Sail-powered spaceships: just a scientific dream or a real possibility to travel to other stars?
After marking the history of our species for thousands of years,
sailing has continued to exert an irresistible fascination even in the years of its decline;
and now, after decades in which it was forcibly relegated to sports and recreational activities
(at least in the Western world), it is even making a comeback in terms of exploration
and commercial use, as in the days when clippers raced the seas in search of ever faster routes.
With a slight change of scenery though... interplanetary space instead of the ocean.
It was the Russian Fridrikh Arturovich Tsander (1887-1933) - in an article published in the
summer of 1924 - the first to introduce the term "solar sail" and to describe the
technical foundations of a propulsion system capable of exploiting the pressure exerted by
sunlight. All based on the studies of the great pioneer of astronautics Konstantin Tsiolkowsky,
who had already theorized the subject in 1921. But already Johannes Kepler, if we want to go back
to times when intuition could not be supported by evidence of feasibility, proved to be able to look
into the future, so much so that after observing the behavior of the tail of a comet, which always
turns in the opposite direction to the Sun regardless of its movement and its position,
he wrote to his "colleague" Galileo in 1610: "Give me ships with sails suitable for the celestial
wind, and there will be people who will not fear for themselves even in front of that immensity".
Returning to our times, in the mid-60s, with the advent of new lighter materials,
NASA began to think about studying their possible practical applications. However, due to ups and
downs related to the management of funds, agency policy, and more profitable programs both in terms
of image and, above all, money, such as Apollo and the Shuttle, the study of a solar sail project was
permanently shelved when at the end of the '70s was discarded to conduct an automated mission
on Halley's comet, which was not realized at all. But by then the idea of the solar sail had
captured the imagination of engineers and scientists and so it was that, since the
early '80s, were mostly private efforts to try to realize the first solar sail vehicle in history.
In 1992, NASA scientist Klaus Heiss proposed to organize, on the occasion of the 500th
anniversary of the discovery of America, a real race from Earth to Mars: the Columbus
500 Space Sail Cup. But the project - started well - aborted for economic reasons.
The race would have involved three spacecraft, one American, one European, and one Asian,
driven solely by the pressure of light. The departure was scheduled for October 12,
1992 (Columbus Day), with destination another “new world”: Mars. The race unfortunately never
started, but several research centers worked to design the vehicles that could have participated.
Even the European Space Agency had a solar sail project, called Odyssee, which in turn,
however, has stopped for over three years to preliminary test deployment of the sail.
More recently, in 2010, the Japan Space Agency launched the IKAROS mission,
the first interplanetary sail probe in history. Able to reach the orbit
of Venus in six months of travel propelled by a solar sail of about 200 square meters.
Which, only in 2019 was followed by LightSail 2, the mission wanted by the Planetary Society
to test the maneuverability of a 32 square meter sail placed in Earth orbit at a height of 720 km.
In practice, only a couple of successes in more than 20 years of missions funded
and then canceled, or aborted due to problems with the rockets during launches...
As it can be guessed, the extreme complexity of such a project,
not to mention the high costs and the extreme economic precariousness with
which these "minor" programs often have to deal, have made these projects uncertain
over the years, with partial results or at least very difficult to achieve.
However, NASA considers fundamental the use of solar sails for space exploration and
illustrates its motivations by referring to the well-known fable "the tortoise and the hare",
where the current rockets are the hare while the solar sail is the tortoise.
In this race, the rocket-propelled vehicle quickly takes the lead, overtaking the solar sail vehicle,
which instead proceeds at a slow but steady pace. Sooner or later, no matter how fast you go,
the rocket will run out of fuel while the sail vehicle will have an infinite supply of energy,
given by the Sun. The latter could even potentially return to Earth, while the
rocket vehicle would have no propellant at all. As the solar sail vehicle continues its journey,
it is able to reach speeds unthinkable for rocket-powered vehicles; such a vehicle would
be able to travel at about 90 km/s, which is ten times faster than the orbital speed of the shuttle
(8 km/s). To give you an idea of how fast it is, think that in a solar sail vehicle that reached
top speed you would be able to travel the New York-Los Angeles route in less than a minute!
If you wanted to calculate the time it takes an interstellar probe powered by
solar sails to reach Voyager 1 (the farthest spacecraft from Earth) it would take eight years,
an insignificant amount of time compared to the forty-five years it takes Voyager
to get where it is. If you then add a laser or magnetic beam transmitter to the system,
NASA has guaranteed that these vehicles would be able to reach speeds of 30,000 km/s, one-tenth
the speed of light. At such a speed, interstellar travel would no longer be a dream but a certainty
Yes, but how would solar-powered navigation work in practice?
Meanwhile, we must immediately clear the field from a misunderstanding in which it is all too
easy to fall: sunlight and solar wind are not the same thing. Despite the name suggests otherwise,
in fact, the impulse received by a solar sail does not come from what we call solar wind
(consisting mostly of emission of particles of matter such as electrons and protons)
but from the elementary particles of sunlight, or photons. This is because the photon pressure
is thousands of times higher, and therefore more profitable than that exerted by the solar wind.
Despite this, it should be noted that the thrust of sunlight is still very modest:
we are talking about a value corresponding to the pressure exerted by a weight of 9 milligrams
on a surface of one square meter. It is therefore evident that, in order to obtain convenient values
of acceleration, it will be necessary to prepare a collection surface as large as possible. In other
words, the larger is the surface of the sail, the greater is the amount of light affecting the sail
and therefore the greater is the force exerted by light. But on the other hand,
the larger the surface area of the sail, the greater the overall mass of the vehicle,
and the more force will be required to move it. Thus, in the economics of solar sail propulsion,
the tradeoff between the sail's surface area and its mass cannot be ignored. This is the reason
why one of the crucial points in the practical realization of a solar sail is the sail material,
which, while maintaining adequate strength values, should be as light as possible.
