JAXA Sail to Jupiter’s Trojan Asteroids

by Paul Gilster on March 15, 2017

I like the way Jun Matsumoto approaches his work. A researcher with JAXA (Japan Aerospace Exploration Agency), Matsumoto is deeply involved in the design of the space sail that will pick up where Japan’s IKAROS left off. Launched in 2010, the latter was a square sail 14 meters to the side that demonstrated the feasibility of maneuvering a sail on interplanetary trajectories. JAXA has talked ever since about going to Jupiter, but the challenges are formidable, not the least of which is the question of generating enough power to operate over 5 AU from the Sun.


Image: A computer rendering shows what JAXA’s solar sail may look like as it approaches an asteroid. The probe is at the sail’s center. Credit: Japan Aerospace Exploration Agency.

But back for a moment to Matsumoto, who has the kind of long-term approach to his work that this site has long championed. I ran into him in an article in the Japan Times that ran last summer (thanks to James Jason Wentworth for the pointer). Matsumoto knows he is tied up in a project that will take decades, and he relishes the notion. Let me quote from the newspaper:

“I am currently the youngest JAXA staffer on this project. But by the time it is completed, I might no longer be part of the team due to retirement,” said Matsumoto, 27, who dreamed of becoming an astronaut as a child. “Wherever I go, people always talk about how to train young people to become the researchers of the next generation.

“Personally, I want to show children that there are adults who aim to explore places no human has ever gone.”


Image: JAXA workers and others attach electric wiring to a thin film that contains solar panels on July 13 in Sagamihara, Kanagawa Prefecture. Credit: Satoko Kawasaki / Japan Times.

It’s been too long since I’ve talked about this mission. The time frames here come from the necessary travel and exploration time, and the lengthy process of designing and building the spacecraft in the first place. The new JAXA sail, which has been in the planning pipeline since before IKAROS flew, will span 50 meters to the side, 2500 square meters that will contain the 30,000 solar panels — thin film solar cells attached to the entire surface of the sail membrane — necessary to operate at the 5.2 AU distance of Jupiter’s trojan asteroids.

Like IKAROS, the sail will use liquid crystal reflectivity control devices as a means of attitude control. But the new sail will also carry a high specific impulse ion engine for maneuvering among the trojan asteroid population. Here we’re at a key issue in the mission, for operating this far from the Sun, generating electrical power becomes increasingly difficult, and the craft will also need to perform numerous trajectory changes. Just as significant as the sail itself, then, will be the operational success of the new sail’s solar panels and ion engine.

The sail is to be made up of 10-micrometer-thick polyimide, with the payload attached to the center of the sail. Current plans are for launch in the early 2020s. The Jupiter trojans are a group of asteroids that share orbits with the giant planet, clustering in its L4 and L5 Lagrangian points. There should be no shortage of candidates, for the total number of Jupiter trojans greater than 1 kilometer in size is estimated at 106. The JAXA sail will perform both flyby and rendezvous operations, with a landing on the surface of a 20-30 km asteroid, operations there and, if all goes well, a sample return to the Earth in the 2050s.

Screenshot from 2017-03-15 08-54-46

Image: A slide from a JAXA presentation by Osamu Mori et al. on the new sail, called Direct Exploration of Jupiter Trojan Asteroid using Solar Power Sail, available online.

Last summer at a gymnasium in Sagamihara, Kanagawa Prefecture, a team of JAXA workers, academics and students unfolded a full-scale model of one of the sail’s four trapezoidal components before an audience of 250. As the article in the Japan Times points out, a mission lasting decades has to look not only to current researchers but the young scientists to whom its fate will be handed off, making Matsumoto’s long-term outlook germane not just to sail missions to Jupiter space but any long-haul ventures with decadal timeframes.


{ 27 comments… read them below or add one }

djlactin March 15, 2017 at 13:06

Jupiter emits very strongly in the radio spectrum. Compared to visible photons, those photons are a lot less energetic, but they are certainly abundant. Could JAXA ‘switch’ to running on RF when near Jupiter?


