Hazards by meteoroid impacts onto operational spacecraft

Advances in Space Research 33 (2004) 1507–1510 www.elsevier.com/locate/asr

Hazards by meteoroid impacts onto operational spacecraft M. Landgraf a

a,* ,

R. Jehn a, W. Flury a, V. Dikarev

b

ESA/ESOC, Robert-Bosch-Str. 5, Darmstadt 64293, Germany b MPI-K, Postfach 103980, Heidelberg 69029, Germany

Received 15 November 2002; received in revised form 25 June 2003; accepted 25 June 2003

Abstract Operational spacecraft in Earth orbit or on interplanetary trajectories are exposed to high-velocity particles that can cause damage to sensitive on-board instrumentation. Asteroids and Comets produce small solid particles as they disintegrate. The meteoroids stay on bound orbits around the Sun if they are larger than a few micrometres in size. Because of their high velocities they create different kinds of hazards to operational spacecraft. Here we discuss the impact hazards that lead to surface degradation, hull penetration, and plasma discharge. Surface degradation is mainly caused by meteoroids in the 0.01 to 0.1 mm range, hull penetration by meteoroids above 1 mm, and plasma discharge by meteoroids between 1 and 10 lm. Ó 2003 COSPAR. Published by Elsevier Ltd. All rights reserved. Keywords: Meteoroids; Space environment; Modelling; Impact hazard

1. Introduction Interplanetary space is filled with meteoroids with masses larger than 10
12 g at an average concentration of 2  10
3 km
3 at 1 AU (Gr€ un et al., 1985). Meteoroids are shed off the asteroids and comets of our Solar system as they collide with each other and as the Sun evaporates their volatile components. The most dramatic display of large-scale meteoroid production are the active comets like Halley and Hale-Bopp. The dust tails of these comets, however, consist mostly of micronsized grains that leave the Solar System quickly on hyperbolic orbits as they are pushed out by the pressure exerted by sunlight. The dominant mass lost by comets and asteroids, however, goes into grains with sizes greater than 1 mm, which are not as visible by groundbased observations of the reflected sunlight or the emitted thermal radiation. These grains initially stay close to their parent objectÕs orbit, and they orbit the Sun with Keplerian speeds, 29 km s
1 in the case of a circular near-Earth orbits. Meteoroids with sizes up to 1 mm will slowly drift away from their parent bodiesÕ orbits over an extended period of time, forming what is *

Corresponding author. E-mail address: [email protected] (M. Landgraf).

commonly referred to as the background meteoroid population. Even larger ones quickly disperse along the initial orbit due to a small variation in the mean motion, forming the parent bodyÕs trail. Those trails are observed as meteor outbursts and storms when the Earth crosses them. The chemical composition of meteoroids is known mainly from the collection of grains that have entered the Earth atmosphere. A selection of small enough grains with small enough entry velocity survive the plunge into the EarthÕs atmosphere and are collected regularly in the atmosphere by high flying aircraft (Brownlee, 1985). We find mainly a stony composition with dominant minerals also known on Earth being Olivines and Pyroxenes, but there are also organic components in meteoroids. There is a whole range of average material densities, with the average being 1 g cm
3 . At typical encounter speeds above 10 km s
1 a meteoroid of any density and tensile strength can damage space hardware. 2. Types of impact hazards Depending on the size, velocity, and location of a meteoroid impact, there are various hazards to operational spacecraft. The processes that have been observed

0273-1177/$30 Ó 2003 COSPAR. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.asr.2003.06.001

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on returned surfaces (Coombs et al., 1992; Aceti et al., 1994) are surface degradation and structural penetration. Another hazard that could not yet be linked directly to meteoroid impacts is plasma discharge. These types of meteoroid impact hazard are discussed below. 2.1. Surface degradation Surface degradation occurs when an impacting meteoroid creates a crater in the target material. The impact destroys the immediate surface and thus changes its optical and thermal properties. Fig. 1 shows a 500 lm crater on the multi-layer insulation (MLI) of the Hubble Space TelescopeÕs solar array boom. The diameter Dp of the impactor can be calculated from the crater diameter Dc when an impact speed m is assumed. From laboratory measurements (H€ orz et al., 1994) the following empirical relationship was derived: Dc ¼ Dp ð0:38ðm½km s
1 Þ þ 0:2Þ: For a typical impact velocity of 20 km s
1 the meteoroid that created the crater shown in Fig. 1 was less than 100 lm in diameter. While an individual sub-millimetre crater will not change the properties of the affected surface, the longterm exposure of that surface will lead to a degradation of its thermal and optical properties.

