PREFERRED RELIABILITY PAGE 1 OF 6 PRACTICES METEOROIDS & SPACE DEBRIS Practice: Deign pacecraft external urface to enure 95 percent probability of no miion-critical failure from particle impact. Benefit: Reliability i greatly enhanced becaue the likelihood of eriou miion degradation or pacecraft lo i ignificantly reduced. Program That Certified Uage: Voyager Center to Contact for Information: Jet Propulion Laboratory (JPL) Implementation Method: Prepare deign requirement which pecify mean velocity, ma denity, and ma ditribution for the impacting particle in term of the integral fluence. Thi fluence (ample unit m ) repreent the expected number of impacting particle per unit area, above everal different ma threhold, for the miion (uing the wort cae trajectory if more than one i contemplated). The deign mut then atify two eparate requirement: (1) that the mallet penetrating particle have a probability of impact below 5%, uing the product of fluence with vulnerable area and a Poion ditribution, and (2) that for maller particle, of which many will impact any given pacecraft urface, the reulting degradation of urface propertie (e.g., optical, thermal, dielectric) doe not exceed allowable range for urface performance (conidering, e.g., pitting, pallation, contamination, etc.). In practice, the firt of thee refer to a um of probabilitie over a variety of vulnerable pacecraft urface (each having pecific value for area and threhold penetrating ma), allocated o a to make effective ue of reource (e.g., hielding ma) and to achieve the deired probability for miion ucce. For thi purpoe, experience dictate that a two-urface configuration, of which the outer urface erve a the thermal blanket a well, provide the leat maive meteoroid protection. Technical Rationale: For a given miion (pecified in term of geocentric and heliocentric poition a function of time, for example), the environment compriing impacting olid JET PROPULSION LABORATORY
PAGE 2 OF 6 particle are both independent of miion control and rather uncertain. The flux and fluence of uch particle can be evaluated from uitable numerical model (here for pace debri and for interplanetary meteoroid, although other may occur, e.g., for Saturn ring particle). The integral fluence typically decreae a ma increae according to a power law, illutrated here uing the exponent ": F ' F 1 m 1 m " (1) Here F and F 1 repreent the integral fluence (ample unit m ) for particle with mae greater than m and m 1, repectively, accumulated over the miion. Table 1 provide example of uch ditribution (where the exponent i not necearily contant over the range of mae of interet) and additionally pecifie mean denity and impact velocity. For large particle, the ditribution repreented by equation (1) or Table 1 imply that the expoed urface area A of a pacecraft ubytem ha a probability P ' 1 & e (&A F) that no particle larger than the ma m (correponding to the fluence F) will hit, where equation (2) i obtained auming Poion tatitic for the particle impact. If the urface i deigned o that no particle of ma m or larger, impacting at the mean velocity, can penetrate or lead to other component failure (e.g., by pallation), then the probability of no failure i alo P (auming that penetration lead to component failure with unit probability). When P i mall for each ubytem (a i the cae when the area-fluence product in eq. 2 i much le than unity), the um P t ' j P p (2) () repreent the probability of failure (P t) of the ytem, where p i the conditional probability that the ytem fail when ubytem fail. If the value of p are not independent then equation () mut be replaced by the appropriate combination of probabilitie. Finally, the probability of miion ucce, conidering particle impact alone, become (1-P ), and deign mut proceed to enure that t thi quantity exceed the 95% probability cited above. To protect a ubytem againt thoe large particle for which equation (2) applie, and for impact velocitie larger than a few km/, hypervelocity impact experiment how that a two-urface configuration (often named a bumper hield) prevent penetration far more effectively than a ingle urface of the ame ma. Thi i o becaue the kinetic energy of impact lead to the vaporization (or liquefaction or diintegration) of the projectile when it hit the outer target urface; the momentum i thereby dipered over a large area a the vapor expand in the pace between the urface, and become le capable of rupturing the econd urface than had the latter been hit directly. Typically a thickne of a few tenth of a millimeter, and a tandoff ditance of a few centimeter, uffice to prevent penetration of a pacecraft tructural wall by a milligram particle arriving normally at 15 km/. For a ample configuration, Figure 1 diplay the threhold penetration