Marshall Eubanks March 15, 2017 at 15:49

Jupiter puts out a fairly highly beamed radio emission, with 1 TeraWatt power being pretty routine (in the beam). Note that this is not visible light, but decameteric (meter wave radio); it comes from the Jovian auroral regions. Suppose you were at the Galilean satellites, at 1 million km (10^9 m) distance. Then (in the beam) the flux is ~ 10^12 / (4 pi * 10^18 m^2) or ~10^-7 W/m^2. A 1 km x 1 km collector (with an area of 10^6 meter^2) would collect about 0.1 Watt. This is in the Jovian system; 5 AU away at the Trojan asteroids near the Lagrange points the flux would be down by roughly a factor of 1 million from this.


djlactin March 16, 2017 at 8:09

hokay. Thanks Mr Eubanks.


djlactin March 16, 2017 at 8:10

Next question: what would energy input from Sol be at that range?


john walker March 16, 2017 at 10:22

Solar irradiance at Earth(orbit) is about 1360 Watts/m2. At Jupiter only 50.


Marshall Eubanks March 16, 2017 at 12:22

Note that at/near the Galilean satellites (well, at least Europa and Io) there are a lot of charged particles flying around, and a strong magnetic field moving by, and so an electrodynamic thruster or power generator is a very real possibility. This won’t work out in the Trojan asteroids, but you can definitely collect power from Jupiter at close range. See the following for an entire mission based upon this:



Alex Tolley March 15, 2017 at 13:39

I think back to Drexler’s argument for a solar sail that could deliver 1 ton to an orbit of Jupiter and return. JAXA’s IKAROS was an innovative design and successful.

Now JAXA have leaped ahead with a much larger 2500m^2 sail (>10x larger in area than IKAROS) that will travel out as far as Jupiter, marrying it with a high Isp ion engine for maneuvering, and carrying a 100Kg lander (Philae sized) that could allow a sample return mission. The sail also produces power for the ion engine.
The mission would reach Jupiter in 4 years, but will last decades as it reaches target asteroids and finally returns to Earth.

Bold thinking and I hope we see it launched. Rest of the world: time to catch up.


George King March 15, 2017 at 16:07

As depicted, that’s a pretty sailcraft.


Alex Tolley March 15, 2017 at 16:10

The solar power will not work with RF energy. Now if the “sail” had dipoles, it could draw power from such RF (I think).


Neil S March 15, 2017 at 16:18

That is a Very impressive mission. Kudos to JAXA! I presume Jupiter’s trojans are well out of the high-radioactivity-danger areas around Jupiter, which is good!

It will surely be a test of reliability of components and perhaps of AI navigation since, according to Siri, it takes 35 to 51 minutes for light to get from Jupiter to earth. Double that and, unless you have a really complete scan of the nearby bodies, you need some brains on the spot to be dodging baseballs or basketballs or wrecking balls. Or islands.


Marshall Eubanks March 16, 2017 at 12:28

At the Trojans, you are on average 5 AU from Jupiter. That means that Jupiter will be no brighter (or larger) from the Trojans that the would be from Earth. Radiation from Jupiter would not be a worry, but it would show a nice crescent in a telescope.


Hop David March 15, 2017 at 16:33

A number of reasons I’m enthusiastic about this mission. Thin film solar arrays have the potential for great alpha, the ration between power source mass and power delivered. Good alpha makes ion drives even more able. A low mass but powerful power source also makes asteroid isru more plausible.

I’m also very curious about the bodies residing at the Sun Jupiter L4 and L5 regions. Science fiction writer William Barton once described these places as the solar system’s Sargasso sea where all sorts of curious flotsam and jetsam have accumulated. I believe he’s correct that a variety of curious things have found their way to these locations. There are likely many volatile rich bodies containing water, ammonia, carbon dioxide and other goodies in these places.


