Europa Clipper mission launches on schedule!

Graham writes ...

Jupiter seen from the surface of Europa. Credit: Getty Images and Ron Miller.

Despite fears that the Europa Clipper (EC) spacecraft might not survive the radiation environment at Europa, the spacecraft launched on schedule. A SpaceX Falcon heavy lift launch vehicle left the famed 39A launch pad (the historic Apollo 11 mission left from here in July 1969) at 12.06 EDT (16.06 GMT) on the 14th of October 2024, just 4 days after the launch window opened on 10 October. However, now we all need to be patient – if all goes well, EC will enter Jupiter orbit in April 2030. Nevertheless, this is a great opportunity to look forward to what it is hoped EC will achieve, at an overall mission cost of around US$5-billion.

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Back in May of this year, the launch looked to be in jeopardy (for details see previous ‘miniblog’ – click on ‘August 2024’ on the RHS of this page) when a problem was discovered concerning the reliability of electronic components already installed in the integrated spacecraft. An issue was suspected with the transistor elements, which are essentially the building blocks of the micro-processors onboard. The Jupiter system, where EC will operate, exposes the spacecraft to intense particle radiation similar to the Earth’s Van Allen radiation belts but 50 times more intense. To survive this the electronic systems need to be housed in a protective radiation vault engineered with 9 mm thick walls composed of aluminium and titanium. In addition, the electronic components are required to be ‘radiation-hardened’ to achieve the spacecraft’s planned 3.5 year mission lifetime. Earlier this year, it was found that identical transistors failed under testing before they should. This posed a real problem for the engineers. Would they have to replace the installed transistors, or would they have to redesign the flight profile to achieve the mission objectives in a shorter time scale? Both of these are major undertakings – either of which would have threatened the readiness of the spacecraft to launch within the bounds of the upcoming launch window. 

Integration, assembly and test of Europa Clipper in the clean room environment. Credit: NASA

Following this potentially devastating news, NASA instigated four months of 24-hour intensive testing at three different facilities – JPL in Pasadena, California, the Johns Hopkins Applied Physics Laboratory in Laurel, Maryland and the NASA Goddard Space Flight Centre in Greenbelt, Maryland. After evaluating spare components from the same batches that were installed on EC, the test engineers found that the spacecraft’s systems would, afterall, perform as required. This conclusion partially rested on the fact that during the first half of its mission lifetime, the spacecraft will be in the most intense of Jupiter’s radiation only one out of every 21 days. 

Schematic of the spacecraft's radiation environment. Credit: source unknown.

To get an idea how this works, the diagram above shows a ‘plan view’ of the Jovian system with Jupiter at the centre and the orbits of the major (Galilean) moons shown in black. A representative elliptical EC orbit is shown in blue and the radiation hazard is shown roughly by the red, gold and beige areas, red being the most intense. As can be seen, in order to execute flybys of Europa the spacecraft must dip into the red zone, but there is a mitigating factor. Due to the nature of elliptical orbits, the spacecraft’s speed at the perijove (point of closest approach to Jupiter) is very much higher, than its speed at apojove (point of furthest distance), so EC will spend most of its time outside of the damaging radiation environment. Hence for the majority of the time, the orbiter will remain outside the region of greatest hazard. Also, during this time the components can be partially restored from radiation damage by gently heating them (a process called ‘annealing’).

The crazed, icy surface of Europa as seen by the Galileo spacecraft. Credit: NASA.

So why is Europa, a body about the size of Earth’s moon, the subject of such intense interest? As I said in my previous blog post, it has long been thought to be a place in the Solar System where conditions may be suitable for life to develop. Looking at Europa – a distant and cold, ice-covered world – it doesn’t look at all like an environment where life could flourish. However, in this case, appearances are deceptive. There is strong evidence that beneath the ice crust there is a warm water ocean, the heat being generated most likely by volcanic vents on Europa’s ocean bed. It is currently believed that the depth of this ocean is up to 100 km, so that Europa may have twice the volume of water compared to terrestrial oceans! So, how does this vulcanism work?  The key to understanding lies with the massive gravitational pull Jupiter exerts on its moons. As Europa orbits the gas giant, tidal forces cause the icy moon’s interior to flex which generates thermal energy within the moon’s rocky core, increasing the likelihood of volcanoes on the ocean bed. Other Galilean moons, Io in particular, exhibit intense volcanic activity on their surfaces driven by the same mechanism.

Depiction of a water plume on Europa. Credit: Corey S. Powell.

Suspicions that there may be an ocean beneath Europa’s icy surface were first raised by imagery acquired by NASA’s Galileo spacecraft which orbited Jupiter from 1995 to 2003. A good example is the image below of Europa’s icy surface taken at a range of about 200 km during a flyby in November 1997. In this region, the surface ice appears to have melted, broken up and then refrozen, suggesting surface thawing caused by heating from below. Furthermore, recent reanalysis of old Galileo mission data suggests that the Galileo spacecraft may have flown through plumes of water vapor emanating from the moon during flybys.

A closeup image of Europa's surface, acquired by the Galileo spacecraft. Credit: NASA/JPL.

​So, what do we hope to learn from the EC mission? Clearly, there is no way that it can perform an in-situ investigation of potential life signatures at the ocean bed. The objectives must be limited by what can be achieved remotely during the planned 50 flybys of Europa, at ranges as low as 25 km. The over-riding motivation for the mission is to do a detailed study of the Europa system to investigate whether the icy moon could harbour conditions suitable for life to exist. To work out the orbital profile of the mission and to determine what payload instruments are required, the mission objectives (in general terms) may be expressed as:
 

• to understand the nature and structure of the concentric ice shell that encompasses the moon,
• to acquire information about the composition of the ocean beneath the ice crust, most likely by attempting to fly through water plumes emanating from the surface, and by remotely analysing the composition of any liquid deposits on the surface,
• and more generally, to attempt to determine the overall composition, structure and geology of Europa.

