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1999 KW4: Close-Up of a Double Asteroid

I’ve argued in these pages that the interstellar effort will be driven as much by planetary protection as by the human exploratory impulse. I count the latter as crucial, but we often think of planetary protection as an immediate response to a specific problem. Let’s place it, though, in context. Now that we’re actively cataloging asteroids that come near the Earth, we have to know how and when to react if what looks like a dangerous trajectory turns into a deadly one. That mandates a continued level of observation and progress on mitigation technologies.

A small nudge counts for a lot with an object that’s a long way out, and we can’t exclude, for example, long period comets in our thinking about planetary protection. So mitigation strategies that begin with changing the trajectory of a small, nearby object will grow with our capabilities to encompass more distant options, and that incentivizes the building of a defensive infrastructure that can operate deep into the Solar System. We need deep space technologies that can not only form a warning network but a defensive screen in case a threat develops.

We’ll build out a Solar System infrastructure one day that can do these things, one with inevitable consequences for the technologies that also foster exploration far beyond Sol. So when I see the recent images of the asteroid 1999 KW4, I’m also reminded of another binary asteroid called Didymos. With its companion (‘Didymoon’), Didymos is to be the subject of a NASA asteroid mitigation experiment, in which the DART spacecraft will impact the small moon in 2022 to gauge orbital changes that can be induced around the larger object. The European Space Agency will follow up with data from the asteroid in 2026 in a mission called Hera.

As to DART itself, the acronym stands for Double Asteroid Redirection Test. We’re talking about an impactor demonstrating the kinetic effects on the tiny object to see just how effective a deflection strategy may be. Didymos is not an Earth-crosser but it does provide a useful venue for the experiment. It’s worth remembering that a variety of options exist for asteroid mitigation, but DART will offer the first kinetic impact test at a scale realistic for planetary defense.

Image: Schematic of the DART mission shows the impact on the moonlet of asteroid (65803) Didymos. Post-impact observations from Earth-based optical telescopes and planetary radar would, in turn, measure the change in the moonlet’s orbit about the parent body. Credit: NASA/Johns Hopkins Applied Physics Lab.

But back to 1999 KW4. which was imaged by the SPHERE instrument (Spectro-Polarimetric High-Contrast Exoplanet Research) on the European Southern Observatory’s Very Large Telescope. SPHERE was designed to be an exoplanet hunter using adaptive optics to screen out atmospheric distortion, but it proved its mettle much closer to home by providing sharp images of an asteroid just 1.3 kilometers wide that flew by the Earth in late May, offering the opportunity for an observing campaign coordinated by the International Asteroid Warning Network (IAWN).

“These data, combined with all those that are obtained on other telescopes through the IAWN campaign, will be essential for evaluating effective deflection strategies in the event that an asteroid was found to be on a collision course with Earth,” said ESO astronomer Olivier Hainaut. “In the worst possible case, this knowledge is also essential to predict how an asteroid could interact with the atmosphere and Earth’s surface, allowing us to mitigate damage in the event of a collision.”

Image: The unique capabilities of the SPHERE instrument on ESO’s Very Large Telescope have enabled it to obtain the sharpest images of a double asteroid as it flew by Earth on 25 May. While this double asteroid was not itself a threatening object, scientists used the opportunity to rehearse the response to a hazardous Near-Earth Object (NEO), proving that ESO’s front-line technology could be critical in planetary defence. The left-hand image shows SPHERE observations of Asteroid 1999 KW4. The angular resolution in this image is equivalent to picking out a single building in New York — from Paris. An artist’s impression of the asteroid pair is shown on the right. Credit: ESO.

The two components of 1999 KW4, separated by about 2.6 kilometers, were moving at approximately 19.5 kilometers per second as they flew past the Earth, a challenge for observers not only because of their faintness and fast motion, but because atmospheric conditions were unstable at the time and the SPHERE adaptive optics system crashed more than once, though operations were quickly restored. The asteroid and tiny satellite reached a minimum distance of 5.2 million kilometers from Earth on May 25, about 14 times the distance to the Moon.


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  • Alex Tolley June 6, 2019, 14:25

    (…)the interstellar effort will be driven as much by planetary protection as by the human exploratory impulse.

    I hadn’t really thought about this, but you are probably correct. Planetary defense is going to be ultimately, a DoD project, which means a lot of spending. DSTAR-type lasers, space based rocket launched impactors of various types, deep-space sensors and vehicles equipped to do gentle orbit perturbations using various approaches. Very expensive, but also necessary, much like global heating cost estimates of $tn’s per nation, but without spending, a potential collapse of civilization from any of the 4 horsemen of the apocalypse.

    In comparison, exploration, and commercial interests like asteroid mining, lunar water extraction, will be small beer, at least in the nearer term this century.

    • Bruce D. Mayfield June 7, 2019, 10:16

      “(…)the interstellar effort will be driven as much by planetary protection as by the human exploratory impulse.”

      In addition to the threat from impactors, there’s the admittedly very long term but inevitably even more deadly threat from the Sun which will slowly grow hotter as it evolves thru its MS lifetime. Without some means of mitigation, this WILL cause the Earth to be rendered uninhabitable by anything in ~ 1 billion years. With mitigation the Earth could be kept in the Sun’s HZ sweet spot for 4-5 billion years.

