On June 24th at 22.30 EST (4.30am BST on June 25th), The Planetary Society (TPS) launches it’s CubeSat space probe called LightSail 2 on board the mighty SpaceX Falcon Heavy rocket. All going well, LightSail 2 will deploy a 32 square-metre Solar Sail, and commence it’s enigmatic mission to orbit the Earth propelled only by the momentum of sunlight, and steered by the Earth’s magnetic field.
If successful, LightSail 2 will use the momentum of the Sun’s light to move about the Earth while using orientation changes in relation to the Sun to tack into a higher Earth orbit, in a manner similar to how a sailing ship tacks through wind to traverse it.
The outcomes of LightSail 2 will herald a new era in innovative propulsion whose value has long been understood and which is now in our sights – that of using sailed spacecraft to beach the enormous distances to the planets, and even the nearest stars, potentially within our lifetime.
Crowd funded by The Planetary Society – the largest non-profit space interest organisation in the World – more than 40,000 people have made the LightSail 2 mission possible: a mission of our time, but one in the hearts and minds of those who founded The Planetary Society in 1980 when co-founders Carl Sagan and Louis Friedman then championed the idea, through to the present day where Planetary Society CEO Bill Nye and Director of Projects Bruce Betts now carry the baton toward a fully operational Light Sail space craft.
Figure 1: An artists impression Light Sail 2 fully deployed in Earth orbit. Credit: The Planetary Society
Sunlight Propulsion – The Background
Although light – a form of energy – has no mass, it does possess momentum. As a result, if a ray of light (or a photon of light – depending on how you regard it) impacts on any surface, it constitutes a momentum collision, where a momentum transfer occurs between the ray of light and the object it collides with, producing what is often termed radiation pressure that causes the object to move.
Although the momentum and resulting pressure from sunlight is tiny, it is not totally insignificant. The radiation pressure upon the palm of your hand facing the Sun is about equivalent to that of a grain of pepper landing gently upon it.
But such is the supply of sunlight, inexhaustible in the vacuum of space, that the affect of such a tiny pressure can build up over time. As just one example, each of the two 1970s Viking missions to Mars weighted in at a hefty 3.5 Tonnes each, yet had NASA not corrected for the radiation pressure from sunlight falling upon each spacecraft as it travelled to Mars, they would have missed the planet by 15,000 kilometres on arrival! Such a tiny pressure, experienced relentlessly in the vacuum of space, builds up to a significant factor in space flight.
Figure 2: A diagrammatic representation of sunlight impacting upon a solar sail, causing it to move Credit: Physics dot org
History and Context
The idea that sunlight might bare a propulsive pressure is not new. Johannes Kepler, as far back as 1619, proposed a pressure associated with sunlight when attempting to explain why comet tails always point away from the Sun; and indeed Kepler is partly correct in this assessment, though he couldn’t have known that the Sun also emits high energy particles, and possesses a vast magnetic field that combine to produce a Solar Wind that also contributes significantly to comet tails being pushed outward from the Sun.
Jules Verne in his 1867 novel “From the Earth to the Moon” also hypothesised the use of sunlight pressure as an alternative to the 900-foot canon used to fire his fictitious ship to the Moon. And indeed, at that same time the Scottish scientist James Maxwell actually produced the theoretical principles that revealed that light does indeed possess momentum; not long after experimentally verified at the turn of the 20th century by the scientists Ernst Fox Nichols and Gordon Ferrie Hull.
Light Sail Propulsion
As the new century unfolded and we began to understand more acutely the vastness of space, the realization of the necessity for new and innovative propulsion systems began to arise if we were to breach the distances to the planets and stars.
Of course since the dawn of the space era spaceflight has been dominated by chemical rockets: burn chemical propellants and they produce enormous quantities of gas at high pressure, released through a nozzle to create a thrust that propels the rocket upwards. From the earliest Chinese rockets over a thousand years ago to the great Saturn V that carried people to the Moon, all have been based on the principle of the chemical propellant rocket.
And while the extraordinary efficiency and reusability of SpaceX’s Falcon rockets will help revolutionize space enterprise in Earth orbit and beyond; and the gargantuan power of Boeing/NASA’s new Space Launch System (SLS) – one and a quarter times as powerful as Saturn V – will be capable of getting people to Mars and back, it has also been known from the outset that chemical rockets can only get us so far into space.