PAGE OF 6 ma a a function of impact velocity. Such a figure can be ued to verify by analyi that the deign doe not fail for the ma neceary for equation (1) through () to provide the required probability; and a parametric et of uch figure panning a uitable deign pace can be ued to elect the deign appropriate for a given pacecraft aembly. In thi deign proce, the uncertaintie in penetration threhold and the variability thereof with angle of incidence (Fig. 1 and related data are commonly preented for normal impact, oblique impact being le well characterized) mut be conidered, poibly by application of margin to ome meaure of hield effectivene (the ue of Poion tatitic for probability of impact i intended to cover only environment uncertaintie, not hielding one). In many cae, thermal blanket of a ingle deign and tandoff will erve a an appropriate bumper hield for much of the pacecraft body. Analytic formulation for hypervelocity penetration, and for bumper pacing and other parameter, hould be elected carefully for relevance to the pecific impact regime (e.g., projectile peed, direction, denity, etc.). The reulting deign hould be verified by teting whenever poible, and the tet hould pan or imulate the range of expected projectile ize and velocitie. For much maller particle, the power-law ditribution (eq. 1) enure that the area-fluence product exceed unity for mot expoed pacecraft urface, and that numerou mall particle will trike the urface. For urface which are hielded a decribed above, thee maller particle are of no conequence, except a they alter the thermal propertie of the urface; the thermal control deign mut provide enough latitude that thee change do not lead to internal temperature beyond the acceptable range for flight. For other urface, concern arie only if a few critical urface propertie mut be maintained; for example, tructural integrity and magnetic cleanline are not threatened by thee mall impact. Among uch critical propertie, optical quality i often the mot eriou, a in lene or mirror whoe performance can be degraded by pitting, eroion, or contamination. For uch component, ad hoc olution to the particle impact problem, poibly involving articulating cover, may be neceary if analyi demontrate that the particle fluence repreent a ignificant hazard to unprotected urface. Typically, if a pecific fluence value i required for deign purpoe in thee cae, a margin of a factor 2 i applied to the nominal value (e.g., Table 1) to account for the uncertainty in the environment of thee maller particle. Impact of Nonpractice: A an example of noncompliance, conider a pacecraft bu, containing critical electronic part, whoe hear plate and thermal blanket are adjacent (i.e., not eparated by the tandoff which characterize a uitable two-urface particle hield). The larget incident particle, namely that for which the area-fluence product i near unity (ref. Table 1) i then, for long-duration miion in low Earth orbit or in the inner olar ytem, capable of penetrating the electronic houing or of producing pallation from the inner urface. In either cae a ingle impact introduce numerou fat-moving fragment into the electronic themelve, everal electronic part will be diabled imultaneouly, and the probability i high that the ubytem' function will be everely compromied, reulting in miion failure if the ubytem i critical. Even if redundant aemblie are provided, they hould not be packaged together, becaue one particle impact may detroy them both.
PAGE 4 OF 6 Even if packaged eparately, the likelihood of two impact (of a penetrating particle) i only modetly maller than that of one impact, o that redundancy i a far more expenive and le effective option than the proviion of uitable particle impact hielding in the firt place. An equally eriou failure reulting from noncompliance i illutrated by the cenario, extenively invetigated for the Galileo pacecraft, that a meteoroid penetration of the propellant tank could reult in lo of fuel, a conequent change in the pacecraft velocity vector, unintended reentry into the Earth' atmophere (intead of the intended cloe flyby for gravitational ait), and widepread atmopheric diperion of radioactive fuel from the RTG (radioiotope thermoelectric generator). In thi cenario, failure to provide adequate meteoroid protection could have both life-threatening and major legal conequence, albeit with mall probability. An exceptionally thorough analyi, in which the velocity ditribution and the time dependence of the meteoroid flux were ued in addition to the appropriate analog of the above equation, wa needed to quantify thi mall probability.
PAGE 5 OF 6 Table 1. Integral fluence of cometary meteoroid a a function of particle ma for three ubet of the Galileo miion (column two and three include interplanetary meteoroid near Jupiter, a focued by Jupiter' gravitational field). Particle Ma-M (1) Integral Fluence Received (2) Integral Fluence Received Miion Integral (gram) during Tranit* during Orbit** () Fluence (Particle-m of ma greater (Particle-m of ma greater (Particle-m of ma than M) than M) greater than M) -12 4 10 1.06 x 10 7.89 x 10 1.85 x 10-10 10 4.27 x 10.17 x 10 7.44 x 10-8 2 2 10 5.7 x 10.99 x 10 9.6 x 10-6 10 21.2 15.7 6.9-1 10 1. 9.6 x 10 2.29 10 8.15 x 10 5.9 x 10 1.41 x 10 10 4.99 x 10.6 x 10 8.59 x 10 10.06 x 10 2.2 x 10 5.26 x 10-1 10 1.87 x 10 1.6 x 10.2 x 10 0-6 -7 10 1.15 x 10 8. x 10 1.98 x 10 4 2-1 -6 Mean relative 15.9 15.9 15.9 peed (km/) Particle ma 0.5 0.5 0.5 denity (g/cm ) (cometary origin) *Tabulated value envelope the Galileo tranfer trajectorie including VEEGA, delta VEGA 2, delta VEGA, and direct. - - ** Fluence reulting from JOI and the firt 5 orbit of the Galileo 79-1 Tour, include gravitational focuing from Jupiter. (1) 95% confidence environment - 2.0 x fluence pectra (2) 95% confidence environment - 5.6 x fluence pectra () 95% confidence environment - 4.4 x fluence pectra
PAGE 6 OF 6 Figure 1. Meteoroid critical ma a a function of impact peed for the Caini propellant tank, including a bumper hield and fluid within the tank. The line are for different denitie of the impacting meteoroid, with and without fluid in the tank, and the power-law egment repreent different regime of failure.