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DART & Hera: Changing an Asteroid’s Trajectory

Asteroids are objects of obvious scientific interest, not only for their intrinsic properties but also our need to understand how we can change their motion in space in case one looks like it will come dangerously close to Earth in the future. OSIRIS-REx is extracting all kinds of valuable data from asteroid 101955 Bennu, but we should also keep in mind that Bennu itself is a potentially hazardous object, with a small chance (1-in-2700, according to current estimates) of striking the Earth between 2175 and 2199. Thus the second ‘S’ in OSIRIS, which stands for ‘security’, and is all about measuring the factors that affect the object’s trajectory.

When we get samples from Bennu, we’ll have a better idea about the asteroid’s chemistry and morphology, useful for understanding the early Solar System as well as assessing how hazardous such an object is. But we need to know more, which is where NASA’s Double Asteroid Redirection Test (DART) mission comes in. Here the purpose is planetary defense from the start, for DART will demonstrate how a kinetic impactor can change the motion of an asteroid in space. The target is a binary near-Earth asteroid called (65803) Didymos.

Image: Simulated image of the Didymos system, derived from photometric lightcurve and radar data. The primary body is about 780 meters in diameter and the moonlet is approximately 160 meters in size. They are separated by just over a kilometer. The primary body rotates once every 2.26 hours while the tidally locked moonlet revolves about the primary once every 11.9 hours. Almost one sixth of the known near-Earth asteroid (NEA) population are binary or multiple-body systems. Credit: Naidu et al., AIDA Workshop, 2016.

DART will carry an imaging instrument called DRACO (Didymos Reconnaissance & Asteroid Camera for OpNav), which is based on the now familiar LORRI high-resolution imager that flew on New Horizons, and will use roll-out solar arrays (each 8.6 meters by 2.3 meters) and a NEXT-C ion engine for propulsion. The plan is simplicity itself: DART will crash into the Didymos moonlet at 6.6 kilometers per second, which should change the moonlet’s orbital speed around the main body by a fraction of one percent, and the orbital period by several minutes.

Flying with DART will be LICIA, the Light Italian CubeSat for Imaging of Asteroid, which will observe the impact ejecta in the early phase of crater formation following the impact. The dynamic changes DART’s impact produces will be measured partly by what LICIA learns about the fallback ejecta on both asteroids and the subsequent Hera observations of unweathered fresh material on the two objects. NASA describes LICIA this way:

The LICIA Cube is a 6U CubeSat provided by the Italian Space Agency. It will be carried along with DART to Didymos and released approximately 2 days before the DART impact. LICIA Cube will perform a separation maneuver to follow behind DART and return images of the impact, the ejecta plume, and the resultant crater as it flies by. It will also image the opposite hemisphere from the impact. LICIA Cube is 3-axis stabilized and has a propulsion capability of 56 m/s. The onboard imager has a 7.6 cm aperture, F/5.2 telescope, and an IFOV of 2.9 arcsec/pixel.

Image: Two different views of the DART spacecraft. The DRACO (Didymos Reconnaissance & Asteroid Camera for OpNav) imaging instrument is based on the LORRI high-resolution imager from New Horizons. The left view also shows the Radial Line Slot Array (RLSA) antenna with the ROSAs (Roll-Out Solar Arrays) rolled up. The view on the right shows a clearer view of the NEXT-C ion engine. Credit: NASA.

The European Space Agency’s Hera mission — powered by solar arrays with a hydrazine propulsion system — is to be the follow-up, making a post-impact survey using high-resolution visual, laser and radio science to map what will be the smallest asteroid yet visited by spacecraft. The DART collision is scheduled for 2022, with immediate results probably hidden by an expected dust cloud. Hera will investigate the asteroid impact crater and surrounding surface in 2026, allowing scientists to refine their numerical models of the impact process. All of this works toward building a deflection technique for planetary defense.

Earth-based observations of the Didymos system gathered during its close approach in February-May 2017 were analyzed at a workshop in Prague in 2018, allowing constraints to be placed on the strength of the YORP effect, which results from uneven heating on the surface that can alter the object’s spin state. Further observations will tighten these constraints, making the effects of the DART impact easier to separate from the pre-impact state of the system.

A key Hera role in all this will be to measure Didymos’ mass, which will help scientists calculate the efficiency of the impact momentum transfer once we’ve also measured the change in the small moon’s orbital period. Thus the two missions will result in accurate modeling of the response to the impact as well as the likely internal structure of the asteroid. Hera will be carrying two six-unit cubesats of its own to provide spectral measurements of the surface of both asteroids, with a Cubesat called APEX (Asteroid Prospection Explorer) actually landing on one of them. A second CubeSat (Juventas) will measure the gravitational field and internal structure of the small moon, doing a low-frequency radar survey of the asteroid interior.