Even sending unmanned space probes to the outer planets of the Solar System using chemical rocket alone is an enormous challenge. The Voyagers to the Gas Giants, Cassini to Saturn and New Horizon to Pluto all needed extra propulsion from gravity assist manoeuvres as they passed the Gas Giant planets, to take years off their travel time.
We have known from the outset, that if we wish for long-term and sustainable engagement with the Solar System; and if we hope to reach the stars, then better propulsion systems are needed, a realisation perhaps more poignantly today as our ambitions towards deep space become ever clearer.
There is no shortage of contenders however: Ramjets, Nuclear Thermal Rockets, Fusion Propulsion and indeed Light Sails among others – all contend for future space propulsion.
While such contenders have remained on the drawing board, the idea of spacecraft using sails to harness the radiation pressure of light has garnered particular interest. Firstly, it does not require an extreme or currently unrealized energy source. And while the idea of propelling a sailed spacecraft by the minute pressure from light may seem at first impractical, means have been realized by which it might become a viable alternative to chemical rockets for long time-scale space missions to the planets and even potentially the nearest stars.
For example, using only sunlight, a solar sail measuring hundreds of metres along each side can generate enough thrust to navigate across the Solar System – endlessly. A solar sail spacecraft with an albeit enormous sail of dimensions 100km square and made of an extremely thin material could achieve extraordinary velocities – in the order of 4 million kilometres per hour – enabling such a craft to reach the our nearest stellar neighbour Alpha Centauri at 4 light years distance in approximately 1000 years. While sounding like a long time, when compared to the Voyagers propelled by chemical rockets and gravity assist and achieving velocities of 20,000 km/h and requiring 70,000 years to reach Alpha Centauri, then the benefit becomes clear.
An alternative to constructing such an enormous sail propelled by sunlight is to construct tiny space craft attached to sails of dimensions of perhaps 5 metre square, then propelled by powerful lasers on Earth or the Moon rather than by Sunlight. Such a system could in principle propel the spacecraft at a staggering 200 million kilometres per hour – one fifth of the speed of light – enabling it to reach the nearest stars in only 20 years or so. The Breakthrough Starshot program, currently underway, is investigating such a possibility, with an aspiration to realising a real mission to Proxima Centauri using light sails in only a few decades from now.
While using light sailed space craft would be impractical for short journeys to the Moon and near by planets, as Bill Nye says: “…[for] interstellar travel, really the only way to do it that anybody can think of right now is with solar sail[s]…”
And so it has been on this basis that alternative propulsion systems, and light sails in particular, have remained of keen interest over the years.
Carl Sagan himself championed the idea in the 1970s, famously showing a model of a light sail on the Johnny Carson Show. Subsequently he, along with Ann Druyan (author, co-writer of the original Cosmos series, principal writer and Executive Producer of Neil deGrasse Tyson’s Cosmos and spouse to Carl Sagan) and NASA Engineer and TPS Co-founder Louis Friedman all progressed the idea from the outset of the formation of The Planetary Society.
Figure 3: Carl Sagan on The Johnny Carson Show, demonstrating a solar sail spacecraft.
And while it is currently unfeasible to construct 100km square sails, or propel sails in space via powerful lasers, it is completely possible to do an enormous amount of preliminary work with a light sail mission: construct light sails with innovative materials capable of being pushed by sunlight; learn how to launch and deploy such sails in space safely and securely; learn to control and steer lights sails deployed in space and develop the mechanisms to do so; and examine the theory of light sail propulsion through experimentation: in short, devise missions that take light sail propulsion off the drawing board and into space. Such preliminary but ground-breaking work is what has underpinned TPS Light Sail.
Cosmos-1 and Light Sail 1
TPS’s first light sail attempt in 2005 was called Cosmos 1: a light sail mission launched in collaboration with Ann Druyan’s company Cosmos Studios. The light sail was launched upon a Russian converted military rocket from a submarine in the Barents Sea. Unfortunately the rocket did not reach its intended orbit and Cosmos 1 could not be deployed.
A planned replacement – Cosmos 2 was subsequently replaced by a new approach using lower mass sail technology, announced in 2009 as LightSail 1. Using a NASA designed Nano-Sail D and deployed into Earth Orbit as a CubeSat in 2015, LightSail 1 successfully deployed it’s sail; albeit in an orbit too low to allow for the Sun’s photons to propel it.
