available to do that job.

Efforts to study the spacecraft charging phenomenon and efforts to successfully mitigate spacecraft charging can be greatly advanced by the SCM. Although the hazards of spacecraft charging are well known, the actual measurement of spacecraft floating potential has proven difficult and is uncommon. Methods now used to measure spacecraft potential depend on relatively bulky, expensive instrumentation and/or complicated data analysis. I believe the breakthroughs embodied in the SCM will revolutionize the measurement of charge in space.

In Low Earth Orbit

In addition to the using the SCM on the ISS the SCM could be used to determine charge on other LEO craft, especially those that carry scientific payloads for plasma analysis. On such missions knowledge of spacecraft charge is especially important. Out of LEO and in the solar wind, the SCM could be used to determine charge as detailed in the footnotes. 3 Around other planets, such as Mars or Jupiter, the SCM could also be used to determine charge, much as it could in LEO by identifying a shift in energy features expected in the ambient electron spectrum 6.

In the Solar Wind

The solar wind permeates the solar system. Many missions are planned that will either fly through or have a final destination in solar wind. For spacecraft that fly through the solar wind, there is no better instrument than the SCM to determine spacecraft floating potential 3.

The SCM would be particularly useful for some of the expected lunar and Mars missions.

Lunar Missions

Recent investigations have revealed a need for a charge monitor such as the SCM for missions to the lunar surface. Unlike the Earth, the Moon does not have a significant magnetic field, and it has no significant atmosphere. Its unprotected surface is thus constantly bombarded by the solar wind. Evidence from the Lunar Prospector indicates that large electrostatic charging due to the bombardment by the solar wind will take place. Such charging could be hazardous to equipment or personnel.

The Lunar Prospector (in flight 1998-1999) was designed for a low polar orbit (100 km to 30 km) investigation of the Moon. Data from the mission indicate that surface potentials of from -500V to +100V can exist on the lunar surface. It has been shown that the occurrence of the charging depends mainly on the ambient electron density and temperature. The largest potentials almost all occur when the Moon lies in the geomagnetic plasma sheet, where the ambient electron temperature is at its highest. A charge monitor such as the SCM could be used to indicate when surface charge is high and the hazard to personnel and equipment is large.

The solar wind electrons that strike the Moon's surface could be energy analyzed by the SCM to find the energy location of the prominent 10eV peak in solar electrons. A shift in the energy location of the peak would then be used to determine the electrical potential of whatever surface is of interest relative to the 10eV peak solar wind electrons.

Why is the SCM well suited for use on the moon? The data from the SCM would be accurate. It would determine charge relative to the ambient space plasma, a reference needed to determine if 'hazardous charging conditions' are present. It is unlikely that any other charge detector now available could measure surface charge relative to the ambient plasma on the Moon. The SCM, by collecting electron spectra, could also determine the ambient electron temperature, which has been correlated to lunar surface charging. Even if an electron spectroscopic charge monitor similar to the SCM were to be built by others, the Goembel Instruments patent will prevent them from using the super-high sensitivity electron optics that enables the SCM to be so small and powerful. Furthermore, the Goembel Instruments product is ready for use, and inexpensive.

Mars Missions

Although the SCM could not determine charge on the surface of Mars (because the atmosphere is too dense at the surface), it could determine charge en-route or in orbit. On the way to Mars it could determine spacecraft charge by locating the 10 eV solar wind electron peak, just as it could determine charge on the moon or anywhere else in the solar wind. In orbit around Mars, atmospheric photoelectrons could be used to determine charge, just as in low Earth orbit.

It is likely that ion propulsion will be used on Mars missions. If so, spacecraft charging becomes a special problem. The ions used for propulsion will likely be positively charged xenon molecules. Thrust is created by accelerating the positive ions through a series of gridded electrodes at one end of a thrust chamber. The ions are prevented from being electrically attracted back to the thruster by an external electron-emitting device called a neutralizer. To produce useful thrust an ion engine must emit large amounts of positive ion current. Yet the total capacitance of a typical spacecraft is only 1 x10-9 Farads. With such a small capacitance the spacecraft will quickly acquire a large negative potential at the rate 9 volts per sec per amp of ion current. Very quickly the entire spacecraft would be charged to such a large negative potential that it would be impossible to eject any additional ions and thrusting would end. This charge-up can be circumnavigated by emitting an electron current into the exhaust stream. Spacecraft charge monitoring is needed for feedback to the 'electron current injection' mechanism to minimize spacecraft charging.

Problems associated with spacecraft charging en-route to Mars include:

  1. For any propulsion system: arcing between differentially charged surfaces and the associated damage from such arcing. Differential charging on a spacecraft is more likely to take place when the chassis potential of the spacecraft differs from that of the ambient plasma. System failures on spacecraft have been attributed to arcing.
  2. For any propulsion system: as on the ISS, shock hazard to astronaut if an extravehicular activity (EVA) is needed (e.g.; to make a repair en-route).
  3. If ion propulsion is used: inefficiency or failure of the propulsion system.
  4. If ion propulsion is used: pollution of spacecraft surfaces due to a positive spacecraft potential. This can affect optical instruments and solar arrays. The pollution can also increase the risk of surface discharge due to differential charging. Spacecraft charging also causes ion sputtering which degrades spacecraft surfaces.

It is unlikely other available charge detectors could measure spacecraft charge on ion propulsion craft as well as the SCM could. The SCM is a low cost, lightweight and sturdy solution.

All in all it is apparent that there are many applications for the SCM. We look forward to the flight of the prototype SCM so that we will be able to prove the utility of this revolutionary new tool for space flight and exploration.

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