Return to Pluto
The brains behind the successful New Horizons flyby to the far-f lung dwarf planet are now planning for a closer look
The brains behind the successful New Horizons flyby are now planning for a closer look
If you had dedicated 26 years of your life to sending the first-ever spacecraft to Pluto, and if you had seen that mission succeed in shedding new scientific light on this far-flung planetary system, what would you do next?
Bask in the glory or make plans to go back? For Alan Stern, the principal investigator of the New Horizons mission, it was a no-brainer. “We rewrote the textbooks in the case of Pluto. In fact, I think we wrote the textbooks from scratch,” he tells All
About Space. And there is no doubt in his mind that planetary science needs to return.
At the end of October 2019, it was revealed that such a plan is being formulated. NASA is funding a study by the Southwest Research Institute into the feasibility, attributes and cost of sending an orbiter to Pluto and its system of moons, with Dr Carly Howett, assistant director in the department of space studies at the Southwest Research Institute in Boulder, Colorado, heading a team that is looking into the development and design of the spacecraft and its payload.
The idea is that the orbiter will analyse Pluto’s system in far greater depth. “We’re looking to bring back the same type of instrumentation that New Horizons had, but to do a more thorough job,” Stern explains. “A first flyby was the appropriate thing to do with Pluto, but we have many questions about its geological activity and the ways in which the planet has evolved over time. There’s a very wide range of scientific topics, and that will make a second mission so strong.”
In order to even get off the ground, the
Southwest Research Institute will need to be persuasive. The scientists need to make it into the next Planetary Science Decadal Survey, which is being prepared later this year. This publication reflects the wishes of the scientific community, and it’s an important step in getting the nod – and a budget – for the mission itself. “It’s just a question of whether this is the right mission to get to the top of that survey,” Stern says.
If it does gain approval, it would continue Stern’s career-long obsession with Pluto. In 1989, the same year he received his doctorate, he helped to set up a group called Pluto Underground with the aim of discovering more about the Solar System’s farthest known planet. Frustrated that NASA’s Voyager mission launched in 1977 had intended to visit
Pluto but ended up leaving it unexplored, he dearly hoped to be able to one day shed light on the planet discovered in 1930 by Clyde Tombaugh.
There were certainly no plans to send a spacecraft to Pluto at that time. But the Plutophiles – as Stern has called them – were a determined bunch. A number of them met in an Italian restaurant in Baltimore to discuss a potential plan,
and within a year a proposal called Pluto 350 emerged that was deemed light in terms of both weight and the potential money needed.
One thing’s for sure, it wasn’t an easy path to success. The following year, as Pluto 350 began to feel sidelined, a plan to use the Mariner Mark II concept of one spacecraft to fit multiple missions was explored and abandoned. A subsequent idea to send a lightweight spacecraft carrying just two scientific instruments was also considered, but this scheme – called the Pluto Fast Flyby that then morphed into the Pluto Express – only proved to be steps to what became New Horizons.
Throughout that time, the Plutophiles had sought support for a mission. They had recommended supporters to sit on advisory committees that would advise NASA about planetary mission priorities, written many letters to colleagues and published papers that promoted the scientific benefit of going to Pluto. But budgets, differing priorities and even White House involvement proved as tricky to navigate as space itself. Indeed, when New Horizons was officially selected for funding in November 2001, the new NASA administrator Sean O’Keefe didn’t go on to include it in the budget for 2003. Thankfully it appeared in the Planetary Science Decadal Survey, and funding was finally given.
That allowed the New Horizons team just four years to design, build and test the spacecraft that would end up being launched in 2006. It was vital that the mission was ready at that point because the window would allow Jupiter to be used for a gravity assist, and Stern’s team was relieved that it got off the ground in time.
Even then, it wasn’t entirely smooth. Hearts were in mouths when on 4 July 2015, contact was lost with the New Horizons spacecraft when it was just ten days away from Pluto. Although it did finally reconnect, all of the computer files needed to be re-uploaded in rapid time – something that would normally take weeks. But then, on 14 July, New Horizons performed the first-ever flyby of Pluto, getting within 12,500 kilometres (7,770 miles) of the surface and giving the world its first close-up look.
The discoveries were spectacular, revealing a variety of terrain and surface ages as well as
“Pluto is so compelling that we should go back with an orbiter like we did with Cassini for Saturn”
Alan Stern
suspected ice volcanoes, vast nitrogen glaciers, evidence of a possible liquid water ocean inside and data that pointed to higher surface pressures in the past. Organic compounds were also evident, and there were startling finds regarding Pluto’s moons, including the reflective surface of the largest, Charon.
But as more and more discoveries continued to be made, they raised ever more questions, prompting a push to return for a more in-depth study. “One of the philosophies since the exploration by New Horizons is that Pluto is so compelling that we should go back with an orbiter like we did with Cassini for Saturn or Galileo for Jupiter,” says Stern. “That’s a vertical, very deep approach, and it has been a conversation that we’ve been having for four years.”
