Dynamics of Meteor Outbursts and Satellite Mitigation Strategies
Glenn E. Peterson
Chapter 1 Assessing the Threat
Meteoroid impacts have represented a very real threat to satellites in orbit about the Earth in the past and certainly will into the future. This chapter quantifies the dynamics of this threat. The particle's orbit about the Sun is computed, and an accurate location of the radiant point (apparent direction from which the particles come when viewed from the Earth) associated with the computed orbit is estimated. This is followed by an analysis of the expected particle fluxes. The analysis to be performed will be general in nature in order to be applicable to any future meteor event, but the examples given throughout this document will emphasize the 1998-1999-2000 Leonids since that scenario poses the greatest threat at the present time.
1.1 Death from Above
Several spacecraft have been known to suffer from meteorite hits, resulting in either catastrophic loss of the vehicle or significant degradation of mission performance. The Perseid meteor showers of 1991 and 1993 caused effective loss of the ISAS Solar A and ESA Olympus I satellites, respectively. The vehicles were not destroyed through a catastrophic impact; rather, their mission effectiveness was degraded in both cases to such an extent that the mission was essentially over. Solar A experienced a puncture of its telescopic shade, which ruined the main experiment. Olympus I experienced erroneous attitude control, causing virtually all the fuel to be expended by the time control was reestablished (Caswell, 1998). NASA's International Sun-Earth Explorer (ISEE) also experienced a degradation of mission goals because of a sporadic meteor hit not associated with any known shower. While meteor strikes are rare, their impact can be disastrous on the mission.
Meteor showers that have the potential to damage spacecraft occur when the Earth intercepts a stream in space consisting of small meteoroid particles. The edges of the debris tube are diffuse and ill-defined; just when the Earth enters and leaves the stream is thus somewhat arbitrary. However, the general concept of the Earth traveling through a tube of small cometary particles holds (Fig. 1.1). The closer the Earth gets to the center of the stream, the more intense the meteor activity is.
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Fig. 1.1. Earth traveling through a meteor stream tube. |
These meteoroids are created by a parent body—either a comet or less likely an asteroid—in a continuous, although nonconstant, process as the parent moves about the Sun. As a comet comes closest to the Sun at periapse, the increase in solar heating causes more and more ice to vaporize and extrude from the surface of the body. At periapse, particle production thus reaches a maximum. As the particles leave the surface, they begin to distribute themselves out along the path of the comet, creating a ring of meteoroids somewhat akin to the rings of Saturn. Eventually, they smooth themselves out so that the particle density is similar throughout the ring. However, for meteor streams whose parent has not yet completely evaporated or broken up, the particle density is, of course, largest close to the parent when that body in turn is closest to the Sun. If the Earth happens to pass close by the parent when the parent crosses the plane of the ecliptic, the resulting meteor shower has the potential of being more intense than usual. If this greater concentration takes a long time to disperse (i.e., multiple cometary periods), then each passage of the comet close to perihelion will create a "streamlet" within the larger stream that is still fairly close to the parent comet. These greater concentrations, either due to streamlets or fresh material, will be referred to as a near-comet type of meteor outburst. Far-comet types of outbursts can occur as well, wherein the generating comet is far from the Earth, but the resulting stream is perturbed such that either the Earth travels closer to the center of the stream or a resonance effect focuses the particles into a region of greater density. A greater-than-normal level of activity thus results. Regardless of whether the outburst is of a near-comet or far-comet type, normally tame meteor showers can turn into monstrous affairs for satellite operators (Fig. 1.2).
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Fig. 1.2. Particle density as a function of comet location. |
This situation has occurred in the past for various meteor showers and is happening again now for the Leonids. The Leonids' parent, comet P55/Tempel-Tuttle, cycles through its orbit once every 33 years. It most recently passed the Earth's orbit plane in early 1998; as a consequence, the Leonid showers of November 1998–2000 are expected to peak at an intensity that may be several orders of magnitude greater than usual. A similar situation occurred the previous time Tempel-Tuttle came around, in 1966. The resulting storm was anywhere from 1000 to 10,000 times greater in intensity than normal. Because there were so few satellites in orbit at that time, the storm did not cause any noticeable effects on the spacecraft. However, with so many satellites aloft at the current time and even though the 1998–2000 events are not expected to be as intense as they were in 1966, the anticipated increase in activity will pose a significant risk to orbiting vehicles.
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