In a little over a year, a new decade will begin, and with it a completely new stream of ideas for NASA missions will open, some closer - like Mars, some further away. Some very distant. Some people expect that the era of robotic travel to worlds that are not just millions - billions of kilometers from us will open for us. These include Uranus and Neptune (the planets we visited in 1986 and 1989, respectively), as well as hundreds of ice bodies outside the region known as the Kuiper belt.
The Kuiper Belt is home to Pluto and thousands of other worlds of varying sizes. Most of the bodies there are made up of the building blocks of our solar system, long ago escorted to distant icy regions. A visit to the Kuiper Belt can provide us with clues to questions about how our planet and its neighbors formed, why there is so much water and other mysteries.
On the borders of the solar system
Uranus and Neptune also hold many mysteries on their own. The more we learn about planetary systems, the more often we see that most worlds are not as large as Jupiter and not as small as Earth. Many of them tend to be similar in size to Uranus and Neptune, "ice giants" that are named for the exotic state of water ice that lies deep under cloudy layers. Studying Uranus and Neptune will not only help us understand the planets in our solar system - it will help us understand the planets that revolve around other stars.
Many of these missions are time dependent. The upcoming Decadal Survey - NASA's "ten-year survey" of when the agency sends spacecraft in the 2020s and 2030s - could create or disrupt these far-reaching plans to explore the outer solar system.
Decadal Survey: How the Decadal Survey will progress
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Starting in 2020, a group from the National Academy of Sciences (with the participation of several stakeholders from the space community) will come together and draw up a list of priority research targets. Scientists will offer their options in the form of written recommendations known as "white papers" (read: white paper).
From these recommendations, a general consensus will emerge on what the priorities should be. These goals serve as benchmarks for the mid-range mission offerings in the New Frontiers category (New Horizons and Juno were in this category). NASA first compiles a list of proposed missions, and then narrows them down gradually to one or two finalists. Once a finalist gets the green light, the team behind them can start planning and designing - and it takes years.
All this can make it difficult to get into a specific window through which it will be possible to explore Uranus or Neptune, as well as look at an object from the Kuiper belt. This is why accurate charts are risky.
Visiting the ice giant
One of the groups, in particular, considered the option of a mission to visit Uranus and Neptune at the same time. The last iteration includes a flyby of Uranus and an orbit around Neptune. Led by Mark Hofstadter and Amy Simon, scientists plan to see a different side of Uranus than Voyager 2 observed in 1986 and study Neptune and its largest moon, Triton. Triton rotates backwards, which may be due to the fact that it was once the largest object in the Kuiper belt - before Neptune pulled Triton towards itself, ejecting many of its original satellites.
Simon says these missions should be deployed over 15 years, including travel and research time. This is due to how long individual parts of the vehicle can survive in space with relative certainty. While a spacecraft can live longer, 15 years is the minimum, during which you can be sure that the mission will complete its scientific tasks to the fullest. But how to make sure that the journey does not waste too many resources in the current phase of research? One way to accelerate a spacecraft is to use the planet's gravitational force to accelerate.
“Typically, to get there in less than 12 years, they fly around planets, typically including Earth and Venus,” Simon says. In such scenarios, you plunge into the planet's gravity well, hoping for a slingshot effect that will accelerate your craft and save as much fuel as possible. Jupiter is also used by the best of the options, as it is the most massive and can greatly accelerate the spacecraft.
New Horizons, for example, used Jupiter's help to reach Pluto. Cassini used four separate overflights to accelerate with Saturn after launch from Earth, receive acceleration from Venus twice, return to Earth, and finally the final jump from Jupiter.
Simon says that in order to get to Uranus on a tight schedule, a flyby of Saturn could be used - for example, in a window between 2024 and 2028, to catch the gas giant in the right place in its 29-year orbit. Such a mission will require quick thinking by NASA standards - usually missions are planned ten years before launch, then planned, designed and launched within five years - so you will have to rely on the next window, a Jupiter flyby between 2029 and 2032, followed by an exit to Neptune. The next chance will appear no earlier than ten years from now.