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Regarding the guidance, even a cosmic vessel, just like a sea sailing ship,
will maneuver by changing the position of its sails with respect to the "wind".
You might think that the obvious solution to get the maximum thrust is to follow a route
that allows to "take the wind at the stern", that is with the back to the
Sun and the sails perpendicular to the radiation. In reality, this would not
work. If you put the sails in that position, the combined action of the pressure of light
(which pushes away from the Sun) and the force of gravity (which acts in the opposite direction),
would lead the probe in an elliptical orbit that would bring it back to the starting point.
To avoid the effect of continuous return, the sails must be oriented to form continuously
variable angles (in interplanetary travels are sufficient one or two degrees of variation per
day) with respect to solar rays. In this way the probe will progressively increase its speed,
migrating towards wider and wider orbits around our star, with a spiral trajectory.
Previous research has therefore suggested that "light sailing" might be one of the
only technically feasible methods to get a probe to another star within a human lifetime.
In order to develop a way for sunlight to propel a light sail to useful interstellar speeds,
researchers analyzed previous scientific research looking for strong, lightweight materials. They
settled on aerographite, a carbon-based foam 15,000 times lighter than aluminum.
The scientists calculated that a hollow aerographite sphere about 1 meter in diameter
with a shell 1 micron thick (about 1% the width of an average human hair)
would weigh just 2.3 milligrams! If such a sphere carrying 1 gram
of payload were released about one astronomical unit from the sun (the average Earth-Sun distance,
which is about 150 million km), sunlight would push it to a speed of up to about 183,600 km
per hour - three times that of Voyager 1... Missions within the Solar System assuming
such a sphere with a shell thickness of roughly 0.5 mm could, according to these calculations,
reach the orbit of Mars within 60 days, arriving at Pluto’s orbit in 4.3 years.
If such a sphere were released about 0.04 AU from the Sun, the more intense sunlight there
would accelerate the spacecraft to nearly 24.8 million km/h. It could travel the 4.2
light-year distance between Earth and Proxima Centauri, the closest star to our solar system,
in 185 years, the researchers said. The larger the sphere, the faster it could go,
or the more payload it could carry. The researchers noted that a few grams
of electronics or other payloads are not a lot to bring aboard a mission. Still, they argued
the payload for these crafts would be 10 times the mass of the spacecraft, whereas the payload
on chemical interstellar rockets would typically be one-thousandth the weight of the rocket. This
means, for example, that a big 1000 tonne starship could easily leave our solar system dragged by a
sphere of at least 2.7 km in diameter... But these, of course,
for the moment are only dreams... Returning to the spheres one meter large,
the scientists estimated that developing a prototype bubble craft
might cost $1 million. They calculated each foam ship might then be built for about $1,000 or less,
and a rocket launch to deploy and test these craft might cost $10 million.
The biggest caveat of this work right now is that no one has ever built an aerographite structure
larger than a few centimeters, while we need something that’s a few meters in size.
Another point of caution about this concept is that there is currently no way to control the
trajectory of the spheres once they are deployed. And In order to reach a certain target, this
needs to be rectified. by an active, autonomous onboard computer, and some form of photon “wings”
or “rudders” to trim the sail. There are so many unknowns that would affect a sail trajectory...
such as interplanetary magnetic fields, the solar wind, interstellar magnetic fields,
limited knowledge of the actual position and velocity (“proper motion”) of the target star...
that we can’t see at this point how one could passively steer an aerographite hollow sphere - or
whatever – to a star at four light years. Even aiming at Mars would be hard with a passive sail,
which is why we talk about reaching the orbit of Mars and the orbit of
Pluto rather than Mars and Pluto themselves…. Only onboard electronics and equipment could
enable active maneuvering. And soon (we must always be optimistic!) this will be possible.
To conclude... The idea of using hollow spheres of aerographite for interplanetary
missions inside the Solar System deserves, without doubt, to be deepened and developed.
The production and development costs are very low compared to other types of space missions
and this in itself is a significant incentive. In addition, compared to chemical propulsion,
the significant reduction in travel time to the periphery of the Solar System makes a mission
based on solar propulsion extremely interesting; especially for the return in terms of scientific
knowledge that it could produce. Moreover, the recent progress in miniaturization of electronic
components is such that it is plausible that the aerographite spheres could be equipped, within
a few years, with both communication tools with the Earth and essential scientific instruments,
such as cameras or particle detectors. What seems much less feasible, unfortunately,
is interstellar travel. In the theoretical simulations proposed by Heller and colleagues,
the solution that guarantees the highest final velocity is to send hollow spheres
with a 1 µm thick shell into space, at only 0.04 astronomical units from the Sun.
But, even then, the time required to reach Proxima Centauri would be just under two centuries.
It is very little in cosmological terms, but it is an eternity from human life,
especially if we relate that time to the speed with which the contemporary world is changing,
from technological, political, social, and economic point of view.
But it is not only a question of speed and time. Solar propulsion is strongly penalized
by the very small mass margin, in the order of few grams, that can be used to implement
scientific and communication tools in probes that must remain ultra-light. To have larger
mass margins for the scientific load, it would be necessary to launch in space huge solar sails,
able to collect, thanks to surfaces in the order of square kilometers,
a sufficient thrust from solar radiation. But this would lead to many other problems not easy to
solve, as, for example, the need to fold countless times a sail of kilometers in size, so that it can
be accommodated in the cargo compartment of a rocket, and then deploy it completely in space,
keeping intact its structural characteristics. We fear that humanity will be able to realize the
dream of interstellar travel only in many decades, when alternative propulsion systems will be
(maybe) finally available, both to chemical and photonic propulsion.