THEO: Designing a Spacecraft to Explore An Icy World

This is a post nearly a year in the making!

Last summer I had the incredible opportunity to participate in NASA’s Planetary Science Summer School, which teams early-career scientists (a.k.a. current and recent grad students) with the Jet Propulsion Laboratory’s Team X to design a planetary exploration mission. In other words: we picked a place in the solar system and designed a spacecraft to go there! Two days ago, our mission concept study was published in the journal Advances in Space Research.

How do you design a mission to explore another world? For our group, it started with the help of Team X. What is Team X? Well… basically, it’s what happens when you lock 20 scientists and 20 engineers together in a room and tell them they can only leave when they have a viable plan for a space mission. Also known as: spacecraft design heaven!

Designing a space mission is a bit of a “chicken-or-egg” problem. The science motivates the mission, often guided by the Planetary Science Decadal Survey (which is basically the scientific community’s prioritized wish list for all things space). Reality sets in, however, once engineering gets involved – “sorry, team, but there’s just no way to fly a spacecraft with 700 instruments, powered by solar panels the 3 times size of the Empire State Building, into orbit around Saturn… for less than 10 dollars.” The two groups must compromise to achieve the most science possible within the bounds of engineering reality. The engineers design to meet the scientific objectives, which adapt to engineering capabilities, which evolve to meet the science objectives… and so on.

There are two options for completing this process: 1) continually go back and forth between the scientists and the engineers – the “serial” version, where each round of science or engineering discussion follows a report from the other team – or 2) put the scientists and engineers in a room together and let them make tradeoffs in real-time – the “parallel” process, in which both teams work together, simultaneously.

Team X scientists and engineers hashing out design trades to design a planetary exploration mission.

Team X has made an art of the latter. Given the requirements for the mission and armed with years of NASA and JPL expertise in building spacecraft, this group of people hashes out the design of a mission with both science and engineering combined. Together, this group of people has designed over 1,000 planetary exploration missions (if only they’d all gotten funded!) for various customers, including NASA. They are the experts of “concurrent design and engineering.”

The JPL Planetary Science Summer School is a miniature version of the full Team X mission design process. To capitalize on a single week at JPL with the Team X experts, my teammates and I spent 10 weeks preparing via multi-hour teleconferences and a truly prodigious number of Google docs.

Step 1 in this process: decide where you want to go! We settled on Enceladus, moon of Saturn. This little moon, once thought to be geologically dead, has turned out to be full of surprises. Images returned from the Voyager spacecraft as they passed by Saturn in the 1980’s showed that Enceladus had a very bright, young surface… meaning that something (a geologic process!) was “freshening up” that surface! The Cassini spacecraft (which is still operating at Saturn today) next flew by Enceladus and observed plumes of water vapor and ice crystals spewing from the tiny moon’s south polar region. Aha! A possible source of the young surface.

Enceladus Plumes
Plumes of water vapor and ice crystals erupting from Enceladus’s south polar region.

Subsequent Cassini flybys have improved our knowledge of the plumes and their sources. We now believe that they are escaping from a global ocean beneath Enceladus’s icy crust! Could this ocean support life? It’s been warm enough to remain liquid for a very long time – one of Saturn’s rings is thought to be made up of ice crystals vented from Enceladus – and Cassini data suggests that hydrothermal reactions, such as those occurring at seafloor vents on Earth, are occurring in the moon’s ocean. Cassini has also detected traces of ammonia and organics. So far, though, the data is not conclusive as to whether Enceladus’s ocean could support life as we know it.

THEO Team w Curiosity
The THEO team in front of an engineering test version of Curiosity.

Fortunately, Enceladus is continually offering samples of its ocean. The plumes, erupting from the south polar region at predictable points in Enceladus’s orbit, are continuously flinging samples of this ocean out into space. Cassini has flown through the plumes a few times, returning a wealth of data on the plume structure and chemistry. As I mentioned above, though, the results of Cassini‘s analyses are not conclusive. The time is right for a spacecraft dedicated to exploring the plumes and determining whether Enceladus could support life!

Armed with a destination, we settled down to answering open science questions.

Step 2: deciding what questions to ask, and figuring out how to answer them. We divvied ourselves into teams based on scientific questions – how are the plumes and subsurface ocean connected? Is the abiotic environment suitable for life? How stable is the ocean environment? Is there evidence of biological processes? – and researched the instrumentation a spacecraft would need to answer those questions.

Step 3: Fit the instruments into a spacecraft!