Figure 4: Artists impression of Cosmos 1 deployed in Earth orbit. Credit: The Planetary Society / Cosmos Studios
Indeed, LightSail 1 was not the first successful solar sail mission – that honour goes to the 2010 Japanese Aerospace Exploration Agency (JAXA) Ikaros mission – a light sail larger than LightSail 1 (14m along each side) that deployed successfully in interplanetary space close to the planet Venus where it established a 10 month orbit about the Sun, sending data back to Earth and providing valuable insights into attitude control of a light sail craft that has helped the development of TPS’s LightSail 2.
JAXA are fully committed to solar sail propulsion, and intend to use an enormous 250m x 250m sail to send a research probe to the Trojan Asteroids out near Jupiter in the early 2020’s, cementing solar sail propulsion as a viable means of travelling to the planets. Both JAXA and TPS maintain active communications and a sharing of expertise and knowhow to strengthen the future for light sail technology.
Figure 5: JAXA Ikaros deployed in space near the planet Venus. Credit: JAXA
Lights Sail 2
And so for TPS the culmination of more than 40 years thought and effort in light sail technology is the Light Sail 2 mission, to launch at 22.30 EST on June 24th 2019.
So what are the objectives of LightSail 2? In short, to fully deploy a 32 square metre (5.6m x 5.6m) solar sail spacecraft at a high enough altitude orbit (720km above the Earth), free of Earth’s atmospheric drag so as to enable the Sun’s photons to move the spacecraft.
Once secure in orbit, LightSail 2 will continually reorienting it’s solar sails as it orbits the Earth so that it only captures sunlight when moving away from it, receiving a momentum push to raise its orbit and demonstrate solar sail propulsion at work.
And so the primary objective of LightSail 2 is to attempt to elevate it’s orbit from an initial 720km by about half a kilometre each day as it orbits the Earth. If this can be achieved, it will successfully demonstrate both the capability of sunlight to propel the spacecraft, and the ability to adjust its attitude with precision in order to enable its orbit to grow.
Figure 6: LightSail 2 reorientation its sail so as to gain a push from the Sun’s light when pointing away from it, delivering energy to the spacecraft to raise it’s orbit. Credit: The Planetary Society
While LightSail 2 is a TPS driven project, no fewer than 40,000 people world wide crowd funded a sizeable part of the $7 million total budget, making it a true citizen-science mission. True to TPS’s ethos of democratizing the exploration of space, LightSail 2 is a mission made possible by citizens across the planet passionate about space exploration. It is a truly international mission.
Under the TPS stewardship of CEO Bill Nye, Director of Projects Bruce Betts and Purdue University’s David Spencer, the funding raised have enabled TPS to employ necessary expert space systems design and testing companies Stellar Explorations Inc. and Ecliptic Enterprises Corp. Meanwhile, many LightSail 2 and associated ground station and software systems have also been designed and tested by students in Georgia Tech and Cal Poly.
With such talent on board from so many quarters, the systems of LightSail 2 have been designed and tested to extraordinary levels. As just one example, even if LightSail 2 looses contact with Earth, it has the ability to reboot its systems and initiate contact with Earth of its own accord – a very sophisticated capability for a mission costing about 1/20th as much as an equivalent NASA mission!
Figure 7: TPS CEO Bill Nye demonstrates the Mylar Sail material
Perhaps the most intriguing aspect to LightSail 2 is it’s tiny CubeSat foot print. On launch, it measures no more than the size of a loaf of bread! Once deployed in space, 4 mini solar panels will open from its sides to power the on-board computers, communications systems, detectors and sail actuators.
The sails themselves are made of Mylar, and at just 4.5 microns thick (one tenth the width of a human hair) are light and reflective enough for sunlight alone to push them into a higher orbit.
Before deployment the entire spacecraft measures just 30 x 10 x 10 cm, while when fully open the sails span an area of 5.6 x 5.6 m or about 32 square metres; and an entire spacecraft weights just 5kg!
Figures 8 and 9: Images showing LightSail 2 Solar Panel Deployed and Solar Sail Deployed. Credit: The Planetary Society
The Falcon Heavy mission upon which LightSail 2 is attached is due for launch on June 24th at 22.30 EST (4.30am BST on June 25th). With a three hour launch window on that day, all being well LightSail 2 will be deployed into an orbit at 720km altitude (by comparison the ISS is at 408km).
