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A Fleet of Sail-driven Asteroid Probes

One of the great values of the Kepler mission has been its ability to produce a statistical sample that we can use to analyze the distribution of planets. The population of asteroids in our own Solar System doubtless deserves the same treatment, given its importance in future asteroid mining as well as planetary protection. But when it comes to main belt asteroids, we’re able to look up close, even though the number of actual missions thus far has been small.

Thus it’s heartening to see Pekka Janhunen (Finnish Meteorological Institute), long a champion of intriguing ‘electric sail’ concepts, looking into how we might produce just such an asteroid sampling through a fleet of small spacecraft.

“Asteroids are very diverse and, to date, we’ve only seen a small number at close range. To understand them better, we need to study a large number in situ. The only way to do this affordably is by using small spacecraft,” says Janhunen.

The concept weds electric sails riding the solar wind with a fleet of 50 small spacecraft, the intent being that each should visit six or seven asteroids, collecting spectroscopic data on their composition and taking images. Dr. Janhunen presented the idea at the European Planetary Science Congress (EPSC) 2017 in Riga on Tuesday September 19.

Image: The single-tether E-sail spacecraft. Credit: Janhunen et al.

Electric sails ride the solar wind, that stream of charged particles that flows constantly out of the Sun. While solar sails take advantage of the momentum imparted by photons on the sail, and beamed energy sails are driven by microwave or laser emissions, electric sails use the solar wind’s charged particles to generate all the propulsion they need without propellant. What Janhunen envisions is a tether attached to one end of a spacecraft, to which is attached an electron emitter and a high-voltage source, all connected to a remote unit at the other end.

The tether makes a complete rotation every 50 minutes, creating a shallow cone around a center of mass close to the primary spacecraft. Each small craft can change its orientation to the solar wind, and thereby alter its thrust and direction. Janhunen’s presentation at the EPSC made the case that a 5 kg spacecraft with a 20 kilometer tether could accelerate at 1 millimeter per second squared at the Earth’s distance from the Sun. Coupled with the boost provided by the launch itself, this is enough to complete a tour through the asteroid belt and return within 3.2 years.

Image: Artist’s concept of the spacecraft. Credit: FMI.

These spacecraft are small enough (Janhunen refers to them as ‘nanosats’) that they cannot carry a large antenna. Instead, the mission concept calls for each spacecraft to make a final flyby of the Earth to download mission data. The financial numbers are compelling. Billions of Euros would be involved in attempting a flyby of 300 asteroids with conventional methods, while Janhunen’s fleet of nanosats could, he believes, fly this mission for 60 million Euros.

The payoff could be substantial:

“The nanosats could gather a great deal of information about the asteroids they encounter during their tour, including the overall size and shape, whether there are craters on the surface or dust, whether there are any moons, and whether the asteroids are primitive bodies or a rubble pile. They would also gather data on the chemical composition of surface features, such as whether the spectral signature of water is present.”

Working with ever smaller payloads is a recurring theme in deep space exploration, the ultimate example being Breakthrough Starshot’s study of a laser-beamed sail mission to Proxima Centauri’s planet, one that would deploy a fleet of sails just meters to the side, each carrying a payload as small as a microchip. In Janhunen’s concept, the spacecraft are capable of carrying a 4-centimeter telescope capable of resolving asteroid surface features, along with an infrared spectrometer that can analyze reflected light to determine the object’s mineralogy.

With flybys at a range of about 1000 kilometers, the spacecraft would be able to image features to a resolution of 100 meters or better, and with multiple targets for each, the catalog of main belt asteroids that we have seen up close would suddenly mushroom. The mission, being referred to as the Asteroid Touring Nanosat Fleet, would help us catalog the various types of asteroids and provide valuable analysis of their composition and structure, all of which would add to our expertise if and when we ever have to nudge an asteroid into a new trajectory.

