I’ve recently opined on the dearth of lunar rovers sent by the space exploration leaders in NASA. It is understandable why there has never been a NASA lunar rover to-date (although one has been recently announced, again) They already put men on that thing and gathered far more data than any lunar rover could do. That’s not to say that there isn’t more to learn.
The challenge of developing a lunar rover differs from the popular Martian rovers in the news. A lunar rover doesn’t get the benefits of power as a Mars rover. Mars has a similar day to Earth (a little more than 24 hours). So, barring a dust storm, a solar powered Mars rover need only conserve power and keep electronics warmed on battery power for twelve hours or so before sunrise.
A lunar rover has 14 Earth days of extreme sunlight. The next 14 days, it has total darkness with colder temperatures guaranteed to force the probe to run out of battery power trying to keep itself warm, long before the sun rises.
The latest Yutu rover from the China National Space Administration appears to be designed as a durable rover. It hunkered down during its first lunar night and, as of my last search, still currently explores the never-before-visited surface of the Moon’s far side.
NASA, ESA, CNSA all have additional Mars rovers in work. ISRO had the ambitious Chandrayaan 2 orbiter/lander/rover spacecraft. It’s lander does not appear to have made it as of September 13, 2019.
NASA set the deep space probe ambition bar very high when they announced that they’re going very big with Dragonfly, an octocopter drone that will fly about the skies of the Saturnian moon, Titan.
But news on landers and probes to anywhere else outside of the Moon and Mars? There’s not much of it.
The lack of such news shouldn’t be surprising if you recall what I just said about how hard it is to keep a rover or lander working for more than a few days on any place other than Mars.
Let’s look at what few rover-like plans have been proposed for all other places outside of Mars, Titan and the Moon.
Landing on Mercury
The smallest planet, Mercury, has similar challenges for landers and rovers as does the Moon. Extreme heat will eventually cook one. The slow rotation of the planet means that a probe will roast longer, freeze longer and be out of communication longer with Earth.
But before a lander even begins a landing attempt, the effort needed to slow down a probe enough to attain orbit of Mercury requires far too much energy without a lot of flybys of Venus and Mercury itself, passing their sunward sides for a reverse-gravity assist, or “gravity brake,” trading time for multiple reverse flybys to let the planet slow you down with minimal fuel.
There were two concepts made public. One could have been part of a Mercury mission that’s now traveling to the planet as of 2019.
The Mercury Surface Element
Meet the tiny Mercury Surface Element (MSE).
MSE would’ve been the fourth of four modules of ESA’s BepiColombo probe, scheduled to arrive at Mercury in 2025.
The MSE looked unusual. A simple ring-shaped lander would separate from the central spacecraft (presumably before the Mercury Planetary Orbiter (MPO) and JAXA’s Mercury Magnetospheric Orbiter (MMO) would separate.
MSE’s shape was designed to make effectively a softer crash landing, coming down on simple gas generator jets at first, before inflating airbags to cushion a landing as fast as 30 meters/second (67 MPH). Serious lithobraking.
The peculiar trapezoidal shape might ensure that the probe rested upright no matter what position it found itself after being released from the airbags.
MSE was to land near the sunrise terminator to maximize its longevity and data collection. Mission objectives included heat flow sampling (using a mole probe very similar to the same one on the Mars InSight lander), magnetosphere study, surface regolith examination and more.
Based on the Soviet PROP-M rover on the failed Mars 3 lander (thanks to commenter Ralph Lorenz for the correction), the microrover, running on small tank-like tracks, would be connected to the MSE by cables, so it wouldn’t get to travel about much. The 1.1 kilogram rover could have held an imaging camera and a couple of spectrometers to analyze the surface composition.
Sadly, MSE was cancelled due to budget constraints. BepiColombo is in en route only with its two orbiters.
Around the time that NASA was flying the MESSENGER Mercury orbiter, scientists in Russia considered dusting off spares and technologies built for other Mars probes for use for a Mercury lander.
The Russian Mercury-P concept would, like BepiColombo, use electric propulsion for its trip to Mercury. However, it would be a dedicated lander mission. A crasher stage would drop a lander close to the surface before detaching it, enveloped by airbag shells. The lander itself would open up like a flower petal, reminiscent of early Luna landers sent to the Moon in the 1960s.
It was hoped that Mercury-P would be picked up to fly by 2024. However, the technologies of which Mercury-P would be derived was to first fly on the Phobos-Grunt Mars probe.
When Phobos-Grunt failed miserably shortly after reaching Earth orbit, later deorbiting, any further considerations of Mercury-P evaporated.
Venus: The “Final Boss” of Lander Science
In the space simulator game, Kerbal Space Program, there is a second planet from the sun of the game’s analogue to our solar system. Similar in size to the Earth-like planet of the kerbals, the purple planet of Eve lacked any other common characteristic.
Eve has 1.5 times the gravity of the homeworld and an atmosphere five times as thick. Players could generally land probes and kerbals with moderate ease on Eve. But good luck getting any thing landed back to the surface without a lot of preparation added to what you landed.
So, despite the game’s very open-world gameplay with no real ending, players commonly called Eve “the final boss,” a term denoting the climax of a game and a final obstacle or villain you must defeat to win. If you can land and return something from the surface of Eve, you’re likely considered an expert in the most advanced skills of the game.
