Environments

SPORADIC METEOROID ENVIRONMENT
 
 
The sporadic meteoroid environment consists of a diffuse background of meteoroids of cometary and asteroidal origin. It represents a continuous risk to spacecraft throughout the year. The constant threat presented by the sporadic meteoroid background must be mitigated by an appropriate spacecraft design, which can lead to significant engineering challenges. A designer must determine how much shielding is necessary for a spacecraft, and what parts of the spacecraft will be most exposed during its mission.

The Meteoroid Environment Office (MEO) has developed the NASA MSFC Meteoroid Engineering Model (MEM) to define the sporadic meteoroid environment for spacecraft in interplanetary space and Earth orbit. It is used as a tool for spacecraft designers.

MEM Software

The MEM software package, MEM Release 1.0, is available from the MEO by request, It consists of two modules: Interplanetary MEM (IPMEM) version 1.6 and EarthMEM version 1.0.

IPMEM version 1.6 defines the sporadic meteoroid environment from Mercury to Mars. It cannot be used near any planet, i.e. within the planet’s sphere of influence. It cannot be used near the Moon. IPMEM calculates the direction and speed of the sporadic meteoroid flux for a spacecraft orbiting the Sun. This flux and directionality is then described relative to a basic cube shaped spacecraft in a body fixed coordinate frame.

To accommodate Earth orbiting spacecraft, a new version of MEM, EarthMEM version 1.0, is now available to define the sporadic meteoroid environment. EarthMEM calculates the direction and speed of the gravitationally focused sporadic meteoroid flux relative to an Earth orbiting spacecraft. It also accounts for planetary shielding effects as a function of orbital altitude. This focused and shielded flux is then described relative to a basic cube shaped spacecraft in a body fixed coordinate frame.

MEMCXP Software

A derivative of MEM known as MEMCxP (so named because it was developed for the Constellation program) is currently going through validation and incorporation into JSCs risk assessment tool, BUMPER. MEMCxP differs from MEM Release 1.0 in that it randomly selects points in time from an orbit to construct an average meteoroid environment with a one sigma variation. These output files are then incorporated into BUMPER for risk analysis.

MEM Background

The core model for MEM was developed from a physical model of the sporadic meteoroid environment. This model incorporates directionality which is absent or incorrect in previous NASA models (i.e. NASA TM4527, SSP30425, and Divine). Initial model development was performed at the University of Western Ontario (UWO) with funding provided by the Space Environments and Effects (SEE) Program. See McNamara et al, (2004) for more details.

The sporadic meteoroid complex as observed from Earth is known to have four major sources in six radiants distributed symmetrically about the celestial sphere. The location of these different sources is described in a Sun-centered coordinate system. Referring to the Jones and Brown orbital survey paper of 1993, the primary sporadic meteoroid sources are the Helion/Anti-Helion, the North/South Apex, and the North/South Toroidal; these three sources are associated with cometary material. Sporadics from the Helion source at ~342º solar longitude appear to originate from the Sun; Anti-Helion sporadics, at ~198º solar longitude, appear to originate opposite the Sun. The Apex source, broken up into North Apex and South Apex branches, straddles the ecliptic plane in the direction of Earth’s motion at ~271º and 273º, respectively. The Toroidal source, divided into North Toroidal and South Toroidal branches, has high ecliptic latitudes above and below the Earth resembling a toroid around the Earth’s orbit. The fourth and least understood source is the Asteroidal source. Observed asteroidal meteoroids are predicted to have inclinations close to the ecliptic poles, at about ±90º, in the apex direction.

sporadic sources

Each of the six sources has a relative strength and a speed distribution. MEM accounts for these varying source strengths and speed distributions and has been validated against radar observations from the Canadian Meteor Orbit Radar (CMOR) and corrected for known biases to the best ability of the developers.

The total average cross-sectional flux as a function of mass as given by the interplanetary model at 1 AU follows the same mass index as the popular Grün/Zook mass index reported in NASA TM4527 and SSP30425. These values for average cross-sectional flux are within 2% of the daily flux values as reported by CMOR. This average cross-sectional flux does contain meteor shower fluxes but in an average sense only. Meteor storms and outbursts should be modeled separately and the risks mitigated operationally.

 
 
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http://www.nasa.gov/offices/meo/environments/sporadic_detail.html