200,000 Euros per asteroid is a strikingly efficient use of resources, and the engineering involved in deploying Janhunen’s fleet of electric sails would give us priceless experience as we work with other mission concepts that involve the solar wind. But can we ride a wind as mutable and unpredictable as this one while ensuring the kind of pinpoint navigation we need? Questions like these will need answering in space through the necessary precursors.


{ 16 comments… add one }
  • Gregory Benford September 20, 2017, 12:22

    “could accelerate at 1 millimeter per second”
    do you mean could accelerate at 1 millimeter per second squared?

  • Paul Gilster September 20, 2017, 14:44

    Yes — thanks for catching that, Greg. I’ve updated the post.

  • David Herne September 20, 2017, 20:35

    I suppose the €60M would cover a single launch cost too, if each of 50 satellites costs €200,000. It is great to see such ideas being promulgated but at that cost, the concept will surely be investigated further.

  • Alex Tolley September 20, 2017, 22:54

    I love the general idea, although I have to wonder if this technology is the best to use. I am rather skeptical that this mission can be done for 60MM Euros. If it can be, then awesome, but I would like to see an independent assessment of the price.

    Can a 5 kg craft with a 20 km tether really be described as a nanosat?

  • Dmitry Novoseltsev September 21, 2017, 0:43

    This design is a bit reminiscent of the design of the probe to collect near-lunar dust, which I once proposed (https://jour.space/issues/issue-3-2013/). Probably, for the collector of dust also would be appropriate to give an electric charge for greater efficiency. And for the probe to asteroids just precession is undesirable, it would be better to make it symmetrical with two electrodes on the sides. And instead of a round cross section strings to use wide tape, they are more resistant to particulate and dust erosion.

  • J. Jason Wentworth September 21, 2017, 9:19

    We happen to have some nearby targets with which Dr. Janhunen’s E-sail spacecraft could be tested. At any given time, the Earth has at least one, and not uncommonly more, temporarily-captured natural (asteroidal) satellites (see: http://www.google.com/search?source=hp&q=temporary+natural+Earth+satellites&oq=temporary+natural+Earth+satellites&gs_l=psy-ab.12…1490.12364.0.14978.….0…1.1.64.psy-ab..0.33.3548…0j0i131k1j0i22i30k1j33i160k1j33i21k1j33i22i29i30k1.WS5Ej449v4M<http://www.google.com/search?source=hp&q=temporary+natural+Earth+satellites&oq=temporary+natural+Earth+satellites&gs_l=psy-ab.12…1490.12364.0.14978.….0…1.1.64.psy-ab..0.33.3548…0j0i131k1j0i22i30k1j33i160k1j33i21k1j33i22i29i30k1.WS5Ej449v4M ), and:

    These moonlets, which are up to a few meters across, spend most of their time in orbit around the Earth at distances of about 1 to 10 times the distance of the Moon. *This* article (see: http://www.ifa.hawaii.edu/users/jedicke/bierhaus/Granvik-submitted.EarthsTemporarilyCapturedNaturalSatellites-FirstStepOnTheLadderToAsteroidResources.pdf ) examines the challenges of sending missions to these nearby objects, and E-sail spacecraft–which could be prototypes of the Main Belt tour probes–might be the solution to reaching Earth's temporary moonlets cheaply (the same spacecraft could even be sent on to the Main Belt if desired).

  • Michael Fidler September 21, 2017, 10:00

    These spacecraft should also be able to do extended missions after the first Earth flyby, with 50 of them, if built right and with unlimited fuel!

    This is just the beginning of using our solar system resources, in the long run the asteroids are the easiest material to develop the solar system for human use. The problem with the large moons and planets is the expensive gravity well.

    This brought to mind that we should be looking at other exosolar- systems to see if the materials from asteroids in their systems has been converted into objects that would be emitting higher levels of infrared radiation. So what would an advanced civilization do with all this dangerous material floating around and possible colliding with their home planet??? Would what they made with these asteroids even be detectable and would there be certain areas that we need to look for them? Just imagine a 1000 years from now what we may be doing with our asteroids.

