Figure 1. The impulse voltage generator (with only 10 stages). The gold balls (on right) are used to transmit the current to the object being tested
To fulfil its mission to collect scientific data from Titan, ESA's Huygens probe must descend through the Titan atmosphere. In doing so, it faces the possibility of being struck by lightning, particularly in the lower regions of the atmosphere where clouds are known to exist. Protection against such a threat had to be built into the design, and the design later had to be tested. It proved to be the first time that a European spacecraft had to be tested for lightning susceptibility. The probe performed well in the testing: it was able to survive all strength of strikes aimed at it. This gives the project team the confidence to forge forward with the assembly of the flight model in preparation for the launch in October 1997.
After an interplanetary cruise phase of some seven years, ESA's Huygens probe will be targeted on Titan and released from its NASA mothercraft, Cassini. Thereafter Huygens will begin its 22-day coast phase prior to entering the Titan atmosphere on an ambitious mission to collect scientific data from the largest moon in the Saturnian system.
During the probe's design phase, concern was raised about the possibility of the probe being struck by lightning during its descent through the Titan atmosphere, particularly in the lower regions where clouds are known to exist (cloud movement causes triboelectric charging). Although there is little hard evidence of spacecraft suffering from lightning strikes during flight, a few facts and incidents deserve to be mentioned:
Lightning is known to exist in the atmospheres of Earth and Jupiter, but what about Titan? No terrestrial-like electrical discharges were observed by Voyager 1 during its fly-by in 1980. This, however, does not rule out discharges with a magnitude, repetition rate and characteristics different to those known on Earth. In fact, the objective of one of the Huygens probe experiments will be to search for and characterise lightning in the Titan atmosphere.
Given the threat, it was deemed necessary to protect the probe from damage by a possible Titan lightning strike. The primary method used was to shield all of the probe instruments in a metal 'cocoon'. This decision imposed certain constraints on the design at both system and subsystem level. The effectiveness of the chosen design solutions consequently had to be tested and verified on the Engineering Model (EM) probe.
Testing of a spacecraft for susceptibility to lightning is unusual for ESA as none of its vehicles have yet been required to perform an entry/re-entry through an atmosphere. In fact, it is believed that the Huygens probe is the first fully functional European spacecraft that may be subjected to direct lightning strikes.
The venue chosen for the test was the Universit t der Bundeswehr in Munich, Germany, which is not far from the facilities of Daimler-Benz Aerospace, the company responsible for the assembly, integration and testing of Huygens. The university's high-voltage test facilities are mainly dedicated to research work, but about 20% of their time is allotted to commercial testing in order to stay in touch with the 'outside world'. Where possible, unusual tests are selected for the commercial apportionment. The testing of the probe fell squarely into this category.
The facility has:
Figure 1. The impulse voltage generator (with only 10 stages).
The gold balls (on right) are used to transmit the current to the
object being tested
Figure 2. The lightning current simulator with its two impulse current generators
A short example helps to put the above numbers into perspective: the facility performed one test that involved generating an electrical arc over a distance of four metres.
The impulse voltage generator was used to test the probe. The test that was chosen consisted of a number of direct strikes of 5 kA maximum amplitude with a pulse rise time of 50 kA/microseconds. The amplitude selected is considerably less than the 200 kA used as the terrestrial model when testing an Earth-bound design. Because Titan is much farther from the Sun than the Earth is, there is less energy available on Titan, less convective motion, and Titan has a more conductive atmosphere.
The test configuration used (Fig. 3) was with the probe mounted fore dome upwards and isolated from the ground plane by standing it on wooden blocks. The current was injected by positioning the electrode with an appropriate spark gap at selected points on the fore dome and on the central ring, with the ground strap positioned on the probe top platform and on the central ring.
Figure 3a. The probe being prepared for testing, with the impulse
voltage generator in the background. The central ring (silver
band) separates the fore dome (on top) from the probe top
platform (copper-coloured, on bottom)
Figure 3b. The probe undergoing testing for susceptibility to lightning.
An electrode (dish shape above fore dome) with a suitable spark
gap is placed at selected points on the fore dome and the central
ring, and current of different amplitudes is injected
The current injection and exit points were chosen to ensure that main current paths through the probe structure would be through or adjacent to areas of known susceptibility. For example, most cables were routed in a bundle around the periphery of the equipment platform, which is directly on the inside of the central ring, so magnetic fields set up by large current pulses travelling around this ring could conceivably give voltages induced into some of the signal lines contained in these bundles.
The test was performed with the probe activated and running in descent simulation mode, with power being provided by its internal batteries. Data communications were via specially- constructed fibre optic links thereby preserving the electrical isolation of the item under test. Prior to the start of the susceptibility testing, the probe had to be initialised via its Electrical Ground Support Equipment (EGSE); this essentially runs through the procedures performed by the orbiter prior to separation with the probe. Following initialisation, the umbilical connections were disconnected and the EGSE was removed from the test chamber since it is built only to commercial standards and is not 'lightning proof'.
The plan was to start the testing with a low-amplitude current level of about 1 kA and build up progressively to the maximum value, noting susceptibility levels if any should occur. Strikes were made first on the fore dome and then on the central ring. Both the audible and visual effects were impressive, even when viewed from a distance of several metres through a protective metallic grid. The probe proved to be virtually impervious to any strength of strike that was directed at it and continued to operate normally throughout the test. There were a few very minor 'hiccups' on the telemetry data stream which resulted in a loss of data for a maximum period of six seconds with autonomous recovery afterwards. This can be tolerated from a systems viewpoint.
The EGSE did not fare quite as well. During one lightning strike, the front end rack stopped completely and needed to be reset. When one considers that this rack was not 'wired' to the probe (communication was via an optical link) and had been moved five or six metres away at the time of the strike, it gives an indication of how effective the probe's lightning protection really is. A full post-test checkout was subsequently performed; it showed that all the probe systems had survived the test.
The authors would like to thank the staff of the Bunderswehr University without whom this particular test would not have been possible, and also the Daimler-Benz Aerospace and A rospatiale teams for their hard work, dedication and cooperative spirit shown throughout the entire EM test campaign.
ESA Bulletin Nr. 85.