Dmitry Novoseltsev March 16, 2017 at 2:49

I wonder if JAXA is planning a mission of such long-lived probes to one of the poles of the Sun? This could be the beginning of the preparatory work for the construction of further Shkadov thraster. For a start, to scout out the situation and working conditions of the solar sail.


Antonio March 16, 2017 at 4:43

WTF? 30 years for a sample return trip? For God’s sake, put a good nuclear rocket on a ship and you can have several trips before you retire!


Neil S March 16, 2017 at 5:56

I had no idea the Jupiter Trojans occupied such a large volume around the L4 and L5 points. https://en.wikipedia.org/wiki/Jupiter_trojan
This reduces my concerns about collisions with larger objects but there seems to be very little known about the numerical density of small objects in those clouds. Maybe an issue of concern will be tolerance to the sail being repeatedly punctured by baseballs and basketballs.

I note NASA has a mission, Lucy, to start heading out there in 2021, arriving in 2027. That’s assuming no strong perturbations in funding, an iffy assumption these days. https://en.wikipedia.org/wiki/Lucy_(spacecraft)


Michael March 16, 2017 at 14:21
Neil S March 16, 2017 at 19:43

Cool. So those near the inner edge are constantly interacting with asteroids in the asteroid belt, if usually only very mildly.


Marshall Eubanks March 16, 2017 at 23:44

The Hildas (the asteroids in the roughly triangular orbits in those movies, which are in a rotating reference frame) must interact with the Trojans to some degree when they come out to near the L4 or L5 Lagrange points during their aphelion, as they then share the same region of space. As an example, 153 Hilda will come to within 0.059 AU (8.8 million km) of 5254 Ulysses on 2058 Jul 25. That is close, but not close enough for these bodies to perturb each other much gravitationally, especially as their relative velocity is 7.7 km/sec (relative velocities will always around this level for a Hilda-Trojan close approach). I suppose that there will be collisions from time to time, but there is a lot of volume out at 5 AU, and the existence of these bodies shows us that the collision probability, even over geologic time, cannot be too high.


J. Jason Wentworth March 16, 2017 at 6:17

Thank you for the acknowledgement for pointing out the article, Paul! What JAXA may lack in cowboy-like brash boldness (of the kind that NASA had in its early years), they make up for in quiet determination. A three-decades-long Jovian Trojan asteroids rendezvous, landing, and sample return mission, sent to a region of the solar system far more distant than any previously visited by JAXA or ISAS (one of JAXA’s two predecessor space agencies [NASDA was the other one]; ISAS flew Japan’s lunar, planetary, and cometary missions) space probes, is an example of a patient, tranquil brashness that gradually grinds away the obstacles in its path, just as a stream eventually wears down solid rock in its path.


J. Jason Wentworth March 16, 2017 at 8:41

Alex Tolley: I hope this JAXA mission lights a fire under NASA! (The NEA Scout, a CubeSat-based small solar sail asteroid spacecraft that will fly as a “hitch-hiker” payload on the SLS test flight next year, is a small but promising start.) The solar sail is a technology that has been allowed to languish for far too long, and it’s a shame that its many advantages remain almost completely un-realized. (Perhaps, like the space elevator and the various space tether concepts, the solar sail is just so counter-intuitive and different from the “correct” and accepted space travel vehicle—the rocket-propelled spacecraft—that engineers and space agencies’ officials have been reluctant to risk their reputations on such a “strange” device.) But if large-scale space travel, settlements on other worlds, and asteroid mining are ever to come to fruition, getting away from the chemical rocket—to the greatest extent that we can—is precisely what we must do. Also:

I’m inclined to agree with you about using dipole antennas in/on the sail to utilize Jupiter’s RF emissions for power. In the early days of radio, people trickle-charged car batteries using the long outdoor wire antennas that crystal set receivers required. These antennas, which were usually “worked against” an RF ground (which was often also a DC ground, such as a metal cold water pipe [although sometimes—especially in areas with poor soil conductivity—a dipole antenna was used]), were connected to the battery through a crystal, which functioned as a diode. The small pulses of DC current from the antenna/ground system (which were rectified by the crystal, from the [originally] AC current that passed through the antenna/ground system) slowly re-charged any battery that was connected to the antenna/ground terminals. In addition:

