Stoking our Curiosity for Other Worlds

In the wee hours of last Monday (Eastern US time), a jubilant Mission Control erupted at the successful landing of the Mars Science Laboratory “Curiosity.”

Curiosity has demonstrated some amazing technological feats. Now, that portion of its mission is nearly over, and the rover will go over to science operations. The hair-raising, fist-pumping, frenzy-whipping part is done – but it’s been great practice!

While the MSL entry, descent, and landing system may seem harebrained and silly, it is in fact quite conservative and driven by fundamental engineering decisions. The engineering triumph of this system demonstrates to me how spacecraft engineers can set extraordinarily technically ambitious goals and achieve them in dramatic fashion. The JPL engineers who devised it are the types of people who design a device to last for three months and find it still happily ticking away six or eight or more years later. This thing was going to work. The toughest part was probably selling the concept to the NASA brass!

So, now we’ve got reinforcing knowledge that we can aim for the stars and hit them (well, planets, anyway). Let’s set out with some crazy-ambitious goals! And let’s set out for some places that let us answer fundamental questions.

This is my core disagreement with the NASA Decadal Survey, which prioritizes a Martian sample return mission above all else: such a sample return will advance the sub-sub-field of Martian geochemistry an incremental amount. This is not an ambitious enough goal to meet our demonstrated engineering capability! I don’t want to discover evidence that some place may have been habitable sometime in the distant past – I want to go someplace where we discover life because it’s staring right back at us.

Not so long ago, I proposed a mission concept for a subsurface probe to Jupiter’s ice moon Europa. Europa is intriguing because we already know that it has liquid water, and we already know that it has a strong energy source from Jovian tides – both of which are key ingredients for life as we know it! Even better, there are certain surface features on Europa, which – if our best models for how those features form are correct – are conduits from outer space to the ocean beneath. I suggested that we might develop a space vehicle that conducts a high-wire act above one of these exposed ice fractures, dropping probes down into the ocean below.

Soft landing on a Europan double ridge

I just ran some numbers to see how feasible a lander on Europa would be. So far, humans have only dispatched a single orbiter to the Jovian system: the Galileo spacecraft. Galileo launched from Earth, performed some inner-planet flybys, traversed half the Solar System, and then captured around Jupiter where it made successive passes over the moons.

Galileo had a dry mass (the mass of the spacecraft not including propellant) of 1289 kg. Capturing in orbit around Europa is similar enough to capturing around Jupiter that I’m just going to assume that about 1300 kg is a basic guideline for the final mass of a space probe we get into Europan orbit.

If our orbit is circular, with an altitude of 300 km over the moon (about the same altitude as the Mars Reconnaissance Orbiter over Mars), then the spacecraft will be travelling 1.311 kilometers per second. From here, our spacecraft can photograph Europa in detail, and our science teams can look for the telltale discoloration of organic substances on the ice marking an active ice fracture. Once we have selected a candidate landing site, a crack in the ice that is opening and closing over the course of the day, mission controllers will uplink the command sequence that starts a robotic descent to the surface.

In order to land safely on the surface of Europa, we’ll need to reduce that 1.3 km/s speed to zero relative to the moon’s ice shell. Europa’s sidereal day is about three and a half Earth days, so its surface rotates at only about 31 meters per second – I’ll just ignore this, then, and say that our probe has to spend 1.3 km/s of delta-v in order to land. On Mars, MSL can do a lot of that with its heat shield and parachute – but with no atmosphere over Europa, our probe is going to have to do all of this velocity change with rocket propellant.

I’ll assume a reasonable chemical rocket engine with a specific impulse of 350 seconds. Going to my rocket performance equations with this delta-v and this propulsion efficiency gives me a ratio of the mass I can land on Europa to the total mass (lander plus propellant) I started with in orbit: mf/mi = 1/exp(dv/(g*Isp)), coming to about 68%. What this figure (the lander’s “payload fraction”) means is that once we decide how much of the 1300 kg spacecraft is going to land, 32% of the lander mass has to be propellant for the descent stage. An interesting factoid: if the entire 1300 kg spacecraft is designed to land on Europa, then we’ll be putting 890 kilograms of useful mass on the surface. This is almost the same mass as MSL! (Though the MSL lander payload fraction was about 89%. Atmospheres are very helpful for braking!)

A Curiosity-sized rover is more than enough for what I have in mind, especially since my mission plan doesn’t require mobility once we land.