There is a general consensus among scientists that revisiting that part of the outer Solar System would be desirable, yet there is a debate over whether the focus should be entirely on Pluto. California Institute of Technology astronomer Mike Brown says further studies should be widespread. “Pluto is a fascinating place, and there are many things we would still like to know about it,” he tells
All About Space. “But to me, what is interesting about the outer Solar System and the Kuiper belt is the diversity of bodies out there beyond Neptune.”
Brown is well known. He calls himself the
‘Pluto Killer’ since he was instrumental in the International Astronomical Union’s decision to demote the celestial body from planet to dwarf planet status in 2006. It’s interesting to note that current NASA administrator Jim Bridenstine has repeatedly affirmed in recent months that Pluto is a planet, effectively siding with Stern in the ongoing sideline debate. In Brown’s eyes, however, it shouldn’t get special attention.
“There is Haumea, which is an oval-shaped rocky body covered in a thin layer of ice spinning every four hours with two moons and a ring,” he says. “There is Makemake, covered in what looks like slabs of methane ice so thick you could ice skate on them, and there is Quaoar, which looks to be a weird transition-type object a little like Pluto and a little like Charon. The list goes on.
“I understand the feeling of wanting to go back somewhere where there were so many interesting things, but rather than filling in the details on Pluto it makes much more sense to continue the basic exploration of the entire region.”
As things stand, Stern is entirely on board with such concerns. “There is an opposing philosophy that we should sample the diversity of objects in the Kuiper belt, big and small, rather than return to Pluto,” he says. “But we have made a breakthrough in this regard. My institution spent close to $500,000 on funding a major study into the development of a Pluto orbiter, and we found that it is actually possible to orbit the planet, use gravity assists from its largest satellite Charon, skate the system and then go back to the Kuiper belt.
“It means the proposed mission is removing any debate over whether we go broad or go deep. We can do both with a single launch of a single spacecraft mission and cover both Pluto and other Kuiper belt objects. It is something we are calling the Gold Standard, and it’s really exciting because we don’t have these two warring factions. We do it all and at a much lower cost than carrying out both missions individually.”
Stern says the combined mission would merely mean a longer operation phase, since the same launch vehicle and flight to Pluto would be involved. It also adds just 20 per cent to the cost of a follow-up solitary Pluto mission. “Imagine those two-for-one deals,” Stern says. “This is two
“These laser ranging-altitude detectors can map the surface at very high accuracy to gather geological topography data. Thermal mappers can also look for hotspots in the glaciers and in the volcanoes in other regions. Mass spectrometers will sample the composition of the atmosphere and a magnetometer can let us know if the planet’s core is alive and churning or dead and frozen solid.”
It’ll be possible to determine how the temperature changes when the Sun goes down on the night sides of Pluto and its moons and, because the orbiter will be sticking around for a while, seasonal changes can be monitored too.
“New Horizons gave us information on a particular day, but an orbiter that stays for years will be able to study atmosphere and geological changes over a long period of time. We’ll be able to compare the geology and the atmosphere in the year 2015 to the data when the orbiter arrives in the 2030s, 2040s or whenever.”
Certainly, the mission is not about to be sending information back imminently. The findings of the Planetary Science Decadal Survey are expected in 2022, which would allow for a launch no sooner than December 2028. On that timescale there would be a gravity-assist flyby of Jupiter in October 2030 and a braking into Pluto’s orbit in 2046 – a process that will take 13 years.
Stern says it could be sped up if nuclear systems can be used to increase the braking thrust so that it slows faster. “The orbiter has to stop and not just fly by, so it will carry a lot more fuel and a different type of propellant that is much more efficient,” he continues. “It would use a Saturn V-class launch vehicle and a nuclear electric stage augmented by a chemical propulsion braking system.”
As well as more instrumentation, higher communication rates are needed too. “With
New Horizons we intentionally sacrificed communications bitrates to save money, and it allowed it to be affordable. But even in the
1970s-era Voyager missions there were ten-times higher data rates at the same distance, and today
we could do faster still – maybe 50 or 100-times as fast as New Horizons.
“When New Horizons swept by Pluto, it took 16 months to get all the data back. But with 50 or 100 times the transmission power, we will be able to empty the data recorders every week and do so again and again over many years. This won’t require a technological breakthrough because it was done in those Stone Age 1970s.”
Even so, what is perhaps most daunting is that to get the mission off the ground, the team will still have to do some persuading and keep within budget. “But that’s just the way life works,” Stern reflects. “I’d like to have breakfast in bed every morning but I’m not going to get it, just as we won’t get everything we want in every mission. We do have many ideas, and there are more than I’m sure there is room for on the next spacecraft, but there’s only a certain amount of money.”
He is definitely convinced of the merit of a second stab at covering that particular area of the universe, though. “Pluto and the Kuiper belt can stand on their own two feet, and they are enormously scientifically attractive, rivalling many other kinds of enterprises in planetary exploration,” he says.
“I’m ready to help make that next stage happen and use the mission to discover what’s on the other side of Pluto, find out more about the small satellites on the other side of Charon and answer questions about geologic activity and the way Pluto has evolved over time. Yet I can’t predict discoveries. Only the universe will tell us what is out there.”