Biological Cleanliness

The United States has obligations under the international 1967 Outer Space Treaty to explore space in a responsible manner that avoids the harmful contamination of celestial bodies. To help meet these obligations, Europa Clipper planetary protection requirements address the biological cleanliness of the spacecraft and the need to not contaminate Europa’s ocean with microbes from Earth. These requirements are intended to minimize the chance that biological material brought from Earth could compromise future scientific investigations of Europa.

Planetary protection categories range from I to V, extending from the fewest protocols to the most. Outbound orbital missions to celestial bodies that may have an environment suitable for life (now or in the past) — including Jupiter’s moon Europa — fall into Planetary Protection Category III.

To meet NASA’s cleanliness standards, engineers in full-body protective gear assembled the Europa Clipper spacecraft in a clean room at JPL where a number of other missions with strict requirements were built, including NASA’s Perseverance Mars rover. Environmental conditions within the clean room are strictly monitored, and the air is thoroughly filtered. The surfaces and floors of the clean room, as well as mission hardware, are frequently treated with strong cleaning solutions to kill and remove living microbes. Mission hardware, as well as tools used for spacecraft assembly, are routinely sampled to calculate the estimated number of microbes on them.

At launch, Europa Clipper is estimated to have fewer than 350,000 bacterial spores on it. If all those spores were gathered in one place, they could fit on the tip of a ballpoint pen.

In addition to keeping microbes off Europa Clipper, engineers must also monitor and control nonliving materials that could shed and collect on the spacecraft and its science instruments, affecting their performance. For some missions, even thin contamination layers can cause problems. A surface contamination layer just 20 or 30 molecules thick — far smaller than the human eye can see — could be enough to degrade spacecraft or science instrument performance.

Europa Clipper presents a unique challenge to cleanliness in a few ways. The spacecraft’s sheer size is one key consideration. Including its basketball court-length solar arrays, there are approximately 9,688 square feet (900 square meters) to clean and monitor. Each piece has to be treated with care every step of the way from the time it enters a clean room to when it is installed on the spacecraft to when it is loaded into the rocket that will send it to Jupiter.

Jupiter itself provides another challenge due to its high-radiation environment. To prepare for these harsh conditions, engineers conducted extensive testing in which they bombarded thermal blankets, paints, and other spacecraft materials with radiation in a particle accelerator at JPL. During and after this exposure, the materials would outgas, releasing a flurry of microscopic material that could affect the operation of Europa Clipper’s sensitive instruments — fogging a camera lens, for instance, or adding noise to a spectrometer’s data.

To minimize those kinds of effects, engineers used this radiation testing to select materials with the least outgassing. These materials were then baked between 194 and 248 degrees Fahrenheit (90 and 120 degrees Celsius) to further reduce outgassing before being installed on the spacecraft.