LightSail 2 is a secondary payload attached to the Falcon Rocket in a small washing machine-sized spacecraft called a Prox-1. Once detached from the Falcon rocket and at a safe distance, Prox-1 will eject LightSail 2 for orbital insertion.
The orbit LightSail 2 initially settles into will be circular, and of low inclination meaning it will remain within about 24 degrees north and south of the Earth’s equator and unlikely therefore to be visible in skies from latitudes beyond 42 degrees north and south.
After about 5 days of testing, the sails will be deployed. Four Cobalt-alloy tape-like booms unroll from within the tiny spacecraft, and attached are the four wedge shapes Mylar sails which when fully deployed cover an area of 32 square metres.
As the spacecraft orbits the Earth, momentum wheels will enable LightSail 2 to change orientation with respect to the Sun: on the part of it’s orbit facing the Sun it will orientate the thin edge of its sails toward the Sun so as not to be affected by the Sun’s radiation pressure, while on the part of its orbit moving away from the Sun it will re-orientate itself by 90 degrees face on to the Sun, experiencing a slight radiation pressure push that will accelerate the spacecraft by a tiny amount of just 0.058 mm/s². While this sounds like a tiny acceleration, it is continuous, unlike with a chemical rocket whose acceleration ceases once it’s fuel is used up, and so in just one one month of constant sunlight, LightSail 2’s speed will increase by 549 kilometers per hour!
In this way it will gain energy and rise up to a higher orbit. Small mirrors on the edges of the sails will hopefully be tracked from the ground using laser-ranging, and if successful, will enable us to track and determine LightSail 2’s orbit with exquisite precision, gaining new insight into the workings of solar sails.
Alas LightSail 2 does not have full attitude control, and so as one side of its orbit increases (apogee) the other side of its orbit (perigee) will decrease, getting ever closer to the Earth on each successive orbit. And so it is expected that within about a year of operation LightSail 2’s perigee will be close enough to the Earth’s atmosphere to succumb to drag forces that will cause it to finally re-enter the atmosphere where it will burn up, ending the mission.
As of June 24th 2019 you can follow the mission daily from the online feeds presented below, and if you are a radio expert you can track it live via radio receiver – again details provided in list of resources below.
The Future – NASA, JAXA and Breakthrough Starshot
While the science and know-how gleaned from LightSail 2 will be invaluable to all interested in light sail spacecraft, The Planetary Society will likely not pursue further light sail missions. Upon completion, LightSail 2 will have fulfilled the dreams of Carl Sagan, Ann Druyan and Louis Friedman of a successful light sail mission.
As a non-profit organisation whose central remit is to enthuse and inspire citizens across the planet in space exploration, TPS will move onto other projects. Nevertheless, to ensure that the results of LightSail 2 help future light sail missions, TPS have been working through the entirety of LightSail 2 with a team from NASA toward conducting a follow on light sail mission called Near-Earth Asteroid Scout – a fully fledged solar sail mission to conduct reconnaissance of an asteroid, to be launched as one of 13 CubeSats on the first SLS mission. Meanwhile the planned solar sail mission of JAXA to the Trojan asteroids and Breakthrough Starshot’s aspiration so using powerful lasers to send no less than one thousand 4 metre squared space craft to Proxima Centauri all mean that the future for light sail propulsion is very bright, and here to stay.
Perhaps most poignant and gratifying of all is that, at a time of increased awareness of the necessity of environmental sustainability on out planet, it so happens that among the most innovative space missions being planned is for a sustainable model of propulsion, based purely on sunlight.
Figure 10: NASA’s Near-Earth Asteroid Scout, based on a similar configuration to LightSail 2. Credit: NASA
Figure 11: Breakthrough Starshot proposes sending no fewer than 1000 light sails, each almost identical in size to LightSail 2 but with their attached spacecraft only grams in weight, and to fire a 1 Giga Watt Laser array situated on the Earth’s surface to accelerate them by 100km/s/s for 10 minutes in order to achieve a velocity of 0.2 the speed of light. With 1000 light sails sent on the journey, the hope is that at least a few would reach Proxima Centauri after their 20 year journey.
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