One a clear night many years ago, I listened to a Jovian thunderstorm—on a pocket AM transistor radio! I had to keep physically turning the radio in order to maximize the audio volume, by keeping the signal source—Jupiter, as it moved across the sky—“broadside” to the radio’s internal, ferrite bar “loopstick” antenna (where such antennas have their greatest reception sensitivity). Being able to clearly hear Jupiter’s thunderstorm static from (at least) nearly half a billion miles away (Jupiter may have been farther away than that at the time; I don’t remember the exact year or date when this happened), which Arthur C. Clarke had mentioned in his book “The Promise of Space,” deeply impressed me regarding the immense natural electrical power which that planet possesses!

George King: Agreed—JAXA’s new sailcraft is like a geometric tapestry! (They ^really^ need to call it “*Something* Maru” [with “*Something*” being replaced by a name that’s relevant to Jupiter, and/or to its Trojan asteroids]. It could even be a transliteration into Japanese from Greek; for example, Japan’s second X-ray astronomy satellite, Astro B, was named “Tenma”—which is Japanese for “Pegasus”—after it was successfully launched.)

Neil S: Yes, JAXA’s probe will be well clear of Jupiter’s radiation belts. In fact, it will be as far from Jupiter (at the selected Trojan Lagrangian point in Jupiter’s orbit) as Jupiter is from the Sun, since the Sun-Jupiter-L4 (or L5) points form an equilateral triangle. From the L4 and L5 points in its orbit, Jupiter has the same angular size as it does when viewed from the Sun (if one could stand on the Sun’s surface and survive the experience, that is :-) ), although Jupiter doesn’t appear as bright from the L4 and L5 points because it isn’t in the full phase when viewed from them.

Hop David: The “Sargasso Sea of the solar system” is a very apt description of Jupiter’s L4 and L5 regions! They are very far away, very large (in terms of volume), and due to Jupiter’s powerful gravity, they could—and likely do—contain enormous quantities of everything from fine dust to substantial asteroids and comet nuclei, most of which we simply cannot see from Earth because of the distance and the dim sunlight way out yonder. Also, while I wouldn’t bet on it, such regions would be places where it would not be surprising to find Bracewell interstellar probes (whether active, dormant, or dead), and/or resource extraction sites (mines) that the members of ancient, alien interstellar expeditions used. For that matter (although I most certainly would *not* bet on this), even ^living^ aliens could be residing in Jupiter’s L4 or L5 regions right now (and they could have done so for millennia, with no worries about running short of raw materials), observing the Earth remotely, and we would never know it. (While our solar system is a tiny, closely-packed grouping in comparison with interstellar distances and the sizes of multiple-star systems [such as Alpha Centauri A and B and Proxima Centauri], it is easy to forget how huge and voluminous it is to us.) Also:

Thin-film solar cells are one of those humble, unexciting (until one ponders its abilities and *their* implications) technologies that make all sorts of exciting and far-ranging things possible. Ditto for electric thrusters of all kinds (ion drives, Hall Effect thrusters, colloid thrusters, etc.), and combining their capabilities with those of thin-film solar cells isn’t merely additive, but multiplicative! Add still another, as-yet-unexploited capability (utilizing solar sails as large antennas, by means of the Fresnel zone concept) to the mix, and a whole new level of spacecraft performance—at low cost and decreased complexity—begins to emerge.


Hop David March 16, 2017 at 12:43

J. Jason Wentworth, while it’s fun to imagine stumbling on ancient alien artificats and settlements, that’s not the chief reason I’m fascinated with the Trojans.