to either a slow loss of pressure or even a catastrophic failure of the containerÕs structure. A good measure for the penetration risk by a meteoroid is the depth p to which a meteoroid with given diameter Dp and impact velocity m can penetrate a given target material. An empirical relationship between these quantities was derived from penetration experiments (Naumann, 1966).  1=2   p m 2=3 1=8 qp ¼ 1:64ðDp ½cmÞ ; Dp c qt where qp is the bulk mass density of the penetrating meteoroid, qt is the bulk density of the target material, and c is the sound speed of the target material. The velocity dependence of the penetration depth is rather weak, and thus we can conclude that the penetration hazard is created mainly by larger meteoroids, with sizes in the same order of magnitude as the containerÕs wall. In order to protect the spacecraft against impactors that are larger than the thickness of the wall, one can utilise Whipple shields that consist of a number of walls. Laboratory experiments as well as numerical simulation (Thoma et al., 2001) of the impact process show that as the outer wall is penetrated, small fragments are created that have a much larger total surface area and are thus easier to stop by the subsequent layers of the shield (see Fig. 2). 2.3. Plasma discharge

2.2. Penetration Pressurised containers are part of almost every spacecraft in orbit today. Mostly used as propellant tanks they also serve other purposes such as gas storage of life support systems, and habitation modules. Due to the mass restrictions of space systems, the walls of these containers are often designed just to withstand the internal pressure. If an impacting meteoroid is large and fast enough, it can penetrate the container wall leading

Fig. 1. Micrometre-sized impact on the MLI of Hubble Space TelescopeÕs solar array boom. The external diameter is 500 lm.

Another, not so well documented, meteoroid hazard to spacecraft is the creation of a plasma cloud upon impact. The plasma consists of material of the impactor as well as the target. This phenomenon is studied in the laboratory and routinely used to detect solid particles in space (Gr€ un et al., 1992). The total ion charge Q that is created by a meteoroid with mass m at an impact velocity m is given by Q ¼ Cmv3:5 ;

Fig. 2. Simulated penetration of a spacecraft structure by a solid particle (Thoma et al., 2001). The left panel shows the situation 15 ls after the penetration of the outer (left) wall. After 150 ls the second wall is hit by the debris cloud (right panel).

M. Landgraf et al. / Advances in Space Research 33 (2004) 1507–1510

where C is a constant that depends on the target material. This localised plasma cloud potentially causes two kinds of failure modes of a spacecraft. First, if the spacecraft is unevenly charged by differential illumination of different spacecraft parts by solar UV photons, the plasma can act as a conductor and create a current between the differentially charged parts, causing disturbance or destruction of the on-board electronics. In particular excessive current in the main computer will likely lead to the end of the operational lifetime of the spacecraft. There is speculation Caswell (1994) that the failure of the Olympus satellite was caused by a plasma discharge. The other potential problem is the short-circuiting of high-voltage instruments. While these instruments are considered payload and are not normally required for the operation of the spacecraft, an instrument failure can strongly diminish the value of a mission. Normally high-voltage instruments are used to detect charged particles. The relatively high density of ions in the impact plasma cloud, however, will create an excessive current in the instrument, potentially leading to a destruction of the electrodes or the amplifier electronics. As a consequence of strong dependence of the ion charge on the impact velocity, especially fast meteoroids are potentially dangerous. As the mass dependence is much weaker, even small meteoroids can cause substantial damage. As they are much more numerous (see below), the plasma discharge hazard comes mainly from small meteoroids.

