The Next Pluto Mission: An Orbiter And Lander?

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Pluto and Charon. Credit: NASA/JHUAPL/SwRI

For decades, we could only imagine what the view of Pluto’s surface might be. Now, we have the real thing.

The images and data from the ” target=”_blank”>New Horizons’ mission flyby of Pluto in July 2015 showed us an unexpectedly stunning and geologically active world. Scientists have used words like ‘magical,’ ‘breathtaking’ and ‘scientific wonderland’ to describe the long-awaited close-up views of distant Pluto.

Even though scientists are still analyzing the data from New Horizons, ideas are starting to formulate about sending another spacecraft to Pluto, but with a long-term orbiter mission instead of a quick flyby.

“The next appropriate mission to Pluto is an orbiter, maybe equipped with a lander if we had enough funding to do both,” New Horizons’ principal investigator Alan Stern told Universe Today in March.

This week, Stern has shared on social media that the New Horizons’ science team is meeting, and part of the agenda is discussing the next mission to Pluto.

But getting a spacecraft to the outer regions of our solar system as fast as possible provides challenges, particularly in being able to slow down enough to enable going into orbit around Pluto. For the speedy and lightweight New Horizons, an orbital mission was impossible.

What propulsion system might make a Pluto orbiter and/or lander mission possible?

A few ideas are being tossed around.

Space Launch System

One concept takes advantage of NASA’s big, new Space Launch System (SLS), currently under development to enable human missions to Mars. NASA describes the SLS as “designed to be flexible and evolvable and will open new possibilities for payloads, included robotic scientific missions.” Even the first Block 1 version can launch 70 metric tons (later versions might be able to lift up to 130 metric tons.) Block 1 will be powered by twin five-segment solid rocket boosters and four liquid propellant engines, with a proposed 15% more thrust at launch than the Saturn V rockets that sent astronauts to the Moon.

But an orbiter mission to Pluto might not be the best use of the SLS alone.

It takes a lot of fuel to accelerate a vehicle to fast enough speed to get to Pluto in a reasonable amount of time. For example, New Horizons was the fastest spacecraft ever launched, using a souped-up Atlas V rocket with extra boosters, it performed a big burn when New Horizons departed Earth orbit. The lightweight spacecraft sped away from the Earth at 36,000 miles per hour (about 58,000 km/ hour), then used a gravity assist from Jupiter to boost New Horizons’ speed to 52,000 mph (83,600 km/h), traveling nearly a million miles (1.5 million km) a day in its 3 billion mile (4.8 billion km) journey to Pluto. The flight took nine and a half years.

“To enter Pluto orbit, a vehicle [like SLS] would have to boost up to that same speed, then turn around and decelerate for half the trip to arrive at Pluto with a net velocity of zero relative to the planet,” explained Stephen Fleming, an investor in several alt-space startups including XCOR Aerospace, Planetary Resources and NanoRacks. “Unfortunately, due to the tyranny of the rocket equation, you would have to carry all the fuel/propellant to decelerate with you at launch … which means accelerating the orbiter AND all that fuel in the initial phase. That requires logarithmically more fuel for the initial burn, and it turns out to be a LOT of fuel.”

Fleming told Universe Today that using the multi-billion dollar SLS to launch a Pluto orbiter, you would wind up launching an entire payload full of propellant just to accelerate and decelerate a tiny Pluto orbiter. “That’s an extraordinarily expensive mission,” he said.

RTG-Ion Propulsion

A better option might be to use a propulsion system of combined technologies. Stern mentioned a NASA study that looked at using the SLS as the launch vehicle and to boost the spacecraft towards Pluto, but then using an RTG (Radioisotope Thermoelectric Generator) powered ion engine to later brake for an orbital arrival.

An RTG produces heat from the natural decay of non-weapons-grade plutonium-238, and the heat is converted into electricity. An RTG ion engine would be a more powerful ion propulsion system than the current solar electric ion engine on the Dawn spacecraft, now orbiting Ceres, in the asteroid belt, plus it would enable operation in the outer solar system, far from the Sun. This nuclear powered ion engine would enable a speeding spacecraft to slow down and go into orbit.

“The SLS would boost you to fly out to Pluto,” Stern said, “and it would actually take two years to do the braking with ion propulsion.”

Stern said the flight time for such a mission to Pluto would be seven and a half years, two years faster than New Horizons.

Fusion Propulsion

But the most exciting option might be a proposed Fusion-Enabled Pluto Orbiter and Lander mission currently under a Phase 1 study in NASA’s Innovative Advanced Concepts (NIAC).

The proposal uses a Direct Fusion Drive (DFD) engine that has propulsion and power in one integrated device. DFD provides high thrust to allow for a flight time of about 4 years to Pluto, plus being able to send substantial mass to orbit, perhaps between 1000 to 8000 kg.

DFD is based on the Princeton Field-Reversed Configuration (PFRC) fusion reactor that has been under development for 20 years at the Princeton Plasma Physics Laboratory.

If this propulsion system works as planned, it could launch a Pluto orbiter and a lander (or possibly a rover), and provide enough power maintain an orbiter and all its instruments, as well as beam a lot of power to a lander. That would enable the surface vehicle to beam back video to the orbiter because it would have so much power, according to Stephanie Thomas from Princeton Satellite Systems, Inc., who is leading the NIAC study.

“Our concept is generally received as, ‘wow, that sounds really cool! When can I get one?’” Thomas told Universe Today. She said her and her team chose a prototype Pluto orbiter and lander mission in their proposal because it’s a great example of what can be done with a fusion rocket.

Their fusion system uses a small linear array of solenoid coils, and their fuel of choice is deuterium helium 3, which has very low neutron production.

“It fits on a spacecraft, it fits on a launch vehicle,” Thomas explained in a NIAC symposium talk (her talk starts about 17:30 in the linked video). “There’s no lithium, or other dangerous materials, it produces very few damaging particles. It’s about the size of a minivan or small truck. Our system is cheaper and faster to develop than other fusion proposals.”

The Princeton team has been able to produce 300 millisecond pulses with their fusion system, orders of magnitude better than any other system.

“The biggest hurdle is the fusion itself,” she said. “We need to build a bigger experiment to finish proving the new heating method, which will require an order of magnitude more resources than the project has been receiving from the Department of Energy so far,” Thomas said via email. “However, it’s still small in the grand scheme of advanced technology projects, about $50 million.”

Thomas said that DARPA has spent much more on many technology initiatives that ended up canceled. And it’s also much less than other fusion technologies require for the same stage of research, since our machine is so small and has a simple coil configuration.” (Thomas said have a look at the budget for ITER, the international nuclear fusion research and engineering megaproject, currently running at about $20 billion)

“To put it simply, we know our method heats electrons really well and can extrapolate to heating ions, but we need to build it and prove it,” she said.

Thomas and her team are currently working on the “balance of plant” technology – the subsystems that will be required to operate the engine in space, assuming the heating method works as currently predicted.

In terms of the Pluto mission itself, Thomas said there aren’t any particular hurdles on the orbiter itself, but it would involve scaling up a few technologies to take advantage of the very large amount of power available, such as the optical communications.

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