Astronomers have found a potential sign of life high in the atmosphere of Venus. It's the latest discovery in investigations of the planet that stretch back to the 1960s. In that year, NASA's Pioneer 5 space probe was originally intended to go to Venus but technical difficulties meant the mission was changed to instead investigate the interplanetary space between Earth and Venus. Pioneer 5 provided the first map of the interplanetary magnetic field. The first successful mission to Venus - and in fact the first successful mission to any planet - was Mariner 2. The Mariner space program was a series of NASA missions to investigate Venus, Mars and Mercury. Mariner 2 launched in August 1962 and completed its Venus flyby in December that year. During a 42-minute scan of the planet, Mariner 2 conducted scans of the planet. The data it collected indicated no significant difference in temperature across Venus and also found that there was a dense cloud layer. The final launch in the series, Mariner 10, was the first to investigate two planets within one mission. It launched in November 1973 and flew by Venus in February 1974. It returned more than 4,000 pictures of the planet as well as other important data. In 1978, NASA sent a multiprobe mission to Venus. Pioneer Venus 2 included a large probe and three smaller probes. Each probe took about 53 to 56 minutes to reach the surface. Two of the three small probes survived the hard impact. The so-called Day Probe transmitted data from the surface before succumbing to the high temperatures. Information from its nephelometer indicated that dust raised from its impact took several minutes to settle back to the ground. Data from the probes indicated that there is almost no convection in the atmosphere of Venus and below a haze layer at about 30 kilometres, the atmosphere is relatively clear. In November 2005, the European Space Agency launched Venus Express, a probe designed to explore the hot, dense atmosphere of the hostile planet. It relayed information until December 2014 when it is believed to have run out of fuel. Now, astronomers looking at the atmosphere in Venus see something that might be the signal of life. They found the chemical signature of a noxious gas called phosphine. On Earth, the only way phosphine exists is with life.