The DART launch window opens in late July of 2021, with launch aboard a SpaceX Falcon 9, with intercept of the Didymos moonlet in late September of 2022, when the system is about 11 million kilometers from Earth. Earth-based telescopes and planetary radar will be able to measure the effects of the impact to back up the findings of the spacecraft on the scene. The results should be small but highly useful in giving us data on how impacts affect asteroids of this size, with the added benefit of enhancing international cooperation on a matter of global importance.

{ 15 comments… add one }
  • Charley January 29, 2020, 15:45

    I suppose it won’t do any good to suggest this idea that for any scheme that you cook up to perform a deflection that you will probably find that every asteroid is different from any other asteroid as one person is different from another.

    It seems to me that if you are talking about attempting to deflect any particular object that the only sure and safe way to achieve your objective is to illuminate the object with sufficient radiation pressure to achieve some degree of uniformity in your illumination such as to displace the object in question. I personally favor the use of multiple nuclear charges to do the deflection simply because you can repeat if necessary, if one is insufficient for the job.

    Now when I say nuclear charges, I’m not talking about placing a nuclear charge on the surface of the asteroid body, but rather setting it off at a distance, which will create sufficient radiation pressures to give an impulse to the asteroid. So there shouldn’t be any discussion that I’m implying that we should ‘blow it up’ as has been suggested before. Nuclear charges are compact and can be tailored to the job at hand. This allows flexibility, if you need to make a decision right there on the fly.

    • Viktor January 30, 2020, 9:14

      Where is the connection with reality in this approach? Firstly, the spatial dilution coefficient under the influence of a remote non-directional explosion will be about 1/10. This reduces the efficiency of the explosion by an order of magnitude. Secondly, the vaporized stream will escape in all directions perpendicular to the semi-surface, but not in desired direction only. This will further reduce efficiency by at least an order of magnitude. Estimates show that to deflect a sub-kilometer asteroid with a warning time around one year, it is necessary to vaporize a layer of several centimeters from its entire semi-surface, provided that all material escapes in similar direction. The mass of this layer is about several tens of thousands tons. In view of the above, it is necessary to increase the thickness of evaporated layer by almost a hundred times (i.e. up to several meters) for required deflection during “radial” escaping. But this is absolutely unrealistic to do by exposure to radiation generated by an accessible explosion. Especially considering that the energy of penetrating gamma radiation is a small fraction of the total explosion one.

      • Charley January 30, 2020, 15:48

        Viktor, I’m afraid you’re just dead wrong about this, and additionally you misconstrued some of what I have been saying. First off, the detonation of a nuclear device is not omnidirectional in its effects. And that’s not just me saying this; a gentleman named Dr. Ted Taylor, who worked at Los Alamos national laboratory in the 1950s and was a actual designer of nuclear weapons in that time period stated that it is possible to enhance certain asymmetries in the explosion of a nuclear device such to favor one direction over another.
        See the book titled ‘The Curve of Binding Energy’ (by John McPhee ) which is a story of Ted Taylor’s life.
        Thus in effect you are achieving some degree of a ‘flashlight’ affect which can illuminate the surface of the asteroid more intensely than you would normally think.

        Additionally, Taylor said that as a rule of thumb you can calculate that the amount of evaporation of the material by a nuclear explosive is directly proportional to the yield of the explosives (i.e. 10 kiloton yield would mean 10 kilotons of material vaporized, more or less).

        From here on out I would state that what I’m saying is my opinion, but as someone who is an engineer himself, I have a high degree of certainty that it is a factual analysis, even if I have nothing to do with nuclear bombs or specialization in asteroids. Certainly such intense illumination with all wavelengths of the electromagnetic spectrum (not just gamma rays) will produce intense heating on the surface of any given asteroid and produce some degree of the ‘rocket effect’ which would even minutely deflect the given asteroid, this would be true even if it’s a relatively loose pile of rocks or a solid object. And you have to remember that a deflection of even a small amount (say, centimeters) and/or velocity change of even a few centimeters/seconds we would be enough it done while still at great distances in its orbit enough to cause an earth crossing asteroid to miss us.

