Juice electromagnetic fields and antennas aligned for Jupiter science
Flying instruments to Jupiter represents a once-in-a-generation opportunity for Europe’s space scientists. But that translated to a challenge for the Jupiter Icy Moons Explorer, Juice, mission: could all the varied instruments aboard, plus antennas and onboard systems, even end up working together properly without interference? Would in-situ instruments really be measuring the space around Jupiter, or just the influence of their host spacecraft?
The lorry-sized Juice spacecraft hosts a total of 10 instruments, ranging almost 15 orders of magnitude across the electromagnetic spectrum in terms of testability on the ground, all the way from the ultraviolet and optical infrared, through the terahertz range right down into radio frequencies. For the overall mission requirements the range is more like 22 orders of magnitude – although that full span can only be measured in space.
But in order to fly everything together safely, Juice had to become one of the most magnetically ‘clean’ spacecraft ever built, with additional countermeasures to minimise the buildup of charged plasma on its surfaces, which might otherwise impact many of its observations as well.
No-spin zone
Juice spacecraft and system manager Christian Erd explains: “We have instruments sticking out of almost every direction aboard, including particle and magnetic instruments which in the past would traditionally be flown aboard a spinning spacecraft, rather than a three-axis stabilised platform like Juice.
“This is because the structure of a spacecraft exhibits its own magnetic and plasma effects, but if a mission is spinning – take ESA’s Cluster magnetic-mapping satellites as one example – then the environmental field shows up on a regular, periodic basis, so the spacecraft's influence can be more easily screened out from the scientifically valuable results.”
A spinning spacecraft was not an option however. Juice’s optical and radar instruments require precise pointing capability.
Science in all directions
The real instruments of concern here were those either intentionally sensitive to or emitting electric, magnetic or electromagnetic fields. Such units extend out from the spacecraft body to maximise their distance from the disturbing units of the spacecraft, allowing them to interact with the environment 'cleanly': the boom-mounted J-MAG magnetometer to measure Jupiter’s magnetic field, the Radio and Plasma Wave Investigation sensors to sample local plasma and magnetic signals, the RIME radar to sound the depths of the Galilean moons, with its 16-m long antenna, the Particle Environment Package detecting charged particles and the Submillimetre Wave Instrument (SWI), investigating the surfaces and atmospheres of the target moons.
To operate properly, these instruments would have to gather data without undue influence from the spacecraft, or unwanted ‘coupling’ with one another. To ensure this, the Juice team turned to ESA’s Electromagnetic Compatibility (EMC) and Harness section, part of the Agency’s Directorate of Technology, Engineering and Quality.
EMC engineer Zoltan Kiss explains the scale of the challenge: “To take the J-MAG magnetometer as an example, if there is an ocean under the surface of Ganymede, then it would produce a signal linked to the rotational frequency of the moon around Jupiter, with a periodicity of a week. So we require stringent EMC stability of the spacecraft on this timescale to be able to measure this – which is really pushing boundaries on the low side of the frequency range.”
Keeping Juice magnetically and electrically clean
Strict EMC requirements were set back at the study phase; meeting those requirements became part of the competition for the potential prime contractors. And in the end those location dependent requirements had to be applied to each and every part, so EMC was being continuously re-assessed and verified even as the first prototype units and subsystems became available, to be mitigated as necessary.
Sam Verstaen from the Juice project team notes: “We know which are the hardest units are to screen, mainly reaction wheels used to control orientation – which operate based on spinning magnets – and power conditioning units which have high currents running through them. So we organised special engineering workshops with their manufacturers.”
EMC section manager Axel Junge adds: “There are various mitigating solutions, such as balancing out parts with opposite magnetic field directions to cancel each other out, de-magnetising individual parts, or replacing them with non-magnetic alternative metals, or else shielding the units with mu-metal alloy – although that incurs a weight penalty.”
Another solution is to ensure extremely steady oscillators were incorporated into critical units, notes Sam: "The idea behind this is that, even if the influence of these emissions cannot be entirely shielded from scientific instruments then they form a very stable and well-known spectral line that can be more easily screened out in post-processing. Another alternative is to select oscillation frequencies that fall outside the frequency range of interest of the sensitive instrument.”
