The Threat of Space Debris and Micrometeoroids to Spacecraft Operations

by Frank Schäfer

The historical practice of abandoning spacecraft and upper stages at the end of mission life has allowed roughly 2 million kg of debris to accumulate in orbit. The debris with sizes ranging from micrometers to meters poses a threat to current and future space missions. In addition, spacecrafts are constantly impacted by micrometeoroids. As a result of extremely high relative impact velocities, millimeter-sized particles can penetrate spacecraft structure walls and severely damage or destroy spacecraft components. Potential outcomes of such encounters in orbit range from temporary perturbations of spacecraft operations to termination of the mission.

To date, there are roughly 9,000 catalogued objects in orbit with a size larger than 10 cm (Figure 1), of which only about 600 are functioning satellites and the remaining ca. 8,400 classified as space debris. Most of the Space Debris mass consists of non-functional satellites and upper stages of launchers. The majority of millimeter- and sub-millimeter-sized debris particles were generated through one of the ca. 170 explosions that have been registered up to now. Such explosions can be caused by spontaneously triggered combustion of residual amounts fuel in upper stages or by overcharged batteries. The micron-sized debris particles mostly stem from the combustion products of solid rocket motor firings and fragments of varnish. The encounter velocities between space debris and spacecraft in low earth orbits are in the range up to about 15 km/s, which corresponds to head-on collisions. Micrometeoroids can have much higher impact velocities, depending on their origin. Even tiny particles possess considerable kinetic energies as a result of the very high impact velocities.

The Effects of Hypervelocity Impacts on Spacecraft
Hypervelocity impacts can affect spacecraft in various ways. Micron sized particles can degrade sensitive spacecraft surfaces and equipment, like mirrors and optical sensors. Larger particles with sizes ranging from tens to hundreds of microns can penetrate coatings and foils as well as solar cells. Damage such as this has been observed on satellite surfaces returned to Earth (LDEF, HST solar arrays, EURECA) and on the windows of the U.S. Space Shuttle which have been replaced many times due to impact damage. Millimeter-sized particles can penetrate satellite structure walls or shielded walls of manned spacecraft, posing a serious threat to equipment, astronauts, or both. To reduce the destructive effects of impacts, all modules of the International Space Station have debris shields to defeat sub-cm objects. Impacts of such large particles may also induce considerable changes in the satellite’s attitude through transfer of momentum. The impact of centimeter- or decimeter-sized particles will typically lead to complete destruction of important spacecraft parts or even to disintegration of the spacecraft. Prominent examples of collisions involving large fragments with spacecraft are the 1996 collision between the French CERISE military satellite and a 1 m fragment that was generated from the explosion of an Ariane 4 upper stage 10 years prior, and the 2005 collision between an American Thor rocket motor with a large fragment of the third stage of a Chinese CZ-4 launcher. Many satellites and manned space-stations are known to have performed collision avoidance manoeuvres with catalogued Space Debris parts, such as ESA’s ERS-1 and ENVISAT satellites, the U.S. Shuttle, the MIR station and the ISS, to name only a few. Besides the effects of structural damage, every hypervelocity impact generates metal vapour plasma that can result in electromagnetic interference or result in plasma-induced discharges: The European Space Agencies’ (ESA) OLYMPUS communication satellite may have failed as a consequence of hypervelocity impact of a Perseid meteoroid in 1993.

Figure 1: Catalogued Space Debris particles in low earth orbit.

Experimental Investigation of the Vulnerability of Spacecraft Equipment
Consequently, measures to protect spacecraft against space debris are being investigated all over the world. Most of the studies concentrate on reducing the vulnerability of spacecraft by introducing external shielding. These studies ignore the intrinsic impact protection capability of the equipment under consideration. To overcome this shortcoming, ESA funded work to investigate the vulnerability of satellite equipment to hypervelocity impacts and the corresponding equipment failure modes (ESA contract 16483, Michel Lambert). The considered equipment was fuel and heat pipes, pressure vessels, electronics boxes, harness, and batteries. All equipment was placed behind aluminium honeycomb sandwich panels (Al H/C SP), representing the typical satellite structure wall. The impact experiments were performed at Fraunhofer Institute for High-Speed Dynamics - Ernst-Mach-Institut - in Freiburg, Germany, under the supervision of Robin Putzar, using their powerful two-stage light gas gun accelerators to simulate experimentally hypervelocity impacts of Space Debris particles in a laboratory environment. Cooperating partners were QinetiQ of Farnborough, UK (Hedley Stokes), and OHB-System AG in Bremen (Rolf Janovsky, Oliver Romberg). One novel aspect of this project was that the equipment was evaluated in its normal operating mode, thus being highly representative of actual spacecraft operation. In the following sections, some results of impact tests on operating harnesses and computers placed behind typical satellite structure walls are provided and discussed.