3. Mass distribution As discussed above, meteoroids of different mass dominate the different hazard types. In order to assess the risk for various missions, the meteoroid flux-mass distribution in the respective environments has to be taken into account. Based on the model by Divine (1993), the flux-mass distribution of meteoroids can be calculated depending on the mission. As an example Fig. 3 shows the results for a spacecraft in low Earth orbit, similar to the orbit of the International Space Station, for a spacecraft in geosynchronous orbit, and for a spacecraft in interplanetary space, similar to the trajectory of the Rosetta mission to comet ChuryumovGerasimenko. The shape of the flux-mass distribution is similar for low Earth orbit, geosynchronous, and interplanetary missions. The absolute flux, however, differs, due to the different geocentric distances of the missions. This is caused by the gravitational focusing effect in the EarthÕs vicinity. As interplanetary spacecraft are outside the sphere of influence of the Earth, the meteoroid flux is not enhanced. At the geosynchronous altitude, the flux

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Fig. 3. Cumulative flux-mass distribution (impacts per unit area and year) of interplanetary meteoroids according to the model by Divine (1993) for three different missions. The solid curve shows the distribution for a spacecraft (assuming a randomly tumbling target) on an orbit similar to the one of the International Space Station, the dashed line corresponds to a geosynchronous spacecraft, and the dotted line refers to an interplanetary mission, similar to the trajectory of ESAÕs Rosetta spacecraft.

is increased by about a factor of 3 and at low Earth orbit by another factor of 2.

4. Sensitive spacecraft subsystems Given the hazard types discussed above, we can identify parts of a typical spacecraft that are specially susceptible to meteoroid impacts. Surface degradation mainly affects functional surfaces such as solar panels and thermal radiators. Also optical surfaces like telescope mirrors and camera lenses can be affected. Propellant tanks and habitation modules as well as gas storage containers are mostly affected by meteoroid penetration. As meteoroids with sizes significantly below the wall thickness do not penetrate the hull, only impact events by larger meteoroids have to be taken into account. For a 100 m2 spacecraft in low Earth orbit less than 1 impact of a background meteoroid larger than 1 mm is expected every year. We can thus conclude that due to the small number of large meteoroids, penetrations of containers with wall thicknesses above a few millimetre is not a very important hazard to be considered. This might be different, however, during meteor storm conditions. The devices that are most affected by impact plasma discharge are all systems that rely upon a high voltage electrical field for their operation as well as highly integrated circuitry. Examples are the on-board computer, charged particle detectors, and electric propulsion systems. Minor plasma clouds can be expected frequently, up to 50 times per year for a 0.1 m2 sensitive area.

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5. Summary

References

We have discussed the types of hazards caused by the meteoroid environment in interplanetary space as well as in Earth orbits. Three main hazard types have been identified: surface degradation, hull penetration, and plasma discharge. Each of these has different characteristics in the relevant meteoroid size regime as well as in affected subsystems. Surface degradation is mainly caused by meteoroids of sizes between 0.01 and 0.1 mm and affects large-area functional surfaces. Penetration that affects pressurised containers is caused by meteoroids of sizes above 1 mm, which are not abundant in the background. The plasma discharge is a problem caused by very small meteoroids in the size range between 1 and 10 lm that affect mainly high-voltage devices. Due to the stochastic nature of individual impacts and the broad directionality of the background meteoroid population, risk assessment and shielding are the best methods of protection. For risk assessment a reliable model of the background meteoroid population is needed. ESA progressively develops a meteoroid model, an initial version of which was introduced by Divine (1993). The ESA meteoroid model is constrained mainly by in situ data from spacecraft measurements. An expansion of the model with more data from ground-based radars as well as space-based infrared observations is under development (Dikarev et al., 2001). Based on observations of meteor streams by optical means as well as radar (Jenniskens, 1994; Yrjola and Jenniskens, 1998) the ESA debris model also provides a prediction of fluxes of stream meteoroids. Protection from meteor stream particles also requires shielding. However, as for most meteor streams the timing as well as the directionality is known, an inexpensive method is to select the spacecraft attitude such that the most sensitive subsystems do not face the stream during the period of maximum activity.

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