In the early days of planetary exploration there were three main targets: the Moon, Mars, and Venus. The Moon is nearby and relatively easy to reach in a short time. Mars was exciting in those early days because it was thought that it could still have life. Venus was a mystery because it was completely covered in clouds, but it was only a little smaller than Earth. It was possible that it was a steamy, jungle planet that could harbor tropical or prehistoric life.

There were other theories as to what might be beneath the clouds of Venus: a global ocean of seltzer water, pools of oil, or a vast scorching desert whipped with sand dunes. Mariner 2 settled the question once and for all when it flew by Venus in 1962 and detected a surface temperature of at least 800ºF, confirming the greenhouse effect predicted by a young Carl Sagan. Venus has been visited by a fleet of spacecraft that flew by, orbited, and landed on it. The Soviet Union successfully landed on Venus during the 1970s and 1980s confirming temperatures as hot as 900ºF with an atmospheric pressure 92 times Earth that is composed of 96% carbon dioxide and laced with sulfuric acid. The photos reveal a barren desert of rock and dust bathed in an orange light from the thick atmosphere.

NASAs last dedicated Venus mission was Magellan that orbited the planet from 1990-94. The European Space Agency had Venus Express in orbit from 2006-2014 and Japan’s Akatsuki has been orbiting Venus since December 2015. The past few decades have been pathetically sparse for Venus exploration, but that may hopefully change with new missions being planned with the hope of a rebirth of exploring Venus.

Venus has been routinely bypassed for missions to other worlds. One reason is that it is intensely inhospitable with surface temperatures high enough to melt lead, atmospheric pressure capable of crushing poorly-made spacecraft, and its cloudy, carbon dioxide-laden atmosphere prevents a direct view of the surface. Mars is favored because of the less hostile, although frigid environment, so NASA has spent billions of dollars exploring it, with the goal of eventually sending humans. Ironically, Earth is more like Venus than Mars. Venus is only 5 percent smaller than Earth and 18 percent less massive. Mars, on the other hand, is 47 percent smaller and 90 percent less massive than Earth.

It is possible that Venus, Earth, and Mars were all habitable around four billion years ago. Yet life only flourished on Earth, so what happened? One theory is that Venus was so volcanically active that the volcanic emissions destroyed it. Massive amounts of carbon dioxide and sulfur dioxide poured into the atmosphere increasing the temperature and eventually triggering a runaway greenhouse effect. If there were any oceans, they were boiled away, and any life was ruthlessly destroyed. Venus became a toxic, pressurized inferno laced with sulfuric acid and off limits to all life as we know it.

Venus is Earth-sized and is the best test case of how the two planets diverged climatically and geologically, unlike much smaller Mars. It is vital to understand Venus if we are ever to understand exoplanets, the planets that are being discovered around other stars, many of which are like Venus or warm super-Earths. Are these worlds more like a desiccated lifeless Venus or more like a flourishing tropical Earth? Figuring out the answer is critical in the search for life beyond Earth. In spite of these questions, Venus constantly gets bypassed for missions. It is a tough world to explore with instruments that would work well for other worlds, but would quickly fail at Venus. Three missions were proposed for the latest New Frontiers selection in December 2017 with the hopes that one of them would fly sometime during the mid 2020s.

VISAGE, the Venus In Situ Atmospheric and Geochemical Explorer, was a mission that lost out to OSIRIS-REx during the last New Frontiers selection in 2012. VISAGE would be an all-in-one descent probe and lander that would analyze the chemical makeup of the atmosphere during descent. Once on the surface there would be two drills which would collect samples, place them into a pressurized interior, and analyze them with spectrometers. Unfortunately, it lost out again last December in the semi-final selection to two possible missions: a return to Titan or a comet sample return mission, of which only one of these will be selected later in 2019.

VICI, which stands for Venus In situ Composition Investigations, is a mission that would consist of twin descent-probe-and-lander vehicles, each to be dropped off by a single carrier spacecraft during two Venus flybys. Unlike VISAGE, the VICI landers take their readings from inside the spacecraft. An instrument known as the Venus Element and Mineralogy Camera, which is based on the Mars Curiositys ChemCam, would measure surface composition by shooting a laser through a small window, and a gamma ray spectrometer reads the rocks beneath the lander. VICI also lost out last December, but on December 20, 2017, the camera was awarded technology development funds to prepare it for future mission competitions. The funds are meant to further develop the Venus Element and Mineralogy Camera to operate under the extreme heat and pressure on Venus. Of the dozens of missions proposed over the past two decades, this one has the best chance of being selected in the distant future.

VICI and VISAGE each target different terrain on Venus. VISAGE was to aim for the rolling lava plains, which are generally smooth and flat, and cover most of the planet. The two VICI landers would land on older, more fractured regions called tessera, providing insight into what the planet was like before lava flooded vast regions of its surface. Tesserae appear to be made of rocks that are significantly different from the lava plains possibly with more water involved in their formation. Examining these rocks could answer important questions about the history of water on Venus.

The Venus Origins eXplorer (VOX) does not involve a lander, but instead would analyze the surface composition from orbit. The two obstacles in directly seeing the surface are the clouds and the thick carbon dioxide atmosphere. VOX would get around this limitation by using narrow wavelength windows where carbon dioxide doesnt absorb much energy, allowing the spacecraft to compile surface imagery with a visible and near-infrared spectrometer. As for the clouds, the orbiter would image them in another wavelength and digitally subtract them from the surface images. The imaging technique is similar to how the Cassini cameras mapped Titan. VOX would radar-map the surface and collect topography data at a resolution better than Magellan. It would orbit Venus for at least three years and would be capable of detecting surface and atmospheric changes from active volcanoes. VOX also has an atmospheric probe that would dip into the upper atmosphere of Venus and collect a small gas sample before skipping back out into orbit. The sample would be measured by a miniature onboard mass spectrometer.

Sadly, none of these missions were selected, and the dire lack of knowledge on how Venus works will continue to hinder our understanding of the climates of exoplanets, and Earth. This is important and is becoming critical with Earth’s climate change almost certain. Venus may have once been like Earth and somehow went bad. It is sobering to think that the Earth may suffer the same fate. Once Earth begins the journey towards a runaway greenhouse, there is no turning back, it cannot be reversed. Venus may hold the key to controlling or preventing it. Our ignorance will certainly destroy the Earth, and us along with it.