Universe Today
Sending a mission to the Solar Gravitational Lens (SGL) is the most effective way of actually directly imaging a potentially habitable planet, as well as its atmosphere, and even possibly some of its cities. But, the SGL is somewhere around 650-900 AU away, making it almost 4 times farther than even Voyager 1 has traveled - and that’s the farthest anything human has made it so far. It will take Voyager 1 another 130+ years to reach the SGL, so obviously traditional propulsion methods won’t work to get any reasonably sized craft there in any reasonable timeframe. A new paper by an SGL mission’s most vocal proponent, Dr. Slava Turyshev of NASA’s Jet Propulsion Laboratory, walks through the different types of propulsion methods that might eventually get us there - and it looks like we would have a lot of work to do if we plan to do it anytime soon.
Let’s get this out of the way first - there is no way we’re going to use traditional chemical rockets and even gravitational assists from large planets to get out to 650 AU in any reasonable time frame. To reach this point in 20 years would require a craft to travel at 154 km/s - which admittedly is slightly slower than the fastest human-made thing ever. The Parker Solar Probe has hit speeds of 192 km/s, but that only happened when it was flying at its closest approach to the Sun, only 6.16 million kilometers from it. Maintaining that speed, or even reaching it, for a 20+ year journey is infeasible with current technology.
But the Sun might still be our best ally when considering how to do it. One of the technologies Dr. Turyshev looked at was solar sails - giant reflective surfaces that use the Sun’s light to push itself. But perhaps more importantly, solar sails could combine both the Sun’s light and the Sun’s gravitational pull using a gravity assist at the same time they are accelerated with maximum force close to the Sun. By Dr. Turyshev’s calculations, that could accelerate a craft to be capable of speeds that would allow for a 30 year transit, or potentially even a 20 year transit.
Fraser discusses the idea of the SGL with Dr. TuryshevThere are trade-offs though. To hit a 30 year transit speed to the SGL, the craft would have to hit a perihelion (the closest distance to the Sun) of 0.05 AU. That is slightly farther out than Parker has achieved (which was a startling 0.04 AU). But Parker was designed specifically to handle close approaches to the Sun. Designing thin film solar sail that can withstand being blasted by the energy that close is outside our current engineering capabilities.
There’s another factor to consider when calculating the feasibility of solar sails - weight - or more specifically, density. Solar sails don’t offer a lot of actual power output, so dragging heavy equipment around isn’t the best use case for them. Unfortunately, at distances of 650 AU, solar energy is essentially negligible to actually power the telescope once it gets there, so it needs to bring along its own power source. A radioisotope thermal generator, perhaps the most likely candidate for such a power source, isn’t exactly light, and would massively disrupt the calculation of the density levels any such probe could hope to achieve.
All that is to say, solar sails might not be the best way, though they would potentially be the fastest if we could figure out somehow to get them to withstand the Sun’s bombardment. But there are other options that have their own advantages. One is Nuclear Electric Propulsion, or NEP, where a fission reactor provides power to high-efficiency electric thrusters. These long-burning engines are slow, but consistent, and in space travel that “specific impulse” improvement makes a huge difference. Dr. Turyshev calculates that a NEP-driven probe with an 800 kg payload on a 20 ton spacecraft could make it to 650 AU in between 27 and 33 years. Not as fast as the Sun-skimming, very light solar sail version, but well within a single human lifetime at least.
Fraser discusses why solar sails are so amazing.NEP has other advantages, though. Once it reaches the SGL, it could help with “station keeping” by using leftover propellant to correct the positioning of the system. Since it obviously uses electricity as well, it can use some of that to power the system to do the actual observations as well. It’s main disadvantage, though, is heat management. The electric generator on board will require a method of getting rid of its waste heat somewhere, and in space the only feasible way to do that is through radiative cooling. So, an NEP system would require giant radiators that might be infeasible to fit into a single rocket fairing.
But perhaps the most interesting idea in the paper is actually the combination of NEP with another nuclear technology - Nuclear Thermal Propulsion (NTP). NTP is much faster compared to its NEP cousin, and uses heat directly from a nuclear reactor to rapidly heat a liquid propellant like hydrogen and expel it out as thrust. If that sounds a lot like a chemical rocket, you’re right - and that also means that NTPs are subject to the tyranny of the rocket equation. They can only carry so much fuel before consuming all of it and losing its ability to provide additional thrust.
But, since the fission reactor is the same basis for NEP and NTP, the SGL craft could use a hybrid system. It would utilize the NTP at certain points in the trajectory, such as getting a gravitational assist from the Sun, in what’s called an Oberth maneuver. After hitting a certain speed using its more powerful NTP engine, it would switch to an NEP cruise mode and slowly build up speed over the course of years using the NEP’s high efficient ion drive.
Fraser discusses how to use NEP for an SGL mission.Such a hybrid system could potentially make a sub-20 year transit feasible to get to the SGL. But what happens once the spacecraft actually gets there? Typically, telescopes stop in a particular position, whether that’s Geostationary orbit, or a Lagrange point, and hold that position until they stop functioning. However, the SGL mission wouldn’t even attempt to slow down at that point when it reaches its distant destination. It would continue on the “focal line” of the solar gravitational lens effect and continue to collect data as it travels for an additional almost 300 AU, capturing more detailed pictures as it goes.
While that allows the trip to be much quicker - since it won’t have to decelerate in order to hold a specific position, it does mean that we only get one shot at imaging a particular world. The way the SGL effect works is that you have to be on the opposite side of the Sun from the exoplanet you want to image, and we can’t simply adjust course to take a look at another one that might be in a completely different part of the sky. So, while we continue working on propulsion technologies that could take us to this astounding place in the solar system, its worth considering what kind of metrics we need to be fairly certain there’s something interesting to look at on a planet once we get there - otherwise the whole 30 year journey, not including all its development time, will be a great scientific exercise, but won’t truly be able to answer the question of whether we are alone or not.
Learn More:
S. Turyshev - Propulsion Trades for a 2035-2040 Solar Gravitational Lens Mission
UT - Using the Solar Gravitational Lens Will Be Extremely Difficult
UT - Ion Engines Could Take Us to the Solar Gravitational Lens in Less Than 13 Years