Figure 2: Temporary errors in data transmission encountered during impact on satellite harness.

Investigation of Data Transmission Degradation within Electrical Harnesses
The main function of harnesses onboard satellites are power distribution and data transmission. Harnesses onboard spacecraft are typically arranged in bundles and can be routed through several paths throughout the spacecraft. Electrical harness can claim large areas at the inner surfaces of the satellite structure wall. On a typical spacecraft, the total weight of harness can amount to several percent of the overall spacecraft weight. Harnesses primarily are critical due to the fact that they are often located just behind the satellite structure wall. An impacting particle which penetrates the spacecraft structure may endanger unprotected harness, since the impact fragments are dispersed into a ‘spray cone’ that may hit and severely damage large parts of unprotected harness. Each harness submitted to hypervelocity impact testing consisted of several operating power- and twisted-pair data cables, and one radiofrequency (RF) line, transmitting a 9.35 GHz signal. An example of data transmission measurements is shown in Figure 2, where the differential transmission method is used. As can be seen, there is temporary data transmission errors during several tens of microseconds, followed by nominal operation of the cables at later stages after the impact again. Larger impact energies can lead to more violent impact damages that can cause longer temporary perturbations up to permanent failure of operation e. g. the severing of cables. It is to be expected that such damages lead to a functional deficiency of the entire spacecraft.

The larger the stand-off between structure wall and harness, the lower the probability of failure is. Therefore, if it is feasible, harnesses should be moved away from structure walls. If additional spacing cannot be realised, wrapping the harness in a moderate amount of protective fabrics, such as Nextel or Kevlar, should improve dramatically the protection performance. NASA has followed such procedures successfully for ISS harnesses routed outside the manned modules.

Study of Hypervelocity Impact Influence on Spacecraft Computer Operations
Computer boxes are needed for on-board data processing/handling (OBDH), control-, monitoring-, telecommunication (TC), telemetry (TM), avionics and payload equipment. A share of 20%-40% of a satellite bus volume consists of computers. Computer boxes contain Printed Circuit Boards (PCB) with analogue and digital components, capacitors, inductors, resistors, and micro-chips, which are enclosed in a milled aluminium box with a thickness of about 2 mm for electromagnetic compatibility and radiation shielding reasons. The criticality of electronic hardware is slightly reduced by the fact that most electrical components are redundant. Moreover, the delicate electronics are shielded passively by their own housing. However, the complete failure of an e-box will result at least in complications until the redundant system has taken over, if a redundant system exists. Otherwise failure of an e-box could mean the loss of a complete subsystem, ie, the OBDH, TC, TM, with potentially catastrophic consequences for a mission. During the hypervelocity impact tests, the computer-boxes were operating in what is considered normal mode, performing basic read- and write-operations. The observed failure modes were temporary failure and permanent failure. The temporary failures caused interruptions in the operation of the processor, followed by nominal operation a few milliseconds later. The reason for temporary failures is assumed to be related to conductive penetrating dust-like fragments causing transient shorts. Any temporary failure ie, temporary loss of operational performance of electronic components may manifest itself to the systems operator as an in-flight anomaly. Such in-flight anomalies, including faulty data transmission and ‘ghost commands’, have been reported by spacecraft operators and may possibly be explained by hypervelocity impacts. The permanent failures manifested as sudden loss of supply voltage or loss of nominal operation of the computer. In Figure 3, a PCB with severe impact damages (memory chip and resistors + capacitances removed, deposits of metallic spray in various locations) and the corresponding CPU signals are shown.

Figure 3: Degradation of computer performance followed by cease of operation shortly after encounter of the hypervelocity particle.

This study was a first step towards a better understanding of vulnerability of spacecraft equipment to hypervelocity impacts. Still, considerable efforts need to be made especially in the experimental area to generate a comprehensive picture of all effects related to the vulnerability of spacecraft equipment to hypervelocity impacts. However, the investigations performed have already led to a drastic enhancement of knowledge that can now be exploited by spacecraft designers. Amongst others, the work performed can be used by spacecraft operators to possibly provide explanations for unexplained malfunctions of equipment operations in satellite missions.

Links:
ESA website: http://www.esa.int
Fraunhofer EMI: http://www.emi.fraunhofer.de

Please contact:
Frank Schäfer, Fraunhofer Institute for High-Speed Dynamics - Ernst-Mach-Institut, Germany
Tel: +49 761 2714 421
E-mail: schaeferemi.fraunhofer.de