NASA's New Horizons spacecraft, which visited Pluto in 2015, is embarking on an observing campaign of the ice giant planets Uranus and Neptune – and scientists need your help.
Although New Horizons is now far beyond both Uranus and Neptune, throughout September it will be turning its Multispectral Visible Imaging Camera (MVIC) back to look at both planets from a vantage point more than 5 billion miles from Earth. In doing so, it will be seeing these worlds from the opposite direction to how we see them from Earth.
To ensure wide coverage of the two planets from a variety of angles, New Horizons will be joined in this endeavor not only by the Hubble Space Telescope, but also by amateur astronomers all over the world who are being asked to contribute observations throughout the month.
Of all the planets in our solar system, Uranus and Neptune are the least understood. They've only been visited once before, by Voyager 2 in 1986 and 1989 respectively, and their sheer distances mean they can't be seen in great detail from Earth. So, there's still lots to learn about them, which is the incentive behind New Horizons' observing program.
New Horizons' primary science objective for these observations is to learn more about how the atmospheres of the two worlds absorb and emit heat energy as well as how thermal energy is transported from the planets' presumably rocky cores toward their outer atmospheres. Uranus, in particular, is odd in this regard because it appears to have hardly any heat flowing from its interior into space. Neptune, despite being a similar planet at first glance, radiates over two and a half times more energy into space than Uranus does.
There are several possible explanations for why hardly any heat seeps out of Uranus. One is that an impact may have knocked Uranus onto its side billions of years ago, causing the planet's core to lose all its heat at once. Another explanation is that some kind of layer deep within Uranus' atmosphere blocks heat from escaping.
New Horizons' observations intend to get us closer to an answer.
"By combining the information New Horizons collects in space with data from telescopes on Earth, we can supplement and even strengthen our models to uncover the mysteries swirling in the atmospheres of Uranus and Neptune," Alan Stern, New Horizons' principal investigator from the Southwest Research Institute in Boulder, Colorado, said in a statement. "Even from amateur astronomer telescopes as small as 16 inches, these complementary observations can be extremely important."
Telescopes with apertures of 16 inches are technically considered quite large for amateur astronomers. With a telescope of this size, observers will be able to image and track activity in the atmospheres of both planets. This activity is driven by the flow of thermal energy, both from inside the planet and from the sun, and comes in the form of huge, swirling storms that appear as bright spots in the atmosphere.
The upcoming coverage from New Horizons, the Hubble Space Telescope and hundreds of amateur astronomers worldwide is planned to be the most complete survey of Uranus and Neptune ever completed.
Given the placement of Uranus and Neptune in our sky during September, New Horizons' observing campaign also comes at just the right time for amateur astronomers.
Neptune is at opposition on Sept. 19, with Uranus following two months later on Nov. 13. Opposition occurs when a planet is directly opposite the sun as seen from Earth, which means the planet is invariably at its best and brightest.
Neptune will appear to reside in the constellation of Aquarius during September, shining at magnitude +7.8. It'll be modestly high in the southern sky for northern hemisphere observers, sitting just two degrees south of the celestial equator. Uranus is expected to be brighter at magnitude +5.7 and much better placed in the constellation of Aries.
Although resolving atmospheric details through the eyepiece will require telescopes with larger apertures, you can also pick out both planets with 10x50 binoculars and resolve them as tiny disks through an 8-inch telescope. So even if you can't contribute to NASA's atmospheric monitoring endeavor, you can still join in and observe the planets from your backyard, or better yet, local dark site.
Observers are encouraged to post their images with supporting info to social media using the hashtag #NHIceGiants, where the New Horizons team will collate them all and select the best for inclusion in the study.