  • Dmitry Novoseltsev September 21, 2017, 23:56

    The scheme shown in the figure is not very good. Himself Pekka Janhunen (at least in the description of a Russian patent for E-sail http://www.freepatent.ru/images/patents/1/2451629/patent-2451629.pdf), noted that “thin solid wire 100 m long existed would be in space for only a few months before it would be cut by micrometeoroid”. That is why the original E-sail consists of a set of strings. And in this scheme, stabilizated by rotation of the two blocks around a common center of mass, any damage to the strings will cause the blocks will scatter by centrifugal force in an unpredictable manner. For the fleet to the asteroid probes had some chances to reach the goal, the blocks need to connect the wide ribbon of woven strings, like the volleyball net.

  • Alex Tolley September 22, 2017, 9:00

    It is a similar problem to that identified for space elevators. The solution was to use a curved tape of strands – a Hoyt tether. Whether this increases the mass significantly is something the author would need to consider.

  • ljk September 26, 2017, 9:55

    Ghost in the machine

    A common theme in space missions is that spacecraft are able to do so much with so little computing power on board. Dwayne Day reflects on what happens when the computing power, and intelligence, of those missions shifts from the ground to future, more capable spacecraft.


    To quote:

    Recently that mythological meme appeared again on the PBS documentary “The Farthest: Voyager in Space.” A scientist pointed out that the Voyager spacecraft, launched in 1977, had less computing power than the key fob you use to unlock your car doors. Voyager program manager John Casani scoffed at the idea. “What’s wrong with ’70s technology? I mean, you look at me, I’m a ’30s technology, right? I don’t apologize for limitations that we were working with at the time. We milked the technology for what we could get from it.”

    • J. Jason Wentworth September 26, 2017, 15:47

      I agree–simple equipment, as long as it does the job, is better (and usually cheaper and more reliable). Some years ago I read an article about NASA’s Kuiper Airborne Observatory, which mentioned that the computer that aimed the telescope and conditioned the detectors’ data was…a Commodore C-64! One of the scientists, queried by the author regarding why they were using a Lionel Playworld “toy” computer to operate a multi-million-dollar telescope, answered simply, “It works.”

      • ljk September 28, 2017, 9:16

        Early Soviet satellites used vacuum tubes. Apparently they are very well adapted to operating in space. I don’t care if twigs and cardboard get us to other worlds, so long as they work.

        Hey, the Ranger 3, 4, and 5 lunar probes used balsa wood to protect a seismometer they hoped to drop on the Moon and operate for a month. Sadly none of them ever worked properly so we don’t know if the plan would have succeeded.


        • J. Jason Wentworth September 30, 2017, 14:06

          Ranger 3, 4, and 5 even reused much of Pioneer 4; the NASA book “Lunar Impact” mentioned that those Rangers’ conical low-gain antennas were the same cone/rod off-center-fed dipole that formed much of the Pioneer 4 lunar flyby probe’s structure, and:

          In addition to balsa, plywood and oak have also been used for space applications. The Polaris A-1 and A-2 re-entry vehicles used plywood as the ablative material. Chinese IRBM and ICBM re-entry vehicles–and Chinese satellite re-entry vehicles–have used (and may still use) oak ablators. If such wooden impact absorbers and ablative materials are usable on space probes (and I’m sure they are–the Viking landers’ aeroshell heat shields even used cork as the ablator), they should be employed, especially when they’re cheaper than synthetic materials.

  • ljk September 28, 2017, 9:19

    NASA’s Near-Earth asteroid CubeSat enjoys test sail

    By Brooks Hays | Sept. 27, 2017 at 7:53 AM

    Sept. 27 (UPI) — NASA’s Near-Earth Asteroid Scout, a CubeSat designed to study asteroids orbiting near Earth, executed a full solar sail deployment during recent tests in Alabama.

    Earlier this month, NEA Scout underwent environmental testing in preparation for its eventual flight on the inaugural Space Launch System mission, currently scheduled for December 2019. Scout is one of six CubeSats NASA selected for the mission.

    Full article here:


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