        • Viktor January 31, 2020, 8:13

          Thank you for your discussion, Charley. But. Firstly, the possibility of a narrowly targeted nuclear explosion has not been experimentally proven (especially in a space vacuum), so it should be considered more like science fiction. Consequently, geometric dilution, which sharply reduces the efficiency, must be taken into account when evaluating the required power. Secondly, Ted Taylor’s statements about the equivalence of vaporized material and explosion energy cannot be taken too categorically. Especially if they are contained in the popular literature. It is possible that this is achievable in terrestrial conditions, under which the evaporation of all surrounding matter occurs and when exposed to all types of radiation. However, when exposed to an asteroid isolated in vacuum, the only deeply penetrating type of radiation is gamma-rays, in which only a few percent of the explosion energy is concentrated. Other types of radiation can evaporate only a very thin surface layer, i.e. of low mass. Moreover, as I have already indicated, the escape of the evaporated material will occur on average radially from the entire semi-surface of the asteroid, which minimizes its “pushing” efficiency in the right direction. As a result, it will be practically impossible to achieve the required tens of thousands tons of material in a narrowly directed jet for real explosion powers. The arguments in favor of the unacceptability of the military method include the following. The nuclear blast method would violate international law, thereby increasing the risk of violent conflicts, and could pose a radioactive threat for hypothetical “super-bomb” during the creation, testing, storage, preparation, and launch stages. Final conclusion (see also my addition comment with links below): solar-concentrating approach now is the best. Because it surpasses all other known ones on a set of key features such as scalability (the absence of principal limitations for successful implementation even for the any type of largest and most dangerous bodies), propulsion value and controllability, energy and total cost minimization, as well as environmental safety.

          • Alex Tolley January 31, 2020, 15:02

            Irrespective of the effectiveness of a nuke, your argument about the illegality of nuclear weapons due to the Outer Space Treaty would seem teh most relevant.

            However, as we are seeing currently, there are mounting attempts to bypass the treaty for commercial reasons, and human history suggests that treaties tend to get ignored when there are reasons to ignore them. The USA has a shameful history in this regard.

            Yet should an asteroid be detected that would impact Earth, I find it hard to believe that a treaty banning nukes in space would be obeyed given the seriousness of the situation. OTOH, we certainly seem to manage to dither and do next to nothing with the existential threat of global heating, so maybe nothing would be done.

            It really is important that we determine the various options and try them out on non-impacting asteroids so that we have a suite of proven techniques to use that will work in almost all circumstances. I’m sure that the US DoD will be interested in funding Lubin’s D-STAR lasers to try to evaporate material off asteroids to destroy or deflect them. Not coincidentally, they make fearsome weapons that are not banned by treaties and could, in theory, be located in space. Breakthrough Starshot is a cheap way to start designing and testing such phased laser arrays that DoD funding might then build-out under the auspices of planetary defense.

            • Viktor February 1, 2020, 9:54

              Unfortunaly, Dr. Lubin’s method (in common with Breakthrough Starshot) are rather science fictions at best. It should be clear to the physicist or engineer, because of even a few minutes is enough to damage the emitting zone of a continuously operating powerful laser without active cooling. Actually, I don’t know why the authors of DE-STAR “forget” to mention that there are no materials in nature whose thermal conductivity is sufficient to remove heat from the core of a quasi-continuous kilowatt-class diode laser. Perhaps, that this is a consequence of publication “only for publication” or for obtaining any grants. Even the air convection, which is dozen times more efficient than radiant cooling, will not be enough. But the convection in open space is absent, therefore, such a fundamental limitation exists regardless of the size or type of the passive heat spreader and heat sink. The only way to cool of powerful (over 100 W) quasi-continuously operating lasers is to use the flowing fluid systems (water loops) and open-type refrigerators, which in open space is obviously impossible.
              Moreover, the authors of such type approaches, in addition to insurmountable problems with cooling, also “forget” to show the dependence of the laser beam cross-section from the distance to the target. Recall, minimum kilowatt-class diode laser as a separate element of the giant array has the beam’s divergence around one angular minute These are key data from which it will become clear that the laser beam will be expanded to values far exceeding the size of the target even at distances near the hundreds of thousands kilometers. So that “the nature is the nature” and laser evaporation of any asteroid, which is enough for its deflection (operational deflection of the sub-km asteroid requires from months to year action of local continuous irradiation at a power density of about several megawatts per square meter of target surface.), isn’t real…

              • Charley February 1, 2020, 16:27

                while I was reluctant to respond to your previous reply, I felt that there had to be some clarification. And the clarification, I’m seeking is on your part. First off, what is your proposal as to what you call “solar-concentrating approach” that you are advocating as an approach to asteroid deflection? Additionally, why doesn’t geometric effects which you call geometric dilution not applied to your ideas? Now I have to tell you without any uncertainty whatsoever that any deflection schemes that you wish to employ necessitates that it should be done while the object is greatly distance from the sun. Celestial mechanics dictates that deflections are much, much easier when the object is greatly distant from the sun, then when it is close. Thus this idea that you have which you call solar concentration or whatever is going to have to be employed while the object is perhaps at the orbit of Saturn, if not greater to affect a change in some kind of earth shattering asteroid or comet. That’s just certain and there’s no uncertainty associated with it.