I want a lander that comes in sideways on one of Europa’s characteristic double-ridge features. As it approaches the ridge, it will drop an anchor on a tether. Ideally, the anchor would catch on the outside of the feature while the lander sets down in between the two ridges. The tether, then, runs up away from the lander, over the crack in the ice, and drapes down over the ridge line, making for a secure hold point. However, these ridges might measure upwards of a kilometer peak-to-peak! Unless scientists identify a suitably small ridge system while the probe is in its orbiter phase, the probe will need to secure the anchor in the upward-sloping near wall of the ridge. A kinetic impact might do the trick! And, fortunately, Europa’s surface composition offers another solution: the anchor may have some pre-charged batteries designed simply to throw off heat from strategically placed vanes, melting and sublimating its way into the ice.

Coming in for a cool landing

Some quick research with The Google suggests to me that the densities of likely tether materials are around the 3000 km/m^3 mark. I’ll assume that the tethers are about two or three millimeters thick and take the upper-middle of the resulting linear density range: about 0.015 kg/m. A kilometer is a nice conservative estimate for the anchor tether length: so, 15 kg of my mass budget goes to the spool of material. I’ll spring for some redundancy and place two anchor/tether systems on my lander, aimed at the other side of the double-ridge but about 30 degrees apart, bringing the total tether mass to 30 kg.

Once the tether anchor is secure, the lander can deploy smaller sub-probes which crawl up the tether until they hang directly over the breach in Europa’s icy crust. These mini-probes could perhaps use the tether to exchange power and data with the main lander. They will photograph the crack and perform some basic remote sensing tests…but their real purpose is to penetrate the icy depths of the moon! Down, down, they will drop, recording measurements and telemetering their experience back to the surface lander.

Architecture of the subsurface probe system

How big should these subsurface penetrators be? Similar mission architectures include the Galileo atmospheric probe (339 kg, of which 30 kg was payload) and the Huygens Titan lander (319 kg; 49 kg of payload). However, both those probes had entry and descent systems that my penetrators probably don’t need. The penetrators also must hang from the tether, which puts an upper limit on their weight. Again, quick Googling suggests that a solid wire tether with my specifications might have a breaking strain of about 4-5000 N. If I model the entire 15 kg mass of the cable in its center, for conservatism, then in Europa’s surface gravitational acceleration of 1.314 m/s^2 the cable will support a total probe mass of about 3000 kg. We’d better keep well under this limit, but clearly even supporting our entire lander leaves plenty of margin in the design! So, let’s just rough up some numbers: put 15 kg of instruments in the probes, twice that for structural supports, an additional 5 kg for computing and communication equipment, 5 kg for batteries, and an additional 5 kg of mass to make an ice-armor-piercing tip. We will get the most benefit out of the probes if they can communicate quickly with the main lander – but running a cable will be tricky if Europa’s ice shell turns out to be tens or hundred of kilometers thick! I think a better solution would be to have the probe’s crash through any surface ice and into the liquid water trigger the release of a buoyant “float” with a shorter communication line leading to the probe. The float will sit on the liquid surface and transmit to the lander while the probe continues to descend. I’ll budget the same as the science instruments for this system (15 kg), plus a kilometer of half-thickness tether for the communication line (about 7 kg). With this loadout, each subsurface probe will have a mass of  82 kg. If I put two probes on each tether, then I’m asking for 328 kg out of the lander’s total 890.

This leaves 530 kg to devote to lander structure, command and control, power, and landing system. I think that the mass of the MSL Curiosity arm and instrument package is about 100 kg, and there are many instruments – the MastCam, navigation cameras, and chemistry laboratory, for instance – within the rover body itself. I haven’t found a good figure, but I suspect I’m in the same ballpark for the payload-to-engineering-infrastructure ratio.

Lander architecture

After the MSL landing…this mission architecture suddenly seems a lot more achievable! (Apparently, Adam Steltzner, the lead engineer for the Curiosity entry, descent, and landing system, also would like to tackle Europa next.)

One final point to consider: could we afford such a mission? Galileo cost $1.4 billion; putting that all at launch in 1989 and adjusting for inflation, that would be about $2.5 billion today. MSL also cost about two and a half billion dollars. Both expenditures were spread out over about a decade, meaning that each took only about 0.007% of the United States federal budget. If every American chipped in one dollar each year for ten years, that would take care of the entire mission cost. But not only is this an insignificantly negligibly paltry sum as far as US expenditures go, it is also important to recognize – especially if your names are Romney or Ryan – that this federal expenditure directly spurs innovation, provides an outlet for technological ingenuity, and raises the raw sex appeal of difficult push-the-envelope disciplines. In a world of recessions, recoveries, tremendous challenges, and global competitiveness, those are all things that this country sorely needs!

So let’s keep on daring mighty things! There is so much to learn and discover in our universe. We’ve shown ourselves that we are more than capable of doing these things. Let’s not ever stop!

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