The myriad mall bodies afford much more surface area than rocky planets or large moons. Not only that, but their entire volume is accessible. In contrast we can only exploit the thin outer shell of rocky planets and big moons. Pressure and heat bar us from burrowing more than a few kilometers.

Some believe the trailing and leading Trojans are a population of small bodies rivaling the Main Belt. They are potentially a huge body of real estate and resources. In particular they likely have lots of water as well as carbon and nitrogen compounds. Being half again as far from the sun as most Main Belt asteroids, they get half the insolation. So they’re colder than the main belt bodies.

There already exists a natural cycler system between the Main Belt and the Sun Jupiter Trojans. They’re called the Hildas. See http://hopsblog-hop.blogspot.com/2016/07/hildas-as-cyclers.html


William R. Alschuler March 16, 2017 at 14:45

I have a question that maybe can be answered by linking me to one of Matsumoto’s articles:
A perfect reflector imparts twice the momentum transfer per photon that a perfect black absorber does. So there seems to be a trade off between sail reflectivity, photon momentum gain, solar cell efficiency and ion engine efficiency and power or specific impulse. Each percent sacrifice in reflectivity sacrifices momentum transfer, and thus the gain from the solar electric ion drive should be worth it. But the cells are fairly black and the efficiency of thin film solar cells is only about 10-14% for commercial cells and 22% is the lab record. Does this trade off work? And the loss of momentum transfer means the sail area density is further constrained.


J. Jason Wentworth March 17, 2017 at 8:31

Hop David: I just mentioned the “alien possibilities” to illustrate how much sheer space and matter (all of which is easy to access once one is out there, as you pointed out regarding the resident objects’ small sizes) that Jupiter’s Trojan regions contain. It would likely be able to support billions of human beings living in locally-made (from local materials) O’Neill-type space colonies, and Earth-intensity sunlight could be provided in them via solar mirrors. (Astonishingly, O’Neill calculated that even out to *3 light-days from the Sun* [which is “Way, way out,” to quote the title of that Jerry Lewis movie! :-) ], Earth-intensity sunlight could be provided for such colonies by means of mirrors of reasonable size and mass–so settling Jupiter’s Trojan regions would be fairly easy by comparison.)


ljk March 17, 2017 at 13:55


Asteroid clay is a better space radiation shield than aluminium

By Abigail Beall

14 March 2017

The huge rocks that hurtle through space may prove to be lifesavers for astronauts. Clays extracted from asteroids could be used on deep space missions to shield against celestial radiation.

Radiation from cosmic rays is one of the biggest health risks astronauts will face on long space missions, such as a proposed trip to Mars or settlement on the moon. A 2013 study suggested that a return trip to Mars would expose astronauts to a lifetime’s dose in one go.

But the heavy aluminium shields currently used for short missions would be too expensive to ship. For a long-term presence on the moon or Mars, we will need to use materials found in space, says Daniel Britt at the University of Central Florida.

“Eventually everything should be able to be produced off Earth if any serious size outpost, base or colony is to be considered,” says Paul van Susante at Michigan Technological University.

Asteroids could provide the answer, says Britt. Clays in asteroids are rich in hydrogen, which is the most effective shielding material for protons and cosmic rays. Britt and his colleague Leos Pohl found that the clays are up to 10 per cent more effective than aluminium – which is used in most current shields – at stopping the high-energy charged particles given off by the sun and other cosmic bodies.

Exactly how the clays could be extracted from the asteroids is still up for discussion. “No current machines exist for actual mining in zero gravity,” says van Susante.

But there are a few ways it could be done. For example, the clays are non-magnetic, so they could be separated from other materials in an asteroid using massive magnets.

“Doing anything in space is not trivial, but there are several paths forward,” Britt says.

