“Nuclear power sources are allowed for the first time under NASA’s Discovery program call,” says Prockter. There’s also another, key reason why Neptune is within the reaches of a robotic spacecraft. (Photo by: QAI Publishing/Universal Images Group via Getty Images) Universal Images Group via Getty Images The nuclear question It is expected that both probes will continue to function until 2020. Voyager 1 was 14 billion km from Earth, making it the most distant artificial object in space. Trajectory of Voyager 1 and 2, The Voyager missions are now over, in principle. “If we don’t launch by 2026, we would have to wait another 12 years for a favorable launch opportunity, or go on a larger, and probably less cost-effective, launch vehicle.” Jupiter orbits the Sun every 12 years, so the next opportunity would be a 2038 launch for a flyby in 2050. “Without gravity assists, we would probably have to use a much larger, more expensive launch vehicle,” says Prockter. “This does not require us to carry a lot of propellant, and therefore enables a small spacecraft and hence smaller launch vehicle.” That's what makes for a relatively cheap mission. “There is a favorable gravity assist opportunity provided by Jupiter in 2026, which allows a fairly simple ballistic trajectory,” says Prockter. The curves will line-up again in 2026–that's why make-your-mind-up time is coming soon. The Trident flyby, more or less modelled on the New Horizons flyby of Pluto in 2015, would occur in 2038 after a 2026 launch. Its target will actually be Triton, which is suspected of being geologically active and almost certainly contains an ocean underneath its surface. but it's not only, or even mostly, going to Neptune. That’s exactly what’s being proposed for the Trident mission. However, the success of New Horizons at Pluto–which discovered the largest known glacier in the solar system, a blue atmosphere, and evidence for an internal water-ice ocean–proves the worth of a brief flyby mission. There’s also only a limited budget for robotic exploration, and getting as far out as Neptune is expensive. It’s basically down to the relative positioning of Jupiter to Neptune. There’s only an opportunity to go to Neptune cheaply once every 12 years. However, there are a number of sensible reasons why Neptune has been ignored for so long. It has been studied with ground-based telescopes and the Hubble Space Telescope ( including finding "Dark Spot 2" in March 2019), but there’s a limit to what they can discover about Neptune from a distance of 29 AU (that’s 30 times the distance from Earth the the Sun). NASA-JPL Why has Neptune been ignored for 30 years? The Great Dark Spot (GDS) seen at the center is about 13,000 km by 6,600 km in size - as large along its longer dimension as the Earth. This color image, produced from a distance of about 16 million kilometers, shows several complex and puzzling atmospheric features. spacecraft as it rapidly approaches its encounter with the giant planet. Neptune's blue-green atmosphere is shown in greater detail than ever before by the Voyager 2. Photos taken by Voyager 2 of Neptune took four hours to reach Earth. The distance from Earth also meant that NASA’s Deep Space Network antennas in Spain, Australia, Mexico and California had to be expanded to 230 feet/70 meters. So Voyager 2’s thrusters fired slightly at its closest approach, rotating the spacecraft to help keep the camera focused on Neptune. Sadly, it was traveling too fast to make that possible without severely blurring the images. In such a low-light environment, Voyager 2’s camera had to take long exposure photographs. Neptune receives only about 0.001 times the amount of sunlight that Earth does. There have been some studies of Neptune’s weather thanks to large ground-based telescopes such as the Keck Observatory in Hawaii, but in terms of atmospheric properties, almost all of what we now is from Voyager 2. “Voyager 2 massively increased our knowledge of Neptune, but since then it’s been a trickle because it's so challenging to see the outer planets with telescopes.” “The vast majority of what we actually know about Neptune came from Voyager–all the close-ups and big shots of Neptune's disc come from JPL’s photo journal, which was compiled after Voyager made its final exit from the inner solar system,” says Tom Kerrs, an astronomer at the Royal Observatory Greenwich, London. Five hours later, Voyager 2 passed about 40,000 kilometers (25,000 miles) from Neptune's largest moon, Triton, the last solid body the spacecraft will have an opportunity to study. Passing about 4,950 kilometers (3,000 miles) above Neptune's north pole, Voyager 2 made its closest approach to any planet 12 years after leaving Earth in 1977. In the summer of 1989, NASA's Voyager 2 became the first spacecraft to observe the planet Neptune.
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