The best way to know a world is to touch it. Scientists have been observing the planets and moons in our solar system for centuries and spaceships have been flying past the spheres for decades. But to really understand these worlds, researchers have to get their hands dirty – or at least the landing pads of a spacecraft.
Since the beginning of the space age, Mars and the moon have got almost all the love of the lander. Only a handful of spaceships have landed on Venus, our other closest neighbor, and none have landed on Europa, an icy Jupiter moon considered one of the best places in the solar system to look for life today (SN: 05/02/14).
Researchers are working to change that. In several conversations at the virtual meeting of the American Geophysical Union from December 1st to 17th, planetary scientists and engineers discussed new tricks that hypothetical future spaceships might need to land on unfamiliar terrain on Venus and Europe. The missions are still in the drafting phase and are not on NASA’s launch plan, but scientists want to be prepared for them.
Navigating in a Venusian glove
Venus is a notoriously difficult world to visit (SN: 02/13/18). Its searing temperatures and oppressive atmospheric pressure have destroyed any spaceship lucky enough to reach the surface within two hours of its arrival. The last landing took place over 30 years ago, although planetary scientists increasingly trusted that the surface of Venus was once habitable (SN: 08/26/16). This possibility of past and perhaps present life on Venus is one reason scientists are anxious to come back (SN: 10/28/20).
In one of the proposed plans discussed at the AGU meeting, scientists have their sights set on mountainous terrain on Venus called Tessera. “A safe landing on Tessera grounds is absolutely necessary to achieve our scientific goals,” said planet scientist Joshua Knicely of the University of Alaska Fairbanks in a talk recorded for the meeting. “We have to do it.”
Knicely is part of a study by geologist Martha Gilmore of Wesleyan University in Middletown, Connecticut, to design a hypothetical mission to Venus that could launch in the 2030s. The mission would include three orbiters, an aerobot floating in the clouds, and a lander that could drill and analyze samples of Tessera rocks. It is believed that this terrain formed where the edges of the continents long ago glided over and under each other, bringing new rock to the surface in a kind of plate tectonics. On Earth, this type of surface renewal may have been important in making the planet hospitable for life (SN: 04/22/20).
Landing in these areas on Venus, however, could be particularly difficult. Unfortunately, the best maps of the planet – from NASA’s Magellanic orbiter in the 1990s – can’t tell engineers how steep the slopes are in Tessera terrain. These maps suggest that most are less than 30 degrees, which the lander could handle with four telescopic legs. However, some can be up to 60 degrees, so the spaceship can easily tip over.
“We have a very poor understanding of what the surface is like,” Gilmore said in a presentation taped for the meeting. “How big is the boulder? What is the rock size distribution? Is it fluffy?”
So the lander needs an intelligent navigation system to choose the best places to land and steer. However, this need for guidance poses another problem: unlike landers on Mars, a Venus lander cannot use small rocket engines to slow down as it descends.
The shape of a missile is tailored to the density of air it is pushing against. That is why rockets that launch spaceships from Earth have two sections: one for the Earth’s atmosphere and one for the near vacuum of space. Venus’s atmosphere changes density and pressure so rapidly between space and the planet’s surface that “a drop from the rocket, which was working perfectly, would misfire and potentially blow itself up,” says Knicely.
Instead of missiles, the proposed lander would use fans to nudge itself around much like a submarine, turning the disadvantage of the dense atmosphere into an advantage.
The planet’s atmosphere is also the greatest challenge of all: seeing the ground. The dense atmosphere of Venus scatters light more than that of Earth or Mars and blurs the view of the surface until the last few kilometers of the descent.
Worse, the scattered light makes it seem like the lights are coming from all directions at once, like a flashlight shining into the fog. There are no shadows that could help identify steep slopes or large boulders for the lander to bump into. This is a major problem, according to Knicely, as all of the existing navigation software assumes that light is only coming from one direction.
“If we can’t see the bottom, we can’t figure out where the safe stuff is,” says Knicely. “And we can’t find out where the science is either.” While the proposed solutions to the other challenges of landing on Venus are nearly feasible, this one remains the biggest hurdle.
Hold the landing on Europe
Jupiter’s icy moon Europa, on the other hand, has no air to blur the surface or break rockets. A hypothetical future European country, which was also discussed at the AGU meeting, could use the “Sky Crane” technique (SN: 08/06/12). This method, which involves floating a platform with rockets above the surface and dropping a spaceship to the ground, was used to land the Curiosity Rover on Mars in 2012 and will be used for the Perseverance Lander in February 2021.
“Engineers are very excited that they won’t have to deal with an atmosphere on the way down,” said spacecraft engineer Jo Pitesky of NASA’s Jet Propulsion Laboratory in Pasadena, Calif., In a taped conversation for the meeting.
Still, there is a lot that scientists don’t know about the surface of Europe, which could affect any land that touches down, said planetary scientist Marissa Cameron of the Jet Propulsion Laboratory in another talk.
The Galileo orbiter offers the best views of the lunar landscape in the 1990s. The smallest features were half a kilometer wide. Some scientists have suggested that Europe might have jagged ice peaks called penitentes, similar to the ice features found in the Chilean Andes, named for their resemblance to hooded monks with bowed heads – although more recent work shows that the lack of atmosphere in Europe is the Should prevent formation of penitentes.
Another mission, the already-ongoing Europa Clipper, will capture higher resolution images when the orbiter visits Jupiter’s moon later this decade, which should help clarify the problem.
In the meantime, scientists and engineers are conducting elaborate dress rehearsals for a Europe landing, from simulating ice with different chemical compositions in vacuum chambers to dropping a dummy lander named Olaf from a crane to see how it holds up.
“We have a requirement that the terrain can be any configuration – jagged potholes, as you call them – and we have to be able to adapt to that surface and be stable there,” says John Gallon, engineer at Jet Propulsion Laboratory. (The dummy lander was named after his 4-year-old daughter’s favorite character in the film Frozen.)
For the past two years, Gallon and colleagues have tested various lander feet, legs, and configurations in a lab by hanging the lander from the ceiling like a puppet. This suspension helps simulate Europe’s gravity, which is one-seventh that of Earth.
Without a lot of gravity, a massive lander could easily jump around and damage itself when trying to land. “You’re not going to hold the landing like a gymnast who comes from the bars,” says Gallon. His team tried to get sticky feet, bowl-shaped feet, springs that are compressed and pushed into the surface, and legs that snap into place to help the lander stay on different terrains. The lander can crouch like a frog or stand stiff like a table, depending on the type of surface it lands on.
Although Olaf works hard to help scientists figure out what it takes to build a successful Europa lander, the mission itself, like its Venusian counterpart, remains only on the wish lists of a few planetary scientists for now. In the meantime, other researchers have been dreaming of travel to completely different worlds, including the Saturn geyser moon Enceladus.
“Some people will pick favorites,” says Cameron. “I just want to land somewhere we’ve never been, that’s not Mars. I’d like that.”
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