In the summer of 2015, a tiny NASA spacecraft called New Horizons zoomed by Pluto, giving us our first ever up-close look at this tiny world at the edge of the Solar System. But many are starting to think that flying by Pluto just once was not enough. A group of scientists argue that we have to go back to the dwarf planet — and this time, we need to put a spacecraft in its orbit.
On Monday, dozens of planetary scientists — including Alan Stern, the principal investigator of the New Horizons mission — gathered together at a workshop in Houston to talk about what a follow-up mission to Pluto could look like. The workshop has been two years in the making, according to Stern, ever since the first pictures of Pluto started coming back from New Horizons. The images revealed Pluto to be a geologically active world, with vast plains of nitrogen ice and mountain ranges reaching 11,000 feet tall. And based on the images and data gathered by the spacecraft, there’s even speculation that Pluto harbors a liquid ocean underneath its surface, despite being located around 3.67 billion miles from the Sun.
“Pluto just wowed people.”
“Pluto just wowed people,” Stern tells The Verge. “It’s been two years and the conversation keeps bubbling up, not just by New Horizons people, but at other scientific meetings... We need to go back.”
Stern and others want to get more answers about all the things that New Horizons saw at Pluto — and they say the best way to do that is to send an orbiter to the dwarf planet. While the New Horizons flyby gave scientists the biggest data dump yet on Pluto, the spacecraft only captured images of one side of the distant world. And since it flew by in just one day, the vehicle also didn’t get to see how Pluto changes over time. “If we send an orbiter, we can map 100 percent of the planet, even terrains that are in total shadow,” says Stern. “And we can watch how things change as Pluto rotates on its axis.”
There’s no definitive plan just yet for what an orbiter mission to Pluto would look like. The workshop was just meant to get the conversation officially started about what scientists would want from follow-up exploration. But Stern and others envision a orbiter that would stay at least three to four years at Pluto and move around the system somewhat like NASA’s Cassini spacecraft, which has been exploring Saturn for the last 13 years. Cassini has taken a variety of paths around Saturn by flying close to the planet’s moons and using them like gravitational slingshots to get it into different orbits around the system. A future orbiter could do the same thing with Charon, Pluto’s largest moon; that way the spacecraft could redirect itself and visit Pluto’s smaller and stranger moons like Nix and Hydra.
“When you have an orbiter you’re in the system for several years.”
New Horizons did get a few shots of these tiny moons during its flyby, but got only a few fuzzy pictures from far away. “We tried to get good images of the smaller satellites,” Amanda Zangari, a post-doctoral researcher on the New Horizons mission from the Southwest Research Institute. “But we got an image or so a piece, because Pluto was the priority and rightly so. When you have an orbiter you’re in the system for several years, and you can do observations that would take a long time to do.”
Not only could an orbiter get closer to Pluto’s moons, but it could potentially get even closer to Pluto itself. The best New Horizons images of Pluto had a resolution of about 230 feet per pixel, according to Zangari. But she says an orbiter could get much closer to the surface, getting images with resolutions of maybe less than 30 feet per pixel. “It will allow us to look at very fine details on the surface and see smaller craters,” says Zangari.
The vehicle could also be equipped with different instruments than New Horizons had, such as one that can directly sample the atmosphere or one to help determine the presence of liquids underneath the surface. Additionally, an instrument that pulses infrared lasers at Pluto could be used to figure out the height differences on parts of Pluto that are in long-term shadow. Pluto takes 248 Earth years to orbit the Sun, which means parts of its surface don’t see sunlight for a really long time. This laser instrument could be used to probe those hard-to-see places when they’re covered in darkness.
Of course, inserting a vehicle into the orbit of Pluto is going to be a more difficult task than sending a spacecraft speeding by the dwarf planet. The probe would need extra propellant to help slow itself down as it approached the Pluto system, and that will require a bigger propellant tank than what New Horizons had. But Stern argues the orbiter could incorporate ion propulsion like NASA’s Dawn mission in the asteroid belt, which is much more fuel efficient than traditional space propulsion methods that use chemical combustion. Additionally, a much different type of communications system will be needed for an orbiter at Pluto, according to Zangari. New Horizons’ flyby data took up a big portion of the spacecraft’s memory, and sending all that information back to Earth took a year and three months. An orbiter will get even more data than what New Horizons collected, and a mission team will need to figure out new methods for getting all that information to Earth in a timely manner.
Perhaps the biggest challenge of a Pluto orbiter mission is competing with all the other ideas out there
But perhaps the biggest challenge of a Pluto orbiter mission is competing with all the other planetary mission ideas out there. The Solar System is a big, dynamic place, and there are many popular destinations that scientists want to explore. There are talks of sending orbiters around Uranus and Neptune, as well as the many moons with suspected subsurface oceans. And NASA doesn’t have an unlimited budget to fund every mission that is proposed. The best that Stern, Zangari, and others can do right now is to continue refining their ideas for a Pluto orbiter mission in preparation for the Planetary Science Decadal Survey — a huge document published every 10 years defining the science community’s biggest priorities when it comes exploring the Solar System. The survey acts as a guide for NASA and Congress to make decisions about which planetary missions that should get funding in the future.
Stern says the Decadal Survey won’t get started for another three years. During that time, he and others will continue communicating their desire for an orbiter mission to others in the science community, in hopes that more experts will agree that a second trip to Pluto is needed. “It’s just the beginning of a discussion, not the end of it,” says Stern. “We don’t have all the answers, but a return to Pluto has become a thing.”