Of all the planets, Venus is the most visible, a bright dot in the evening sky, but one where no human is ever likely to set foot.
It’s hot, with temperatures around 480 degrees and an atmosphere of sulphuric and phosphoric acid clouds 90 times more dense than Earth’s atmosphere.
Much of what’s known about Venus stems from a series of Soviet probes. Venera 1, launched in 1961, was the first probe sent from Earth to another planet.
The most successful was Venera 13, which made it to the Venusian surface in 1981, transmitting data for a record 127 minutes before cutting out.
That produced the first colour images of the planet’s surface.
In 1990, the US Magellan probe started orbiting Venus, producing detailed radar maps of the surface over the next four years.
With attention focusing on human return to the moon in 2024, followed by a crewed mission to Mars international he 2030s, Venus has gone off the radar.
But it hasn’t been forgotten. NASA is still thinking about Venus, what it could tell us about Earth and how it could be explored using various probes.
"Venus is like the control case for Earth," said Sue Smrekar, a planetary scientist with the Venus Exploration Analysis Group (VEXAG) in NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, California.
"We believe they started out with the same composition, the same water and carbon dioxide. And they've gone down two completely different paths. So, why? What are the key forces responsible for the differences?"
In her office at JPL is a 30-year-old image of Venus' surface taken by the Magellan spacecraft, a reminder of how long it’s been since an American mission orbited the planet.
The image reveals a hellish landscape – a young surface with more volcanoes than any other body in the solar system, gigantic rifts, towering mountain belts and temperatures hot enough to melt lead.
Now superheated by greenhouse gases, Venus' climate was once more similar to Earth's, with a shallow ocean's worth of water.
It may even have subduction zones like Earth, areas where the planet's crust sinks back into rock closer to the core of the planet.
Venus isn't the closest planet to the sun, but it is the hottest in our solar system. Between the intense heat, the corrosive sulfuric clouds and a crushing atmosphere that is 90 times denser than Earth's, landing a spacecraft there is incredibly challenging.
Of the nine Soviet probes that achieved the feat, none lasted longer than 127 minutes.
From space, an orbiter could use radar and near-infrared spectroscopy to peer beneath the cloud layers, measure landscape changes over time and determine whether or not the ground moves.
It could look for indicators of past water as well as volcanic activity and other forces that may have shaped the planet.
"We know very little about the composition of the surface of Venus," she said.
"We think that there are continents, like on Earth, which could have formed via past subduction. But we don't have the information to really say that."
There could be other ways to study Venus. How about balloons?
JPL engineers Attila Komjathy and Siddharth Krishnamoorthy imagine an armada of hot air balloons riding the gale-force winds in the upper levels of the Venusian atmosphere, where the temperatures are close to those on Earth.
"There is no commissioned mission for a balloon at Venus yet, but balloons are a great way to explore Venus because the atmosphere is so thick and the surface is so harsh," said Krishnamoorthy.
"The balloon is like the sweet spot, where you're close enough to get a lot of important stuff out but you're also in a much more benign environment where your sensors can actually last long enough to give you something meaningful."
Balloons could be equipped with seismometers sensitive enough to detect quakes on the planet below.
On Earth, when the ground shakes, the motion ripples into the atmosphere as waves of infrasound, the opposite of ultrasound.
Krishnamoorthy and Komjathy have demonstrated the technique is feasible using silver hot-air balloons, which measured weak tremor signals above areas on Earth.
That could work even better through Venus’ dense atmosphere.
"If the ground moves a little bit, it shakes the air a lot more on Venus than it does on Earth," Krishnamoorthy said.
Balloons would still need to contend with hurricane-force winds.
VEXAG has determined that the ideal balloon could control its movements in at least one direction.
Krishnamoorthy and Komjathy propose that balloons essentially ride the wind around the planet at a steady speed, sending their results back to an orbiter.
Among the many challenges facing a Venus lander are the thick clouds, which limit use of solar power. The planet is too hot for other power sources to survive.
"Temperature-wise, it's like being in your kitchen oven set to self-cleaning mode. There really is nowhere else like that surface environment in the solar system," said JPL engineer Jeff Hall.
Any lander’s life will be cut short by the electronics starting to fail after a few hours.
Hall said the power required to run a refrigerator capable of protecting a spacecraft would require more batteries than a lander could carry.
"There is no hope of refrigerating a lander to keep it cool. All you can do is slow down the rate at which is destroys itself,” he said.
So, NASA is interested in developing hot technology that can survive days or even weeks, in extreme environments.
The latest concept is a heat-resistant drilling and sampling system that could take Venusian soil samples for analysis.
Hall is working with Honeybee Robotics to develop the next-generation electric motors to power a drill in extreme conditions. JPL engineer Joe Melko is designing the pneumatic sampling system.
They test their prototypes in JPL’s very own version of Venus, the steel-walled Large Venus Test Chamber, which mimics conditions right down to an atmosphere of 100 per cent carbon dioxide.
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