Figure 1. Vesta, as photographed by Dawn on 23 July, 2011.

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Solar Sail / Ion Drive Hybrid Propulsion

Abundant power in the outer solar system

Abstract

Can Solar powered Ion drives operate beyond the orbit of Jupiter? By use of a light weight mirror to collect and focus light on the Solar cells, the answer is conclusively, "Yes."

March 26, 2013

Introduction

A recent article,1 discussed the possibility of sending a probe to catch up with the Voyager 1 spacecraft. The article cited a NASA/IEEE paper2,3 that described light sail propulsion to reach the Kuiper Belt, approximately 200 AU, in a relatively short time. But I think the paper wastes the best resource of the mission: the 300 m diameter solar sail. If such a sail had a parabolic shape, it could be made into a powerful radio telescope. That could be used along with radio telescopes on the Moon, Mars, or asteroids, to produce an interferometer of greater resolving power than anything else. This could resolve exoplanets in unprecedented detail. It also might be possible to use such a mirror in the far IR or microwave frequencies, to detect the infrared signatures of new Kuiper Belt objects.

This system could also be used to make parallax measurements of nearby stars that are 100 times beyond the range of parallax measurements made from Earth. This would permit a great improvement in the accuracy of our knowledge of the age and the size of the universe, and of the Hubble constant.

With a parabolic light collector like that, could solar cells be used out beyond the orbit of Jupiter? Let's say it is approximately at the orbit of Neptune, or roughly 31.4 AU from the Sun. The sunlight is 1000 times weaker than at Earth's orbit, but you have a ~71,000 m2 sail collecting light and reflecting it back toward your solar cells. That gives you power about equal to 70 m2 of solar cells in Earth orbit: Approximately 94 KW of light energy falling on the solar cells. One may also assume they have degraded in the decades the mission took to get out that far, and only produce 20% power conversion. That gives 18.8 KW, which is enough to run a powerful ion drive.

Ion Drive Head (C) NASA
Ion Drive Head (C) NASA

The potential for rapid communications with the outer Solar system using such a mirror should not be overlooked. 18 KW is enough power to produce a powerful microwave beam for communications. Focused microwave or infrared beams would permit megabit per second data rates, as opposed to the approximately 1 kbps data rates that Voyager and the deep space network manage now. A series of such probes could also relay signals to one another, permitting higher data rates to the Kuiper Belt or the Oort Cloud.

If the power from the Solar cells could be stored, it might be possible to produce radar pulses that could be reflected off of KBOs, to learn more abouth their topography, rotation, and composition.

Conclusion

Use of ion drives and rapid communications with space probes in the outer Solar system are desirable goals. I think we are on to something here.

--- By F. E. Harris

References

1. "With today's technology, would it be possible to launch an unmanned mission to
retrieve Voyager I?" XKCD What If #38, 2012. http://what-if.xkcd.com/38/
2. "Interstellar Probe Mission/System Concept," Wallace, R.A., Ayon, J.A. and Sprague, G.A.
2000 IEEE Aerospace Conference, Paper No. 53 http://trw-new.jpl.nasa.gov/dspace/bitstream/2014/13644/1/00-0010.pdf
3. "Ion Propulsion" NASA Dawn Mission Technical Reports, 2008.
http://dawn.jpl.nasa.gov/mission/ion_prop.asp
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