Journal reference: Advances in Space Research, DOI: 10.1016/j.asr.2016.12.028


ljk March 21, 2017 at 11:58

I really don’t have anything against ARM, except that I was concerned it would not go anywhere or do what it was supposed to do, which is teach humanity how to utilize the planetoids as the key to permanent space settlement.



ljk May 5, 2017 at 11:47

The Planetary Society Blog

Jason Davis

May 4, 2017

Old documents shine new light on NASA’s plan to send a solar sail to Halley’s Comet

In 1976, when Carl Berglund was almost 50 years old, a plan to send a spacecraft to Halley’s Comet landed on his desk.

Berglund, an engineer at NASA’s Jet Propulsion Laboratory, was used to seeing bold ideas, but this one was particularly ambitious. A spacecraft equipped with a square sheet of Mylar nearly a kilometer wide would harness the pressure of sunlight for propulsion, spiral closer to the Sun than Mercury, throw itself out of the plane of our solar system, and rendezvous with the world’s most famous comet, which was returning to Earth’s skies in 1986.

Berglund’s formal project title was lead designer, but to hear him tell it, he was simply a “cog engineer.” At JPL, he looked at preliminary spacecraft designs and helped figure out how specific components would work. He doesn’t remember having a specific reaction to what he soon learned was a “solar sail”—he just went to work on the project like anything else.

“A lot of work, you know, we just did it because it came along,” he told me last year.

Around the time Berglund joined the project, Carl Sagan appeared on the Tonight Show with a model of the kite-like spacecraft—one of two designs being considered. The program manager was the scientist-engineer Louis Friedman, and JPL director Bruce Murray supported the effort. Together, Sagan, Friedman and Murray would found The Planetary Society in 1980.

Berglund only worked on the solar sail for a few months. But during that time, he amassed a treasure trove of meeting notes, schematics and overhead slides—all of which he has saved to this day.

The documents provide new insights into what would have been the world’s first solar sail, which laid the groundwork for The Planetary Society’s Cosmos 1, LightSail 1 and LightSail 2 spacecraft.

Full article with links to all those great documents here:


A crime that the US did not send its own space probe to Comet Halley in 1986. The USSR, ESA, and Japan did not miss the rare opportunity, at least. Then again the 1980s were pretty paltry when it came to US planetary probes, at least ones that were actually built and launched in that decade. The Reagan Administration even seriously considered shutting off Voyager 2 before it flew past Uranus and Neptune in order to save some money. The rational, intelligent mind boggles at such ideas.


J. Jason Wentworth May 5, 2017 at 15:33

I wonder what happened to that original, Halley mission solar sail model that Carl Sagan demonstrated to Johnny Carson on “The Tonight Show?” (The wall-mounted one that Bill Nye showed in that same YouTube video–which also includes a clip from that 1976 “Tonight Show” segment–is, I think, a model of The Planetary Society’s LightSail vehicle.) Also:

Carson’s joke (in response to Sagan’s remark that the sail should have some emblem on it) about putting a McDonald’s advertisement on it (“20 billion served”) suggests a way to fund solar sail missions. Just as some rockets–including at least one of the Soviet Phobos missions’ Proton launch vehicles–carry logos of sponsoring companies (two, in the case of Phobos) to help defray their cost, solar sails could do the same, and on a much larger scale. (Ejectable wireless cameras, like those [DCAM 1 and DCAM 2] that photographed JAXA’s IKAROS solar sail in space, could capture still images and video that the sponsoring companies could use for advertising purposes.) In addition:

Public awareness of and support for solar sail applications could be increased–and some funding for missions could be raised–by the sales of promotional items such as model solar sail suncatchers (these could use actual scrap sail material) and model solar sail kites. (A model of a square-rigged solar sail, if equipped with a tail [perhaps made of transparent polyethylene sheet plastic, for a more realistic appearance in flight] affixed to one corner of the aluminized Mylar [or maybe aluminized Kapton] sail-kite, would fly quite well, and its glittering, highly-reflective material would give it a striking appearance in flight. The other strut-braced solar sail designs, such as the triangular, hexagonal, and “butterfly” types [these are covered in Jerome Wright’s 1992 book “Space Sailing”], would also make good kites.)


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