SCOPE: This quantum dot spectrometer can hasten Uranus and Neptune orbital missions

Among their other deliberations, the 2022 Planetary Science Decadal Survey noted that Uranus and Neptune were far behind their 'peers' when it came to exploration.

"This is the only class of planet in the solar system that hasn’t had a dedicated orbital tour," Robin Canup of the Southwest Research Institute, one of the co-chairs of the steering committee for the survey, told Space News. “Understanding the composition and the properties of either one would revolutionize our understanding of ice giant systems and solar system origins."

Studying Uranus or Neptune could also provide insights into exoplanets, considering the number of ice giants discovered around other stars.

"Exploring outer planets has been extremely limited, mostly due to the cost, long travel time, and narrow window for mission implementation," Mahmooda Sultana, an aerospace engineer at the NASA Goddard Space Flight Center, tells IE. 

The stars must align for outer planet missions

NASA often had to plan for years or decades for a mission to the outer planets to meet a narrow launch window.

"Currently, outer solar system missions often use gravity assist to make the travel time shorter than it would be otherwise. In order to use gravity assist, we need certain planetary bodies to be at specific positions, and if we miss the window for launch, we have to wait for a long time for another launch opportunity, which increases the overall mission cost," says Sultana.

And that is one reason why in our last 60 years of space exploration, we haven't explored the outer planet system as much as we would like. She adds that the recent Planetary Decadal Survey pointed out a glaring gap in our exploration of outer planets.

Limitations of a solar sail

This is where the solar sail may come into play.

A solar sail uses the pressure of solar radiation to propel a spacecraft.

Its use could offer a new approach to space exploration wherein missions could be both low-cost and fast-transit. According to recent studies, solar sails could reach speeds greater than 10 AU/year, allowing a craft to get Uranus in less than two years and Neptune in around three years, unprecedented with today’s propulsion technology. 

However, due to stringent mass requirements, solar sails have "limited capability for science payloads compared to flagship-class mission spacecraft". Keeping this in mind, Sultana proposed SCOPE: ScienceCraft for Outer Planet Exploration, "a game-changing mission concept that integrates a science instrument and spacecraft into one monolithic structure".

Sultana was among the 12 researchers who received Phase I grants in 2022 from the NASA Innovative Advanced Concepts (NIAC) program for her visionary idea.

Employing spectroscopy to explore the outer world

By printing a quantum dot-based spectrometer (developed by PI Sultana) on the solar sail material (developed by Artur Davoyan), the team intends to employ spectroscopy - the study of the absorption and emission of light by matter - to explore the outer solar system scientifically.

"Originally I was looking to develop a highly miniaturized and compact spectral imager. [A spectral imager combines spectroscopy and photography to sample image data at many wavelength bands.] A few years ago, I was working on a mission concept development using CubeSats. And it was extremely difficult to find a conventional spectral imager that would fit inside CubeSats," says Sultana.

Conventional spectrometers use optical elements (like a prism) to separate the wavelength of light, which in turn requires a relatively long path length to achieve high spectral resolution. "Conventional spectrometers are big and don't often fit within the resource envelope of small satellites. That's when I started working on the idea of a quantum dot-based spectrometer," she says.

A 'cool mission concept' in the works

For this, Sultana collaborated with Professor Moungi Bawendi, at the MIT Department of Chemistry. "He's a pioneer when it comes to synthesizing quantum dots. We had a shared graduate student, funded through the NASA fellowship program, who worked on developing the synthesis of quantum dots. Once we received the quantum dots at NASA, we developed a process to print and integrate them with other components to make a highly compact and miniaturized spectral imager. So that was very exciting," explains Sultana.

Around the same time, she wondered if it could be used for a 'cool mission concept'.

"That's when I thought about printing the quantum dots on the spacecraft - I wasn't thinking of solar sails in particular, but more along the lines of spacecraft or balloons."

If the quantum dot was printed on the spacecraft's exterior, a large aperture spectral imager could be made without "costing much regarding mass and power to the spacecraft".

Race to Triton before the window closes

Last year, the Planetary Science Group considered several missions for selection. The process included two missions to Venus and two to other planets, says Sultana.

"But when two Venus missions were selected for the discovery program, the outer planet groups became disappointed. One of the interesting object in the outer solar system is Triton [a moon of Neptune] and it has a narrow window that closes around 2045 due to its orbital position concerning the Sun and Neptune. We wouldn't be able to look at both poles of Triton if we can't get there by 2045, which is needed to address some of the key questions about the origin of its plumes, and that opportunity wouldn't be available again until 100 years later, which is not in our lifetime," says Sultana.

That's when it struck Sultana that as a secondary payload, the mission could be accelerated at a significantly lower cost (much less than a flagship mission) and provide several launch opportunities. That would get them to the Neptune - Triton system before the window closes. 

"And that's how I came up with SCOPE," she says.

How different is SCOPE from the features of a conventional solar sail?

As aforementioned, solar sails have a limited ability to carry mass for payloads.

"With SCOPE, the quantum dot-based spectrometer is printed directly on the solar sail. The printed dots are ultralight, with an aerial density in the order of nanogram per meter square. So that leaves us a lot of room in terms of mass to put additional components that are required to operate the spacecraft. That's a big advantage," explains Sultana.

The second aspect involves utilizing 'extreme solar radiation'. "We make sure that the ScienceCraft gets close to the sun, at about 0.2 - 0.3 AU, where we have a high solar radiation density."

The closer to the Sun the spacecraft is, the higher the impact of the solar radiation density. 

"This pushes the ScienceCraft ahead such that we reach outer planets in just three to five years. That's a huge advantage. But this involves a number of innovative technologies - we have to come up with the solar sail architecture, as well as a solution for maintaining the temperature of the spectrometer to roughly room temperature even when we get close to the Sun," explains Sultana.

The solar sail is being created in collaboration with Assistant Professor Artur Davoyan, a mechanical and aerospace engineer from UCLA. While he focuses on the architecture of the solar sail, Sultana is in charge of the spectrometer, the overall spacecraft, and the mission.

How else can the presence of a quantum dot spectrometer add to the mission?

Because of weight and space constraints, just one instrument will need to suffice to make various scientific measurements in the Neptune-Triton system.

"UV spectroscopy can be done to look at Neptune's rings. We can use visible wavelengths to conduct spectral imaging of the Triton surface. That would be great as Triton is one of the most active bodies in our solar system - it has geysers and predicted ocean that we haven't been able to image before. It has a very energetic ionosphere. Finally, we can use IR wavelength to investigate atmospheric chemistry," explains Sultana.

Ironically, speed is a challenge

Sultana is currently looking at an orbital mission.

"In order to have an orbital mission, we'll need to slow down the spacecraft once we reach our target - Neptune - and that turns out to be extremely challenging. We've several ideas that we're working on right now. Our alternate plan is to do a fly-by mission, which can still give us a good amount of data. But of course, an orbital mission is more desired," she says. 

In Phase I, the team intends to demonstrate the feasibility of the concepts they develop, regardless of the nature of the mission (orbital or fly-by). 

"In Phase II of NIAC, which is a two-year project, we can develop the mission concept further and demonstrate some of the technologies in question," she says.

The team is also planning for a potential Phase III, wherein they could do a demo of SCOPE (although not necessarily to other planets). "At that point, we'll be able to submit the mission concept to other planetary science opportunities - like the Discovery Program," adds Sultana.