There are plenty of crazy ideas for missions in the space exploration community. Some are just better funded than others. One of the early pathways to funding the crazy ideas is NASA’s Institute for Advanced Concepts. In 2017 and again in 2021, it funded a mission study of what most space enthusiasts would consider only a modestly ambitious goal but what those outside the community might consider outlandish—landing on Pluto.
Two major questions stand out in the mission design: How would a probe arriving at Pluto slow down, and what kind of lander would be useful on Pluto itself? The answer to the first is one that is becoming increasingly common on planetary exploration missions: aerobraking.
Pluto has an atmosphere, albeit sparse, as confirmed by the New Horizons mission that whizzed past in 2015. One advantage of the minor planet’s relatively weak gravity is that its low-density atmosphere is almost eight times larger than Earth’s, providing a much bigger target for a fast incoming aerobraking craft to aim for.
Much of the NIAC Phase I project was focused on the details of that aerobraking system, called the Enveloping Aerodynamic Decelerator (EAD). Combined with a lander, that system makes up the “Entrycraft” that the mission is designed around. Ostensibly, it could alternatively contain an orbiter, and there are plenty of other missions discussing how to insert an orbiter around Pluto. Hence, the main thrust of this paper is to focus on a lander.
After aerobraking and slowing down to a few tens of meters a second, from 14 km/s during its interplanetary cruise phase, the mission would drop its lander payload, then rest on the surface, only to rise again under its own power. The answer to the second question of what kind of lander would be useful on Pluto is – a hopper.
Hoppers have become increasingly popular as an exploration tool everywhere, from the Moon to asteroids. Some apparent advantages would include visiting a wide array of interesting scientific sites and not having to navigate tricky land-based obstacles. Ingenuity, the helicopter that accompanied Perseverance paved the way for the idea, but in other words, the atmosphere isn’t dense enough to support a helicopter. So why not use the current favorite method of almost all spacecraft – rockets?
A hopper would fire its onboard thrusters to reach the area on Pluto’s surface and then land elsewhere. It could then do some science at its new locale before taking off and doing so again somewhere else. The NIAC Phase I Final Report describes five main scientific objectives of the mission, including understanding the surface geomorphology and running some in-situ chemical analysis. A hopper structure would enable those goals much better than a traditional rover at a relatively low weight cost since Pluto’s gravity is so weak.
Other objectives of the report include mathematical calculations of the trajectory, including the aerobraking itself and the stress and strain it would have on the materials used in the system. The authors, who primarily work for Global Aerospace Corporation and ILC Dover, two private companies, also updated the atmospheric models of Pluto with new New Horizons data, which they then fed into the aerobraking model they used. Designing the lander/hopper, integrating all the scientific and navigation components, and estimating their weights were also part of Phase I.
The original launch window for the mission was planned as 2029 back in 2018, though now, despite receiving a Phase II NIAC grant in 2021, that launch window seems wildly optimistic. Since the mission would require a gravity assist from Jupiter, the next potential launch window would be 2042, with a lander finally reaching the surface of Pluto in the 2050s. That later launch window is likely the only feasible one for the mission, so we might have to wait almost 30 years to see if it will come to fruition. Sometimes crazy ideas take patience – we’ll see if the mission team has enough of that to push it onto the surface of one of the most interesting minor planets in the solar system.
Full Report “Pluto Hop, Skip, and Jump”
This is the Final Report from Global Aerospace Corporation on this NIAC effort (Grant Nos.: NNX17AJ71G and 80NSSC18K0062) to develop the Pluto Hop, Skip, and Jump mission concept. We sought out to establish the feasibility of using a large inflatable drag device to decelerate and land on Pluto from interplanetary speed (~14 km/s) using only the Pluto atmosphere and just a few kilograms of propellant. The design and analysis efforts in Phase I indicated that this is feasible. Aerodynamic heating and loads were found to be orders of magnitude less than typical planetary entries due to the ultra-low ballistic coefficient craft and the low density and large scale height of the Pluto atmosphere. The deceleration system is capable of delivering a 200-kg lander-hopper to the surface or inserting an orbiter of a similar mass using aerocapture. Mission analysis work led to a reference mission with Earth launch in 2029, Jupiter assist in 2030, and Pluto arrival in 2040.
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