Space grade electronics: How NASA’s Juno survives near Jupiter
Five space missions that reminded us space exploration is hard.
|Mar 15, 2018||1|
November of 2011 was a particularly exciting time for planetary scientists. Russia had launched its Phobos-Grunt spacecraft designed to land on Phobos, the first ever attempt to do so on Mars’ largest moon. What was even more exciting is that the spacecraft was to return 200 grams of samples from Phobos’ surface. Science!
The launch was successful but the spacecraft never left Low Earth Orbit (LEO). The spacecraft failed to fire its engines and its orbit decayed in a matter of weeks. Pulled by the Earth, it plunged into the ocean, resulting in a total mission failure. Along with the spacecraft, China’s first Mars orbiter Yinghuo-1 and The Planetary Society’s LIFE experiment onboard were lost too.
So what went wrong? The failure report revealed that the spacecraft’s electronic components were neither qualified to be used in space nor tested prior to launch. The loss of Phobos-Grunt and the science it could do was a bitter reminder of the unforgiving nature of space exploration. Cutting corners in spacecraft development and testing to save some costs can end up costing us even more.
In this article we take a look at how electronics of spacecraft are built to survive the harshness of space environments and some interesting peeks into different space missions.
Hardening electronics for space usage
Electronic components used in spacecraft must be built to survive the harsh space environments and function reliably in it. The US Department of Defense mandates over 100 tests to ensure reliable operation under mechanical stress, wide temperature fluctuations and intense ionizing radiation. All space grade electronic components must be individually qualified as opposed to the sample testing common in commercial or industrial applications.
To ensure the desired reliability in space, system designers can’t use the latest and greatest chips. If we take a look at CPUs used in spacecraft, we will find that most of them are very old designs, some even dating back to 1990s. It’s critical to use something that has been tried and tested, that we know will work.
Electronics components are hardened for space usage using some of the following techniques.
Launch time vibrations from the rocket can induce mechanical stress on the electronics and damage it. The process of potting involves filling the electronic assembly with a solid/gelatinous compound to resist shock and vibration.
2. Silicon on insulator
Chips for space usage are manufactured on an insulating substrate instead of a silicon one, allowing them to be more radiation resistant and fault tolerant.
3. RAM types
4. External shielding
An external shield (like lead) around the electronic components reduces exposure to radiation, thereby increasing the mission’s life span. This is particularly useful in long-term missions like New Horizons which is currently exploring the Kuiper Belt.
As a result of these techniques, the sizes of electronic components qualified for space usage are usually much larger than commercial/industrial ones.
Effects of radiation on spacecraft electronics
Even with all of these modifications to make electronics space-grade, they can still suffer in space from intense space radiation. A wide range of effects, known as Single Event Effects (SEE), can cause operational issues.
Ions in space radiation interacting with the chip components can flip bit states and cause memory errors.
A high energy ion or proton passing through inner transistor junctions can cause latchups, leading to short circuits.
Similarly, high energy particles can also let electrons lose in a circuit, causing irreversible damage.
Memory losses, code execution sequences going haywire, latchups, etc. are all undesired elements in a successful space mission. Some problems can only be overcome at the expense of a hard reset while some cause permanent damage. Spacecraft electronics must be built with all these factors in mind.
Apart from the Phobos-Grunt incident, let’s have a look at space missions which act as a constant reminder of the harsh, unforgiving nature of space exploration. This part of the article was inspired by Emily Lakdawalla’s piece on sample collection.
Interesting cases of electronics failures in space missions
1. When you lose your star sensors
India’s first lunar orbiter Chandrayaan-1’s star sensor failed to work after a few months in lunar orbit. The extreme exposure to solar radiation combined with other factors caused the backup star sensor to fail too.
The rest of the mission had to be completed using the onboard gyroscopes and constant corrections from the ground station. The mission was ultimately successful but it was a reminder that even space grade components can falter.
2. When you have six computers to complete one job
The Galileo spacecraft designed to orbit and study Jupiter had not one but six CPUs. Surviving in the radiation riddled Jovian environment — which is orders of magnitude more intense than Earth’s — mandated that each major subsystem be controlled by its own CPU for fault tolerance. That way if one CPU dies, it only disables one major instrument.
Each of the CPUs used were the 8-bit RC 1802, clocked at 1.6 MHz. They were fabricated on sapphire (silicon on insulator), which is radiation-hardened and suitable for the intense Jovian environment. The processing capabilities of the Galileo spacecraft was equivalent to the classic Apple II computers sold a decade prior.
The use of redundant backup modules is standard practice in spacecraft development to minimize issues. Despite these protective measures, Jupiter’s harsh environment caused more than a dozen anomalies to the Galileo spacecraft over time, including frequent resets of the onboard computers.
3. When solar flares damage your solar panels
En route to the asteroid Itokawa in 2003, the Japanese spacecraft Hayabusa was hit by one of the largest solar flares in recorded history. The flare damaged the solar panels, thereby reducing their output. Not just that, the flare also took out one of the four ion engines of the spacecraft. The mission duration had to be reduced as a result.
The excessive radiation damaged the junctions in the solar cells and was also a cause of concern when designing Juno, NASA’s spacecraft currently orbiting Jupiter closer than any prior spacecraft. Juno engineers knew the solar panels would degrade with time due to the intense Jovian radiation. The risk was minimized to acceptable levels by using solar panel glass twice as thick than usual and increased solar panel size for greater output.
4. Radiation hardening using RAD computers
One of the latest generations (in space grade terms) of radiation-hardened chips is the 32-bit RAD750 manufactured by BAE, based on the PowerPC 750 designed by IBM. It is designed to minimize losses even when facing extreme radiation from solar flares. Having a wider temperature range and 10x better performance than the previous generation RAD6000, the RAD750 has been used in over 150 space missions since its availability, including everyone’s favorite Mars rover Curiosity.
Some of the other popular space missions powered by RAD750s include Lunar Reconnaissance Orbiter, Kepler space telescope, Solar Dynamics Observatory and of course, the Juno spacecraft. Each RAD750 costs about $200,000.
As noted before, many missions have backup computers for redundancy and robustness against the failure of this crucial component. The Curiosity rover has two RAD750 CPUs, one acting as a backup unit which took over when the first one faced issues with its flash storage.
5. The case of Juno and Jupiter
Designed to study Jupiter’s poles and go closer to the planet than any spacecraft before it, Juno is faced with challenges. It turns out that using individually tested space grade components, one of the most radiation-hardened CPUs and making use of redundant components is still not enough to face the harsh environment near Jupiter, the planet with the largest magnetosphere in the solar system.
The RAD750 CPU is designed to withstand up to 1 million rads of radiation on its own, which is remarkable. But the mission is expected to expose Juno’s components to 20 million rads of radiation over time. That’s quite intense.
To protect the electronic components, the engineers built a protective titanium armor shield that has centimeter-thick sides. This cubical vault reduces the radiation exposure to the electronic components by a factor of 800.
The shield is so important to the functioning of Juno that it gets its own name — Juno Radiation Vault.
Without its protective shield, or radiation vault, Juno’s brain would get fried on the very first pass near Jupiter
– Scott Bolton, Juno’s Principal Investigator
Even then, Juno is expected to cease functioning eventually due to multiple close passes to Jupiter, deep into its radiation belts. A reminder that space is hard.
But it’s worth exploring.
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