A mission to Uranus can use traditional propellants and engines to get to the acceleration points faster - be it an Atlas V rocket or a Delta IV Heavy rocket. But because Neptune is so far away and the exact trajectory does not line up as perfectly as we would like, the mission to this planet will rely on the Space Launch System, NASA's next-generation rockets with increased payload (and it has not even flown yet). If it's not ready in time, we'll have to rely on another next-generation technology: solar electric propulsion, which harnesses solar energy to ignite ionized gas to accelerate the vehicle. Until now, it has only been used on the Dawn spacecraft on missions to West and Ceres and on two missions to small asteroids.
“Even in the case of solar electricity, chemical engines are still needed in case solar energy becomes ineffective and for braking in orbit,” Simon says.
Thus, the schedule is quite tight. But if we move more actively, both of these missions can serve a different purpose: to reach the unexplored worlds of the Kuiper belt.
Big unknown
Another paper, written by three members of the New Horizons team, examines the possibility of returning to the Kuiper belt after a successful probe walk to Pluto. “We saw how interesting it was and wanted to know what else was out there,” says Tiffany Finley, chief engineer at the Southwest Research Institute (SWRI) and co-author of an article published in the Journal of Spacecraft and Rockets.
The Kuiper Belt contains ice remnants from the formation of the solar system, and objects in it include a huge variety of different materials. Pluto, for example, is slightly larger than Eris. But Pluto is made of ice, so it has less mass. Eris is made up of rocks for the most part, so it is more dense. Some worlds appear to be composed of methane, while others contain a lot of ammonia. Somewhere in the backyard of our solar system, there are many dwarf planets and small worlds that hold key points for our understanding of how planets come about - and whether other planetary systems might be like ours.
Scientists used narrow constraints: they limited the mission to 25 years and looked at 45 of the brightest Kuiper belt objects, comparing them with respect to different scenarios of planetary flyby. Jupiter, surprisingly, has discovered most of the targets on the list. But Jupiter's window opens once every 12 years, making Jupiter's missions time-dependent. A simple flyby of Saturn provides a pretty good list of Kuiper belt targets.
But when you pair these worlds with Uranus or Neptune, you get the chance to discover new facts about our mysterious, most distant planets and even some dwarf planets in one fell swoop.
The slingshot effect will help to reach these worlds, first from Jupiter, and then from another planet. Each of these planets aligns with Jupiter in a narrow window in the 2030s, and fits neatly into different parts of this decade. For example, to get to the list of worlds on the path with Neptune, you need to get to Jupiter in the early 2030s, and getting to the Kuiper belt via Uranus would require a launch in the mid-2030s. Jupiter and Saturn align in time for a slingshot into the Kuiper belt in the late 2030s.
The list of goals offers many interesting possibilities. Varuna, an elongated world that has acquired this shape due to its fast rotation rate, is perfect for flying around Jupiter-Uranus. Neptune, as already mentioned, provides a glimpse of Eris. The mission through Jupiter-Saturn will allow observation of Sedna, a large dwarf planet with an orbit that could point the way to the as-yet-undiscovered planet ten. Jupiter-Saturn will allow you to stop at one of the most interesting dwarf planets: Haumea.
Like Varuna, Haumea is egg-shaped, while most of the large dwarf planets in the Kuiper belt are usually round. But Haumea got this shape from an ancient collision that gave her two moons, a ring system and a tail made of debris. When asteroids have a similar composition, they are called the "collision family." Haumea produced the only known family of collisions in the Kuiper belt.
Whatever we choose, we won't have much time. Therefore, if we want to see the rings of Haumea or even the red, alien light of Sedna, work needs to start as soon as possible. These worlds are so small that there is only one way to find out their secrets: to get to them.
Ilya Khel