Is this the Best Time for Outer Solar System Missions?

 

Planetary missions can't launch at any point. Instead we must wait for the stars to align, literally! Low-energy trajectories to the planets which maximize the amount of science payload that we can take along for the ride are only available at certain configurations of the earth and the destination around the sun. However, by using flybys of other planets to provide gravity assists, we can start missions at a wider variety of times and, in some cases, can travel to the outer solar system even more efficiently.
Above: Illustration depicting Cassini’s trajectory to Saturn with multiple gravity assists
(Image Credits: NASA JPL)

By Ankita Das

A few days ago, I was speaking to a colleague of mine about outer solar system missions. We discussed how there are so many unexplored moons, each unique in its own way but only a handful of spacecraft have ventured into the depths of the outer solar system. In the conversation, my colleague, whose research involves studying the plumes of Saturn’s moon Enceladus, said in an upset tone “I don’t think we will have another Cassini anytime soon unless it is privately funded." So, when I was asked to write my first blog at PVL, this was the first topic that came to mind. 

I have always found the outer solar system to be an exciting place to conduct scientific investigations. When I was growing up, Cassini was the only mission that was actively orbiting and studying the Saturnian system. Previously, Galileo had studied the Jovain system in detail and the Voyager missions had flown past the gas giants. The data from these missions informed us about the diversity of the moons and the possibility of these moons having subsurface oceans, possibly indicating habitability. In relatively recent times, the New Horizons mission and Juno were added to the slowly growing list of outer solar system missions. Despite the data from Cassini and Galileo, as a young teenager I often wondered why we didn’t send more missions to explore these moons. Today, as a graduate student having studied interplanetary missions to certain depth, I can see why sending spacecraft to the outer solar system can be challenging. But, I am even more convinced that there is precious science that awaits us there. 

The first challenge that I could think about was the challenge of finding a good power source for the spacecraft. Most of the interplanetary missions within the inner solar system are solar powered. The issue with having a solar powered spacecraft in the outer solar system is that the power received diminishes drastically with distance and it gets harder to run an elaborate suite of scientific instruments with limited power. Mathematically speaking, it diminishes as (1/distance squared). Mars orbits approximately at 1.5 AU, while Jupiter and Saturn orbit further at ~ 5 AU and 9.5 AU. Thus, the solar power received at Jupiter is approximately 1/25 that of what is received on Earth. While, the power received at Saturn is almost 1/100th of what is received on Earth. It is due to this constrain that most of the existing outer solar system missions are powered by Radioisotope Thermoelectric Generators (RTGs). Simply put, RTGs are powered by the radioactive decay of heavier elements like Plutonium into relatively lighter elements. This decay produces energy which can be used to power spacecraft which have limited access to solar energy. So why aren’t we sending a whole bunch of missions powered with RTGs to the outer solar system? The answer is cost and limited availability of the Pu-238. Power from RTGs, however efficient, does come at a price. Another drawback of using RTGs to power spacecraft is that the power produced decreases over time as the abundance of the heavier element decreases. 

Keeping in mind the approximate distances mentioned above, while designing an interplanetary mission, we also need to take into account the vast distances a spacecraft needs to traverse in order to reach orbits beyond Jupiter. The larger the size of an orbit, the greater its energy. Therefore, such trajectories require higher quantity of propellant, which might result in a decrease of the mass budget of scientific instruments on the spacecraft. So, how did spacecraft like Cassini and Galileo make it to the outer planets? The solution is known as gravity assist where the gravity of a planet is used to increase the relative velocity between the spacecraft and the Sun. Typically, the trajectory employed is known as Venus- Earth Earth Gravity Assist (VEEGA). However, in the future, with the advent of more powerful launch vehicles like the Space Launch System (SLS), the number of gravity assist maneuvers required will be reduced, potentially leading to a shorter time spent in the cruise phase. 

The challenges arising from vast distances between Earth and Outer Solar System planets doesn’t end here. Communication with the spacecraft starts to become an issue at such distances. Although the communication between the spacecraft and receiving stations on the Earth happens through radio waves which travel at the speed of light, it can take hours for communications to reach these distances. This implies that operations of such missions must be planned carefully and requires an elaborate team operating round the clock to monitor and operate the spacecraft.

So why invest in such costly missions? Without doubt, the icy moons of the outer solar system show great potential when it comes to scientific discoveries and exciting research. The chances of habitability in the inner solar system planets (apart from Earth obviously) are thin. In contrast to this, the moons in the outer solar are promising candidates for a habitable environment to say the very least. Moons like Titan are rich in complex organics . Moons like Europa and Enceladus have possible oceans beneath the surface which could harbor life. In addition to this, such environments also provide exciting opportunities to study small body interactions – between moons and within the rings of Saturn.  Despite being investigated by missions like Cassini and Galileo, gas giants are still poorly understood. The interiors of these planets very much still remain a mystery. Understanding these gaseous planets will also improve our knowledge about mechanisms in the interiors of exoplanets and young stars. 

These are just few of the many reasons why we should explore the outer regions of the solar system more actively. With the given improvement in technology, we should invest more in missions like JUpiter ICy moons Explorer (JUICE), Europa Clipper, and Dragonfly which will be studying the Jovian system, Europa, and Saturn’s moon Titan, respectively, in the upcoming decade. Until then, we will keep wondering about these ice-rich and organics-rich worlds.