At this point in the process we all traveled to JPL to meet our Team X mentors (and each other). With their help and expertise, we started piling items onto the balance between scientific goals and engineering reality.

This is what it looked like initially:

Sci Eng Balance_initial

We arrived at JPL with a list of scientific objectives that would have made Santa Claus think twice! In the Team X setting, though, the scientists and engineers are side-by-side, and one quickly realizes that the scientific goals must be balanced by engineering capabilities. This begins the painful process of “descoping” your team’s mission design.


 Step 4: descoping. The process was a bit like “Survivor: space!” Each instrument was listed on a whiteboard, along with its strengths and weaknesses. Which science questions can it address? How much space does it take up? Will it drain our limited power? Is it exorbitantly expensive?

As the questions were answered and the pros and cons were weighed, the list of science instruments slowly whittled down. Some beloved instruments fell by the wayside and were left behind, but nobody cried – at least, not in front of everyone else!

Sci Eng Balance_descoped

Slowly but surely, the science objectives synced up with engineering capabilities. On real missions, sometimes this is accommodated by technological developments that expand the envelope of engineering reality. For us, it meant descoping until we were within the realm of possibility.

The final result of these efforts is a spacecraft concept called THEO: Testing the Habitability of Enceladus’s Ocean. (Disclaimer: this design was completed as an educational exercise and is NOT an actual mission proposal, nor is it in any way a “real” mission being funded by NASA.) The spacecraft, which would travel to Saturn via gravitational “slingshots” around Venus and Earth, would eventually settle into orbit around Enceladus to begin mapping the plumes and sampling their chemistry.

We settled on a suite of four instruments (plus a freebie) to do this.

A sub-millimeter instrument. The Rosetta mission (remember the cute little Philae, who landed on a comet?) carried one of these and used it to map the temperature and composition of the surface of comet 67P. THEO would use the instrument similarly, mapping the temperature of the vents from which the plumes emanate, mapping the chemistry of the plumes themselves, and combining with the cameras to map the density of particles in the plumes.

THEO conversation
A THEO design trade discussion in action.

A mass spectrometer.  “Mass specs” gobble up particles as they fly through space and flings them through an instrument which measures the mass of the particles. This can then be converted to chemical species. Mass specs have been used on previous missions, most notably on Cassini, but the one on Cassini cannot detect some of important markers of habitability. Thus, THEO would fly an improved mass spec – the one being designed for NASA’s Europa Mission – with the ability to detect key signs of habitability within the plumes.

A camera. You have to take the pretty pictures, right? Cameras are also incredibly useful for mapping geologic features, such as the plume sources, in great detail. By combining two images of the same feature taken at different angles, planetary scientists can create “digital topographic models” of the surface, which allow the height of a feature to be measured. THEO would map Enceladus’s intriguing South Polar Region more comprehensively than any mission yet flown. The cameras would also be used to image the plumes themselves, giving us an idea of the ratio of ice and vapor in them.

Two magnetometers. Magnetometers are sensitive to magnetic fields, and could be used to explore the depth and extent of Enceladus’s subsurface ocean. They are also useful in understanding the movement of ions in the plumes themselves!

Radio science. This is often referred to as the “freebie” instrument because all you need is a radio antenna – and you have to have one of those to communicate with your spacecraft! Radio science uses the doppler shift of a spacecraft’s communications with Earth to measure any change in its orbit. Just like the pitch of an approaching fire truck rises and falls as it drives past you, the rate of data coming back from a spacecraft is “nudged” by tiny shifts in its orbit caused by changes in gravity over Enceladus. These miniscule changes in frequency could be measured on Earth, allowing us to map Enceladus’s gravity field.

All of this would fit snugly into a little spacecraft capable of making the voyage to Saturn.

THEO configuration
The THEO spacecraft concept.

Last but not least: a cool thing about the engineering of the spacecraft itself! As I mentioned earlier, sometimes engineering breakthroughs facilitate even more science. The solar arrays on THEO would be roll-out arrays, which curl into tiny shrouds during launch. This allows a relatively large solar array – like, say, the one you’d need to gather enough light at Saturn! – into a rocket for launch. THEO would therefore use solar power at an unprecedented distance from the Sun.

So: science AND engineering came together in a very short timeframe to produce a viable mission to the outer solar system! If you want to read ALL about it (all 30 pages of it!) check out the article here. And if you’re a graduate student or early career scientist/engineer, I strongly recommend that you apply for the Planetary Science Summer School. It’s an incredible learning experience!


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