The atmosphere is almost entirely carbon dioxide, trapping all the planet’s heat. As a result, its surface temperature stands at over 450 degrees Celsius, making it hotter than even Mercury, which is far closer to the Sun. It is so hot on Venus that the planet’s sulphuric acid rain (that’s an acid with a boiling point of 337 degrees Celsius) evaporates before it hits the ground.

How could two planets born around the same time, almost equidistant from their star, be so different? As our planet warms, answers to Venus’s past could hold clues to the future of Earth, researchers say.

Getting close enough to seek those answers, however, poses a series of challenges.

Venus is less than 60 million km away from Earth (for perspective, the moon is about 400,000 km away). But it maintains its searing temperatures even at night, owing to the runaway greenhouse effect created by its carbon dioxide. Its atmospheric pressure is at an intense 93 times that of Earth at sea level.

As a result, only four spacecraft have made it to the surface so far (all part of the Soviet Venera missions), and they only survived briefly -- from a few minutes to a few hours -- before they were cooked and crushed by the planet.

A hot mess

As we’ve peered at our neighbour through the decades, though, the view has been changing.

Humanity’s first interplanetary mission headed to Venus, given its proximity. This was the US National Aeronautics and Space Administration (NASA)’s Mariner 2 fly-by, in 1962. It gathered the first granular data on the planet. The Mariner 10, launched in 1973, relayed the first close-up images.

With each fresh set of data, estimates of surface temperature and atmospheric pressure were revised upwards. Then the Venera landers were crushed in the 1970s, and humanity moved on to places with more promise: back to the Moon, and on to Mars (which is roughly half the size of Earth and at least five times farther).

In recent years, however, orbiters have returned. The European Space Agency’s (ESA) Venus Express, launched in 2005, found evidence of granite-like rocks, which caused some excitement because on Earth they only form in the presence of water.

The Japan Aerospace Exploration Agency’s (JAXA) Akatsuki, the only spacecraft currently orbiting this neighbour, has been studying Venus’ mysterious atmospheric dynamics since 2015, using cameras set to different wavelengths. Other missions — including Hubble, the Parker Solar Probe and ESA’s Solar Probe — have flown past and added to the data.

What we’ve learnt, from these fly-bys and from ground observations via radio telescopes and climate models, is astounding .

Scientists believe that Venus once held an abundance of water, possibly for as much as two to three billion years, long enough for the planet to have been habitable. (The solar system, incidentally, is about 4.6 billion years old.) They have found traces of phosphine in the planet’s atmosphere. This is a compound that is produced as waste by bacteria on earth, and is hence considered a biomarker.

Could there have once been life on Venus? Could Venus have once been a lot more like Earth?

Venus fly trap

New missions are seeking answers. Four, a mix of orbiters and atmospheric probes, are headed there in the coming decade, including one by the Indian Space Research Organisation (ISRO).

NASA’s DaVinci and Veritas missions are slated for launch between 2028 and 2030. ESA’s EnVision mission is due for launch in 2031.

ISRO’s Shukrayaan 1 is set to head out as early as December 2024 or, failing that launch window, in 2031. This four-year orbiter mission is designed “to probe the surface, sub-surface, atmosphere and upper atmosphere, and understand its seasons as well as the Venus-Sun interactions,” says Anil Bhardwaj, director of the Physical Research Laboratory in Ahmedabad, a unit of the Government of India’s Department of Space.

Shukrayaan 1 will be the closest ISRO has ever come to the Sun. It will travel 40% of the distance between us and our star, Bhardwaj points out. A key challenge will be keeping the craft from being sucked into Sun’s orbit.

JAXA’s Akatasuki, for instance, was launched in May 2010; failed to enter orbit around Venus owing to a technical mishap; then got dragged into the Sun’s immense gravitational pull and went spinning around it for five years before it could be manoeuvred back towards its original destination in orbit around Venus.

If all goes well, radar, infrared cameras and other instruments on board Shukrayaan 1 will collect data on Venus’s composition and atmospheric characteristics, in the hopes that these will provide new clues into the theory of an Earth-like early life.

“There are fundamental reasons to look into our neighbour’s backyard,” Bhardwaj says. “There is a curiosity to understand what happed to a planet that is terrestrial like Earth, is almost the same size, and is a similar distance from the Sun… What happened for it to end up so different? Understanding that could help us understand what could happen here on Earth, millions of years from now.”