Instruments, subsystems and eventually the entire spacecraft underwent EMC verification test campaigns – the latter at test facilities at Airbus EVT Toulouse (previously Intespace) – to check the electric and magnetic emissions of the fully operational spacecraft.
Zoltan adds: “Despite the world-first novel magnetic test methodology that has been applied at spacecraft level, testing across the entire frequency range of interest was simply not practical, so we have had to rely on system modelling at the lower end of the frequency range. Similarly, the challenging low frequency electric field emission verification at spacecraft level was also supported with a model-based approach.”
Mitigation work was meanwhile essential to minimise plasma charges gradually being accumulated on spacecraft surfaces, which might also perturb in-situ measurements. For example, the cover glass safeguarding the 85 sq. m solar arrays was coated with a nanometre-scale layer of Indium Tin Oxide, along with tiny wires. This electrically conductive, optically transparent layer will prevent buildups of ‘electrostatic bubbles’ that might disturb Juice's instruments as they sample the space about them.
Other spacecraft surfaces were similarly made conductive – either by choosing conductive multi-layer insulation, or applying conductive paint, as in the case of Juice's High Gain Antenna.
"What we want to avoid is differential charging, where one part of the spacecraft might charge up more than other parts," adds Sam. "There are times when the spacecraft will fly through environmental plasma and will charge up to tens of kilovolts at most. Such a buildup might disturb science measurements during these occasions, but this has been anticipated since the beginning. Overall this is quite acceptable, and will be down to tens of volts most of the time – which will be fine for science observations.
“The real killer for science however is differential charging, which is why we built a completely conductive spacecraft, to equalise the charging across all of Juice.”
Arranging antennas
The spectral range of Juice instrumentation threw up additional challenges, ESA antenna engineer Luis Rolo recounts: “RIME, with its 16-m long antenna, makes use of 33-m wavelengths so long that they can interact with the entire spacecraft, so we found that the antenna performance changes based on the orientation of the solar arrays.
“We had to use a combination of software and physical models to validate the final configuration of the antenna relative to the arrays, selecting a dipole antenna set perpendicular to the solar arrays.”
The test that went into the cold
The SWI instrument went to the other extreme with a wavelength of 0.2 mm, utilising such high quasi-optical frequencies that it became apparent the low temperature prevailing at Jupiter would cause sufficient shifts in alignment to alter its performance.
Luis adds: “We needed to test it at as close as possible to operational conditions, which in the end demanded the creation of a dedicated facility called Low-temperature Near-field Terahertz Chamber, or Lorentz.”
Based at ESA’s ESTEC technical centre in the Netherlands, Lorentz is able to test high frequency radio frequency systems in sustained cryogenic vacuum conditions.
Antenna engineer Paul Moseley adds: “SWI has very sensitive optics, if it is not designed correctly changes in temperature could cause the instrument to become defocused. All the simulations performed predicted that the optics would be in focus once it is in the cold conditions of Jupiter – but given the eight years it will take to arrive, we had to try that for real in order to be sure it would work.”
EMC bill of health
The final spacecraft, bristling with instruments and antennas, has ended up very clean in electromagnetic terms, especially in comparison to other ESA spacecraft.
“It is not perfect, but we are hitting the limits of what is technically possible within the given budget and time constraints," adds Sam. "There are some things that couldn't be tested on the ground, so instead we have had to rely on calculations, modelling and analysis. But we're not completely in the blind, and based on all we have been able to measure we look to be in pretty good shape.
”The proof of the pudding will be the in-flight behaviour, so we are eagerly awaiting the initial measurement results, which should be available within the first three months after launch.”
The end result of this sustained modelling and testing effort is that all the instruments should be fully operable throughout the mission, if all goes to plan. The final combination of activations at any one time will be overseen by the mission controllers, who will function like orchestra conductors to obtain the richest possible picture of Jupiter space.
