Today marks not one but two milestones in planetary exploration. It is the 25th anniversary of Voyager 2's flight past Neptune, the most distant planet ever seen up close. It is also the exact day that the New Horizons spacecraft is crossing Neptune's orbit on its way to Pluto, the mysterious world that marks the boundary between the solar system we know and the one we don't.
Flight path of New Horizons. Don't look for Earth's orbit--it's too small to see. (Credit: NASA/JHUAPL) The known solar system has planets that come in three well-studied varieties: rocky (like Earth), gas giant (like Jupiter), and ice giant (like Neptune). Beyond Neptune, things get complicated and confusing. There's Pluto, but there's also the whole Kuiper Belt, a vast collection of other, related objects. Most are the size of small planetary moons, but a few are roughly the size of Pluto and some, yet unseen, might be even bigger. Beyond that is a region called the "scattered disk," where recurring comets come from. And beyond that comes the really shadowy territory: The Oort Cloud, an inferred swarm of dormant comets stretching almost halfway to the next star. Voyager 2's images of Neptune and its satellites are still stunning and largely under-appreciated. More fascinating even than Neptune itself is its giant moon Triton, which is similar in size and density to Pluto but has lived a very different life. Triton may have started out as Pluto's near twin, but it got captured into a backward (clockwise) orbit around Neptune. Gravitational interaction between satellite and planet generate heat and keep Triton active. Its surface has few craters and looks geologically active. Triton offers a hint at what awaits in the Kuiper Belt. Pluto will show us for real what's out there. I'm boycotting the whole debate over whether Pluto is a planet or not, because it misses the point. The Kuiper Belt and scattered disk cover account for about 99.8 percent of the volume of the solar system (the Oort Cloud is so ridiculously large I won't even get into it). We've seen only a few examples of the objects that wander sunward and become comets, and we've never seen any of them in their native environment. We are still strangers in our own solar system, and Pluto is the first step in getting to know the rest of the neighborhood. Planet? Dwarf planet? I just call it damn interesting. So I'm looking back at some of the signature images from Voyager 2 at Neptune, and looking forward to what New Horizons will be seeing just 11 months from now. Follow me on Twitter: @coreyspowell1. Rings of Neptune. That's right, Neptune has rings, but they're thin and faint, nearly invisible from Earth. Here Voyager 2 looks at them backlit by the sun, with the planet severely overexposed by the two, combined 10-minute exposures. All images credited to NASA/JPL unless noted otherwise.
2. Clouds of Neptune. Despite its great distance from the sun, Neptune has dynamic weather, including the highest wind speeds recorded anywhere in the solar system. The "great dark spot" seen here has since disappeared, showing how quickly things change out there.
3. Triton in full. Paul Schenk at the Lunar and Planetary Institute reprocessed the old Voyager data to bring out more detail and correct colors. This is the result: the best-ever view of Triton's two hemispheres, though much of the moon's northern half is missing because it was in shadow at the time.
4. Geysers of Triton. This Voyager close-up contains two surprises. First, there are almost no craters, indicating that the surface is only about 10 million years old--very young in geological terms. Second, what's up with those bizarre streaks? They appear to be eruptions of some sort, perhaps from gases warmed by the sun, that shot out clouds of fine dust; the extremely thin atmosphere then carried the particles downwind.
5. Neptune from Triton. By combining Voyager images with topographic measurements, NASA researchers put together this composite perspective view. The smooth plains suggest that Triton has been reshaped by icy volcanism.
6. Goodbye--for now. Three days after encounter, Voyager 2 looked back at the receding crescents of Neptune and Triton. Soon New Horizons will be on its way to pick up where Voyager left off, completing the solar system "grand tour" originally proposed by NASA in 1964 (!).