In the coming decades, several space agencies plan to return astronauts to the moon (or send them there for the first time) and launch the first crewed missions to Mars. Between this and the explosive growth we see in Low Earth Orbit (LEO), there is no doubt that we are living in an era of space re-exploration. It is understandable, therefore, that old and new concepts for interstellar travel are also being considered these days.
A significant focus is currently on light sails that generate their own propulsion through radiation pressure or are accelerated by lasers. These concepts pose all kinds of engineering and technical challenges. Fortunately, Coryn Bailer-Jones from the Max Planck Institute for Astronomy (MPIA) recently carried out a study in which he advocated a “Sun Diver” light sail that picks up the required speed by diving near the sun.
The study, entitled "The Sun Diver: Combining Sun Sails with the Oberth Effect," was recently published in the American Journal of Physics. As Bailer-Jones noted, one of the major draws of awnings and light sails is the fact that they free the spaceship from having to carry its own propellant. This ensures a lower mass, which ultimately allows the spacecraft to accelerate to greater speeds.
This is a consequence of Tsiolkovsky's famous rocket equation, which describes how a spaceship applies acceleration by ejecting some of its mass (propellant) as thrust. A consequence of this equation is that the amount of propellant a missile must carry to increase its velocity (delta-v) increases exponentially with increasing delta-v, mainly because most of the propellant is used to accelerate the unused propellant is used.
To date, a number of concepts have been developed and tested to explore the solar system, including LightSail and Lightsail 2 from the Planetary Society and the IKAROS satellite from JAXA. In addition, various organizations are exploring the idea of light sails accelerated by laser arrays as a means of interstellar travel – such as NASA's Breakthrough Starshot, Project Dragonfly and Starlight.
In both cases, a light sail offers advantages. As Bailor-Jones emailed Universe Today:
“When exploring the solar system, you can use solar sails to perform many different maneuvers or get into orbits that would otherwise require a lot of propellant. For interstellar travel, we would use a solar sail to dive as close to the sun as possible and thus get the maximum acceleration from the sun.
A phased laser array, perhaps in the high desert of Chile, propels the sails on their journey. Credit: Breakthrough Initiatives
This method is advantageous because conventional solar sails rely on radiation pressure, which has some disadvantages. According to Bailor-Jones, this includes being totally dependent on the sun for modest acceleration. "In particular, the maximum acceleration you can get from the sun with a solar sail decreases with the square of the distance from the sun," he said. "So you are not very useful when you are very far from the sun."
Since the radiation pressure (and the resulting delta-v) is low in these cases, the light sail would have to be either extremely light or extremely large. One way to get around this problem is to rely on powerful directed energy laser assemblies (lasers) to accelerate the sail. This is exactly what the Starshot, Dragonfly, and Starlight concepts require, but such an array would be very expensive to build and operate.
Another possible solution, according to Bailor-Jones, is to create a lightweight sail that can take advantage of the Oberth effect. As he explained:
“The principle of the Oberth Effect is that you apply your thrust when you are moving fastest in relation to the body you are orbiting. This is the sun in the case of the sundiver. The closer you are to the sun in your orbit, the faster you will be. To use the Oberth effect, you have to get as close to the sun as possible. "
The Planetary Society's LightSail-1 solar glider is scheduled to go into orbit in 2016 on a SpaceX Falcon Heavy rocket with its parent satellite Prox-1. Photo credit: Josh Spradling / The Planetary Society.
For this process, a spaceship with a sail stowed in it and a small amount of propellant would take a circular orbit around the sun. The spaceship could then do one of three things. First, it could use all of its propellant to perform a retrograde boost (which would slow its orbital speed) and fall as close to the sun as possible before unfolding its sail.
Second, it could forego the dive altogether and apply full progressive thrust (increasing its orbital speed) when the sails are deployed. Third, it could do a combination of the two. He also considered a scenario in which the sails would open on the initial circular orbit and then apply the full fuel boost at the end.
After considering all of these scenarios, Bailor-Jones concluded that the highest speed would be achieved if the spacecraft performed a full retrograde thrust to dive as close to the sun as possible and then opened its sail as soon as it did reaches the perihelion. According to his calculations, a sail could reach a top speed of around 350 km / s in this way – which corresponds to 1.26 million km / h.
This is much faster than what chemical missiles can do. Currently, the record for the fastest spaceship is held by the Parker Solar Probe, which reached a maximum speed of around 150 mph. With a top speed of 1.26 million km / h, a light sail would take another 2,865 years to reach Proxima Centauri.
IKAROS space probe with solar sail in flight (artist representation) with a typical square sail configuration. Photo credit: Wikimedia Commons / Andrzej Mirecki
As Bailor-Jones suggested, this is due to the limits of materials science, which limits a spacecraft's proximity to the sun. “(I) If your spaceship could withstand an approach to the sun than the dive you could get with the available Delta-V, you could tilt the sail so that it curls as close to the sun as possible, and then using the Delta-V as a boost, ”he said.
In addition, his study focuses on what can be achieved with a single transfer lane. As an extension, he claims that an optimal combination of the three impulse methods he identified during two transmission paths could also lead to higher speeds. Even so, there are many mission profiles a sun diver could accomplish, especially closer to home:
“The use of the Oberth effect can be limited in practice, however, as you are not only interested in speed on many missions. You also want to control the direction you are traveling and when you reach certain destinations. Therefore, the correction of these goals may not be compatible with the maximum possible boost from the Oberth effect. "
Interstellar missions have to wait for advances in materials science, regardless of whether it is a "Sun Diver" or a propulsion concept with directed energy or not. Fortunately, such advances (and many more) are expected in the years to come, and the implications for space exploration are sure to be profound!
Further reading: arXiv