On better scrutiny, Eve is more like a “super-Earth” than it is an analogue of Venus, like some of the exoplanets we’re finding with telescopes like Kepler and TESS. Super-Earths have similar atmospheres, perhaps some amount of water and temperate environments that could make it suitable for life. But super-Earths tend to be larger and so more massive. As a result, if anyone from a super-Earth visits us Earthlings, they’d likely be more like Kryptonians such as Clark Kent/Superman, having much denser and durable biology. Their technology has to be ass-kicking, just so they could leave their planet.
The first successful interplanetary probe, Mariner 2, chugged its way past Venus in 1962. Attention to the cloudy world shifted from it to the Moon as the Apollo lunar objectives drew closer. With little to see and depressing data that suggested Venus was very inhospitable, attention outside of Apollo was on flybys and orbiters to Mars.
By the 1970s through the 1990s, Venus had a lot of visitors from NASA and others. The former Soviet Union, tragically unlucky with other interplanetary probes, still holds the trophy for “Most Badass Landers” with their many Venera landers.
Why “badass?” In case you don’t know, Venus is a hellish place for any kind of surface landing attempt. The clouds are partially comprised of sulfuric acid. The surface atmosphere is 93 times as dense as that on Earth. If your probe isn’t dissolved or crushed, it will soon bake and melt at over 800 degrees Fahrenheit.
The standard designs for NASA and ESA Martian or lunar rovers simply would die seconds after arrival, if not before. While the Venera landers provided the first Venusian surface data and pictures, the best of them were only operational for about two hours.
The prospect of spending several million dollars on developing a lander that works for just a very short time, with a high probability of failure before it could reach the surface, has probably steered NASA away from Venus and to more tolerant science targets.
But a couple of studies have been released that would take on the Solar System’s terrestrial planet “final boss” with the hardiest landers imaginable.
Conventional silicon-based semiconductor technology used for all manner of computers today simply won’t work long in the super-hot Venusian atmosphere. Power will be a problem as standard batteries could overheat or ultimately run out of power. Solar energy is nil with the thick cloud layers.
While a paper filed with NASA suggests that a mission could use a radioisotope thermoelectric generator of the types used by Cassini, the Curiosity rover and many other spacecraft, the two concepts compel the forces of Venus to generate a lander’s power. Such systems would save on mass and complexity.
Long-Life In-situ Solar System Explorer (LLISSE)
Everything is stacked against a Venusian rover, literally and figuratively. The use of wheels are out. There wouldn’t be a lot of immediate need to move about in general as there is potential to gain lots of data just standing still. The Russians (first space agency to use rovers, with the Lunokhods) skipped over rovers and just made a durable lander.
The best Venera lasted just shy of 2 hours on the Venusian surface. Scientists today have reasoned out that a stronger lander requires more than physical durability but the capacity to fully withstand the planet’s atmospheric heating as well as pressure.
NASA engineers looked into the idea of toughening up the electronics.
Traditional semiconductors, the material used for modern electronics, simply cannot keep itself cool enough on Venus.
So the engineers have worked on electronics based on silicon carbide as the semiconductor. In tests inside a device that simulates Venusian atmospheric pressures and heat, these devices work dutifully.
So the electronics could be adapted to survive. But how will you get power and communications for a Venusian lander?
That’s where design comes in. LLISSE, the Long-life In-situ Solar System Explorer concept, has several designs to generate power from Venus itself.
Using silicon carbide electronics, landers like LLISSE can’t use solar power. The Venusian atmosphere’s thick clouds block most of the light, and solar panels would crack or melt under the atmospheric pressure, heat, or sulfuric acid droplets.
But the thick atmosphere does generate wind. So probes like LLISSE could use a windmill. Power is stored in a high temperature tolerant battery system. Or, a long-life non-rechargeable battery could be used to simplify things.
LLISSE is a consideration as a separate project and as part of a Russian Venera spacecraft.
NASA appears to be chewing on a Venus landing as they ponder options for a flagship mission. Those are good words: “Flagship” proposals form the high-profile spacecraft like the Voyagers, Cassini and the Mars Curiosity rover. Discussions on how and when to do a landing include the Venusian veterans of the Russian Federation along with France, Germany, the Netherlands, and Japan.
AREE: The name is more a screech, really
NASA engineers are also considering a Venusian rover.
Don’t underestimate the size of AREE. It’s gigantic. It has to be to withstand the Venusian environment.
Two versions have appeared. The first version is the curious multilegged lander you see in the NASA video.
The second version looks like (and is the size of) a tank.
To give you a sense of scale of the tanker bot:
AREE 1 would go the wind turbine route for power.
But the AREE tank concept would supplement wind power using an unusual generator with some solar energy.
Communications would be just as unconventional if silica carbide electronics aren’t wholly used. One idea for the AREE tank is to use radar signals to communicate. While most military aircraft try not to be seen with radar, AREE 2 could use radar targets that are highly visible. Orienting the targets could form a type of signalling “light”, flashing on and off radar signals to send data in a Morse Code-like fashion.
I suspect things are about to heat up, literally and figuratively, with a NASA mission announcement about Venus. Stay tuned.