 
Based upon these broad objectives the engineers were able to determine the characteristics of the spacecraft. In other words, they can decide on the payload instruments needed to achieve the objectives, and what services  are required on board to support these instruments. The ‘services’ are usually referred to as subsystems such as electrical power, communications, data handling, attitude control (required to point the instruments as appropriate), propulsion, structure, and so on. 

Europa Clipper reflected in the solar array, during a solar array deployment test. Credit: NASA and Frank Michaux.

This process led to the design that we see in the final integrated spacecraft, which is now on its way to Europa. With a dry mass of more than 3.2 tonnes, a height of roughly 5 metres, and a width of more than 30 metres with its solar panels fully unfurled, EC is the largest spacecraft that NASA has ever built for a planetary mission. To handle the large transmission rate of payload data over such large distances, the spacecraft is equipped with a 3 metre diameter communications dish. All previous NASA deep-space missions have been much more compact due to the use of radioisotope thermal generators (RTGs) to provide electrical power. These devices use the heat from radioactively decaying elements to produce electricity. The use of solar arrays in this instance is a significant change in design philosophy for NASA. Given that Jupiter is 5 times further from the Sun than Earth, the amount of power per unit area is 25 times less (the ‘inverse square law’!). This means that the incident solar power at Jupiter is only about 55 W per square metre, so lots of array surface area is needed to supply the spacecraft’s electrical needs resulting in the spacecraft’s large size, as seen below.

The size of Europa Clipper compared to a basketball court. Along the array axis the spacecraft is in excess of 30 m. Credit: NASA/JHUAPL.

A major payload component is an ice-penetrating radar to determine the structure of Europa’s icy crust and to attempt to acquire direct evidence of sub-surface water.  The spacecraft's is also equipped with two sets of cameras, one operating in the visible part of the spectrum and the other in the infra-red (thermal imaging) to produce a high resolution map the moon’s surface and look for potential plumes. The thermal sensor will help pinpoint locations of warmer ice and perhaps recent eruptions of water. EC also carries spectrometers, again operating in different parts of the spectrum to determine the composition of the surface and atmosphere. Spectrometers measure the intensity of reflected light from Europa across a band of frequencies (or colours, if the instrument is operating in the visible part of the spectrum). Particular surface elements or compounds will absorb light at particular frequencies (colours) leaving dark lines in the surface spectrum, revealing their presence. In addition, a magnetometer will measure the moon’s magnetic field, and perturbations to the vehicle’s trajectory during close flybys will provide information about its gravity field, both of which will offer clues about Europa’s internal structure.
 

Depiction of an Europa Clipper flyby of Europa. Credit: NASA.

Stepping back and looking out into the Solar System in general, there is a growing trend with current and proposed missions to give the search for life a high priority. When I was growing up and learning about our planetary system this was not the case. In those times, it was firmly believed that Earth was the only seat of life in the Solar System, and the remainder of the planets were considered to be sterile wastelands (both hot and cold). This was entirely understandable given that at that time everything we knew about our planetary system was acquired remotely through terrestrial telescopes. However, with the advent of the space age, an armada of spacecraft has ventured out into the Solar System, visiting each of the major planets.
 
The recent emphasis on the search for life is epitomised by our current investigations of Mars. It is now recognised that in its early history Mars was a water world, and that life may have developed there. Current rovers are equipped with instruments to detect possible bacterial life that may still dwell in the rock beneath their wheels. However, Mars is not the ideal laboratory to undertake this investigation. It is not often acknowledged that Mars is not quarantined from Earth, so that any life found there may have originated here (or indeed vice versa). To understand how this is possible, we have to go back maybe 3 or 4 billion years when life was thought to be stirring on our world. Alongside this, the early Solar System was a very hazardous place with lots of debris left over from its formation. Large impact events on Earth occurred often, throwing rocks off the planet and into solar orbit. It is possible that ‘earthling bacteria’ could have hitched a ride on this ejecta, to ultimately arrive on the Martian surface. Interestingly, the recent discovery of underground reservoirs of water on Mars (see August 2024 blog – click on the date on the RHS of this page) may reshape the way we think about Martian life.

Europa's mysterious interior. Credit: Nature.

This is why I find missions like Europa Clipper so exciting. If we find life there, it is very likely that it has nothing at all to do with life on planet Earth. The postulated Europan ocean is isolated by a crust of ice which is thought to be somewhere between 3 and 30 km thick. Though no sunlight can penetrate that shell to power life, there is a good theoretical basis to believe that vents that release heat from the moon's interior exist on its ocean floor. Indeed, similar vents in Earth’s deep oceans teem with life. There is a school of thought that advocates these terrestrial vents could have been where life originated on our planet.  If, ultimately, we do find life on Europa we have no idea what form it will take. Will it be based upon a DNA-type organisation, but with a different code? Will we even recognise it as life at all? The discovery of non-terrestrial life is a whole new ball game that we have yet to play!
 
In passing, it is worth mentioning that there is another prime suspect in our Solar System where life may reside. A moon of Saturn called Enceladus is a body which exhibits a lot of similarities with Europa. Here NASA’s Cassini Saturn orbiter mission (2004-2017) identified water plumes emanating from an ice crust, below which is believed to lie an ocean warmed by tidal vulcanism. However, this is another story!
 
Now that I have acquired the grand old age of 74, I’m not sure that I will be around to see the outcome of this mission! However, I do hope so – it could very well be a profound milestone in our understanding of whether extra-terrestrial life is extremely rare (and precious) or very abundant throughout the Universe.
 
Graham Swinerd
 
Southampton, UK
October 2024