                Now I agree with your analysis regarding lasers because lasers do require cooling and they must be usually shot at very short intervals are otherwise you will get cracking in your rods requiring replacement. I’m not sure how I see how your idea works to actually (which you never explained fully) gets to the point of achieving deflection. So you attack our ideas, but you don’t have any way in which you defend what you are proposing.

                As for international treaties and what have you, I agree with Alex that in times of crisis, those will be the first thing thrown out the window. Even in times of noncrisis we are seeing treaties being violated left and right if it serves the purpose of the violator. Take a look at the Iran nuclear deal which has been completely abrogated by the United States, even though the Iranians do not seem to have done any violations. So, let’s not be too pedantic about whether or not treaties are something that is sacred and cannot be ever breached if the occasion arises, or the emergency demands it.

                • Viktor February 2, 2020, 7:07

                  Charley, all clarifications regarding the solar concentrating concept you can find in my scientific paper: Viktor P. Vasylyev, Deflection of Hazardous Near-Earth Objects by High Concentrated Sunlight and Adequate Design of Optical Collector, published in “Earth, Moon and Planets”, Volume 110, Issue 1-2, pp 67-79, 2013; http://link.springer.com/article/10.1007/s11038-012-9410-2
                  as well as in short demo-video https://www.youtube.com/watch?v=9u7V-MVeXtM

                  • Charley February 2, 2020, 16:33

                    I have to say I am DEFINITELY not convinced that this is a workable solution. Without getting into a long and drawn out reason as to why I feel this way I can simply say that given the needed actions to induce an impulse to a wayward body (especially if it’s massive) definitely suggest that your method would have negative aspects to it, which under the circumstances probably is not a tolerable risk.

                    To return to the continual suggestion that only gamma rays act on the body in question and are the only part of the electromagnetic spectrum, which is useful I would reply that nuclear underground test absorbed the full energy from the blast and by necessity required that the rocks would be melted in vaporized by the action of the explosive. It can’t just be gamma rays alone, which decides this.

  • Thomas R Mazanec January 29, 2020, 18:52

    Will Hera be able to tell if the moonlet has returned to tidal lock after the period was changed slightly?

  • Viktor January 30, 2020, 5:47

    It is unlikely that the kinetic impact will work because of the internal structure of near-Earth asteroids is crumbly: “We think they’re very loose aggregates. They’re not solid through and through” said Melissa Morris, OSIRIS-REx deputy program scientist at NASA Headquarters in Washington, D.C. The detailed photos and probe impacting of Bennu and Ryugu reveal rubble-pile natural properties of the NEOs, which will prevent shock wave propagation and proper impulse transfer. Therefore, AIDA-DART-Hera will not be an anti-asteroid mission, but rather an “anti-budget show”, unfortunately.

  • Andrei January 30, 2020, 7:00

    Thank you for a good article of the mission, and asteroids can indeed be dangerous in different ways.
    The obvious one is of course it is solid rock. But also a loose aggregation of rocks could pose quite a threat. It would be torn apart somewhere just inside the roche limit many would think that would be time to call off the alarm then.
    Actually that can develop into an even worse scenario, such a large number of fragment could heat up the atmosphere via ram pressure to such a degree it could cause massive forest fires below. That adding to the dust from the asteroid itself.
    Now Didymos is not an object that would pose a threat major threat of that kind or to Earth as a whole, but it could wipe out a major city.

  • Viktor January 30, 2020, 9:06

    Actually, most of proposed approaches to planetary defense are neither effective nor scalable even to asteroids capable of country-wide destruction. As of now, it appears that asteroid ablation using highly concentrated sunlight is the only method that meets all of the following criteria: scalability up to global-threat sizes and any type of hazardous bodies as well as low cost and environmental friendliness. This method converts the asteroid to a “natural rocket”, providing more than enough thrust without fuel and energy concerns.

    An improved concept for such solar-based deflection using an innovative solar collector was proposed and substantiated in 2013 – see https://link.springer.com/article/10.1007%2Fs11038-012-9410-2
    and also a short demo-video

  • Harry R Ray February 11, 2020, 11:27

    Breaking news: Pallas is surprisingly round, but just not quite round enough to be designated a dwarf planet. It also is as heavily cratered as Ceres is, and, it also has a “bright spot” like Ceres dies, but in this case, the spot lies just outside a major crater instead of being inside Occator Crater, as is the case with Ceres. To view the new images, go to news.mit.edu/2020/pallas-golf-ball-asteroid-0210

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