BOX 4.1
Meteor Storms and Spacecraft Safety

The NASA meteoroid program established during the 1960s provided estimates of the background meteoroid impact environment, which was not found to be a show-stopping hazard to the Apollo program and subsequent crewed missions in low Earth orbit. That effort determined that the dominant impact threat from meteoroids was from the random background of particles, rather than the visually spectacular but less numerous (at the small dust sizes of concern to spacecraft) meteor showers.

However, one possible exception to this rule of thumb is a rare phenomenon termed a “meteor storm.” Such meteor storms happen on average only once every few decades. The only major meteor storm of the early space age happened on November 17, 1966, when the Leonid meteor shower rained over western North America, providing spectators with a once-in-a-lifetime sight of up to tens of thousands of visible meteors in less than an hour.

Although the number of meteoroids potentially hitting spacecraft spikes during a meteor storm, an additional potential danger exists in the speed of shower meteors, which tend to be many times faster than the average sporadic meteor. The Leonids, traveling at 71 km/s, are near the top of this scale, with other potential storm-producing showers like the Perseids (60 km/s) and Lyrids (43 km/s) also packing a substantial punch. The added velocity is a danger to spacecraft, not only for the added mechanical impact damage produced, but also because, at such high speeds, large amounts of plasma can be produced, which can damage sensitive spacecraft electronics.

No similar storms were seen or expected after the 1966 Leonids until 1993. In 1992, the possibility of a storm from the well-known Perseid meteor shower became a sudden and real possibility with the surprise discovery of the Perseid parent comet, 109P/Swift-Tuttle, as it passed through the inner solar system. The prospect of a substantial storm the following year led to the reorientation of the Hubble Space Telescope near the time of the predicted storm peak and to a delay in the launch of space shuttle mission STS-51. The 1993 shower resulted in a strong surge in meteor numbers but fell short of a storm. Nevertheless, the Olympus telecommunication satellite suffered an impact, likely from a small Perseid meteoroid at the height of the shower, which ultimately led to the termination of that mission.

After the experience of the 1993 Perseid shower and the Olympus impact, the space community became sensitive to the impact damage possibly associated with meteor storms. The Perseids proved a warm-up for the much more spectacular returns of the Leonids, which produced a strong shower in 1998 and true meteor storms in 1999, 2001, and 2002. Several major research efforts recorded the Leonid storms using video cameras and radars, some providing Leonid meteor numbers in real-time to space operators—the first real-time meteoroid “weather” reports. Many satellites were turned to present a minimal target area to the oncoming stream, and several satellite operators took additional precautions, such as turning off high-power subsystems at the time of the predicted peak and ensuring that extra ground support was available in case of an emergency. While major satellite damage did not occur during any of the Leonid storms, in part perhaps because so many satellites took precautions during the height of the storm periods, some smaller anomalies were reported by operators, which have been linked to the sudden increase in numbers of small, fast Leonids.

In the short term, the 2011 October Draconids are predicted by some forecasters to produce a possible strong shower (or maybe even a storm) on October 8, 2011, which is likely to be the last meteor storm for at least a decade.

properties (mass, bulk density).5 In addition to ground-based observations of meteors, direct in situ measurements are also made by means of space-borne dust detectors, analysis of the surfaces of returned spacecraft, and laboratory measurements of a select suite of meteoroids in the form of airborne collected interplanetary dust particles (IDPs). In situ measurements must also be modeled and calibrated for proper interpretation of the impact signal.

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5 For example, see S. Close, M. Oppenheim, S. Hunt, and A. Coster, A technique for calculating meteor plasma density and meteoroid mass from radar head echo scattering, Icarus 168:43-52, 2004, available at http://soe.stanford.edu/pubs/Icarus_scattering_sigridclose_5373.pdf; J. Borovička, Physical and chemical properties of meteoroids as deduced from observations, pp. 249-271 in Proceedings of the International Astronomical Union, Vol. 1, Cambridge University Press, Cambridge, U.K., 2005.



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