Engineering topics
Publications and source records attributed to Althea Valkyrie Moorhead.
The purpose of this document is to provide a forecast of major meteor shower activity in low Earth orbit (LEO). Most major showers are expected to exhibit typical activity, but the Andromedids, Aurigids, and “Finlayids” (a possible new shower originating from comet 15P/Finlay) may produce outbursts.
A meteor shower can be modeled, to first order, as a parallel stream of particles encountering the Earth. Gravity bends the trajectories of these particles inward, producing an increase in flux both near the planet and along the anti-radiant line. This effect is known as gravitational focusing, and the interception of a portion of meteoroid trajectories by the Earth or another massive body is known as planetary shielding. For a perfectly parallel meteoroid stream, gravitational focusing produces a flux singularity along the anti-radiant line, near which intense enhancements in flux occur. In reality, meteoroids will exhibit some dispersion in their motion that removes this singularity. We present a modified analytic treatment of gravitational focusing and planetary shielding that takes this dispersion into account and discuss its ramifications for several problems in meteor astronomy.
A meteoroid environment model must describe, at a bare minimum, the local density or flux of meteoroids. To accomplish this, many models make use of analytic equations to convert orbital distributions to number density, or to compute the effects of a planet's gravity on flux and speed. However, these equations often contain singularities that are unrealistic. For instance, the orbit-to-density conversion used by Jones (2006) is unbound at peri- and aphelion, and the gravitational focusing algorithms presented by Staubach et al. (1997) are unbound along the anti-radiant line. We present methods for removing these singularities that result in more realistic descriptions of the meteoroid environment.
The best way to obtain information about a meteoroid's composition is to measure meteor spectra at a resolution fine enough to distinguish between emission lines. However, simple color filters are cheaper and easier to use than a grating or radiometer, and it is therefore tempting to try to use a set of filters to characterize a meteor's "color" despite the extremely coarse resolution of this approach. To test whether color filters can provide useful information about meteoroid composition, we convolved the light curves from a catalog of meteor spectra (Vojáček et al., 2015) with U, B, V, R, and I Bessel filters. We find that the Borovička et al. (2005) meteor spectra classifications cannot be retrieved from color alone, with the possible exception of iron meteoroids.
Astronomers sometimes use color – the relative brightness of an object seen through two different color filter – as a proxy for other quantities. For instance, a star’s color indicates its temperature because stellar spectra vary in a predictable way.Meteor spectra are dominated by emission lines and reducing them to a “color” is not as straightforward.We combine meteor spectra from the literature with standard Bessel filter functions to test whether color measurements can probe a meteor’s spectral type.
Astronomers sometimes use color – the relative brightness of an object seen through two different color filter – as a proxy for other quantities. For instance, a star’s color indicates its temperature because stellar spectra vary in a predictable way.Meteor spectra are dominated by emission lines and reducing them to a “color” is not as straightforward.We combine meteor spectra from the literature with standard Bessel filter functions to test whether color measurements can probe a meteor’s spectral type.
Orbital debris poses a substantial threat to many Earth-orbiting spacecraft, particularly those near Sun-synchronous altitudes. Man-made debris is not equally prevalent at all altitudes, however; geostationary satellites likely experience more damage from meteoroid impacts than from debris. We have conducted a comparison between debris- and meteoroid-induced damage on cubic aluminum spacecraft orbiting the Earth at a variety of altitudes and orbital inclinations using NASA's latest models of these environments: ORDEM 3.1 and MEM 3. We use the results to determine in which regions the risk is debris- or meteoroid-dominated, and find that debris dominates the risk between altitudes of about 600 and 1300 km above the Earth, while meteoroids dominate at altitudes less than 270 km or greater than 4800 km.
The sporadic meteoroid environment displays distinct groupings of meteoroid orbits that are referred to as the sporadic "sources." These sources are linked to different dynamical classes of parent bodies; for instance, the so-called helion and anti-helion sources are attributed to Jupiter-family comets. Because the sporadic sources have different orbital distributions and thus encounter the Earth's atmosphere at different speeds, meteor observing biases tend to obscure the relative contributions of the sporadic sources to the overall meteoroid flux. We present a de-biased map of both meteoroid radiants and speeds and compare this map with two meteoroid dynamical models: Jones (2004) and Wiegert et al. (2009).