      • Alex Tolley June 7, 2019, 12:11

        Even with a Stapledonian POV, that is a long way off. Humans, if there are any, will long since have evolved into new species. Metazoan life on Earth is not even 1 bn years old, with the Cambrian explosion just about 1/2 that time. If star flight is possible, I suspect we will have long since colonized other stars with terrestrial life long before then. If star flight is not possible, just as with Stapledon’s Last Men, we will have colonized our system, either on other planets and moons, and/or with habitats as far out as the Oort cloud.

  • Charley June 6, 2019, 15:51

    So precisely what methods will be ‘allowed’ to permit nudging of these bodies? I assume certain techniques will be off-limits.

  • Gary Wilson June 6, 2019, 16:57

    What sized solar sail applied for what amount of distance would be required to effectively nudge Didymoon? And how much of a vector change would have to be applied to Didymoon to significantly affect the trajectory of Didymos? Have any calculations been done to see how much Didymos would have to have its trajectory changed?

    • Michael T June 6, 2019, 23:58

      Interesting question. Am I wrong, but wouldn’t the detection only affect DidyMOON’s orbit, and not the trajectory of the system as a whole (i.e., the barycentre)? As the moon orbits the primary, any deflections caused by it will be neutralised on the opposite side of the orbit.

      I can only see Didmoon’s orbital change affecting the trajectory of the whole system if the latter happened to be close to another body at the time.


  • Charley June 6, 2019, 19:04

    I’m always a little mystified by these missions; wouldn’t the people who plan these things try to work to get the most scientific information that they can get out of them?
    It seems extremely elementary to me, but I would think that one of the first things that you’d want to do is to soft land a radio transmitter on the asteroidal body, such that you could perform tracking for position/velocity measurements so that you can ascertain over time, whether or not your deflection maneuver actually succeeded – at least that’s what I would plan if I was planning the mission.

  • ljk June 7, 2019, 9:32

    I post this link largely because I want you to see the headline…


    And it looks like the image they used to represent the steroid – I mean asteroid – is actually Earth’s moon rendered very warped.

    Ah, modern journalism.

    • Alex Tolley June 7, 2019, 12:16

      The article would have been more reassuring if the author had provided some comparisons to previous asteroid impact sizes and whether this asteroid could impact or where it would break up in the atmosphere and the fragments burn up as meteors. It is the lack of context that I find annoying. [Although I must say that image is hilarious.]

      • ljk June 10, 2019, 9:05

        CNN and its kind are usually not my first choices for quality space news. The above is a good example of this.

  • Douglas Muir June 7, 2019, 14:53

    “the interstellar effort will be driven as much by planetary protection as by the human exploratory impulse. ”

    I’d sharply disagree with that.

    In the last 500 years, exactly two people have died from an asteroid/comet impact — the two reindeer herders who were too close to Tunguska.

    (Asteroids and comet. If we add in meteorite deaths, that figure is a lot higher! Dozens to hundreds — yes, really. But meteorites arrive at terminal velocity and don’t represent any kind of threat to civilization generally.)

    Yes, impacts will arrive randomly on a mass / frequency curve that we don’t well understand yet. And yes, a very big impact could be devastating. But our detection techniques are so good now that we KNOW we won’t be hit by a very big impact — here defined as a body with diamter 500m or more — in the next century. We know that to a very high degree of certainty.

    Yes, a 300m impactor could still be pretty terrible. But we’re finding all of those, too; we’ll have bagged them all within the next decade.

    Once you get below 300m you’re out of the “threat to civilization” range. A 200m impactor would be very roughly as destructive as Tsar Bomba, the ~50 megaton nuclear bomb that was the largest ever tested. Tsar Bomba exploding over, say, Yorkshire would be pretty bad for Yorkshire but end civilization it would not.

    So, no. I know a LOT of people in the space community are very emotionally engaged with the idea of asteroid impacts. But they’re not actually much of a threat.

    Doug M

    • ljk June 10, 2019, 9:04

      In all my reading about Tunguska, this is the first time I have heard that any humans were killed by the impact. Knocked off their porch chairs miles away, yes, but not outright killed.

      Where did you get this information and how did they know about this considering the time (1908) and place (remote Siberia)? Russia didn’t even get an official expedition to the region until 1928.

      I can imagine planetoids being turned into interstellar arks by those humans who do not mind a slow boat to Alpha Centauri, or cannot afford or find a faster means to reach the stars.

      • Douglas Muir June 17, 2019, 16:09

        There are multiple reports of either one, two or three Evenk going missing at or around the time of the Tunguska event. These were of course compiled years after the event. The closest thing we’ve got to evidence is a 1926 conference of several Evenk tribes that got written up as an appendix in the later Soviet report. The Evenk name three people as disappearing, but one of them had already been absent for some time and may have died of natural causes.

        The Evenk were nomadic reindeer hunters who moved around a lot. (Some still are, of course.) In winter they form up into tribes, but in summer they split up into hundreds of small groups, some as small as a single family, to spread out and exploit the brief period of lush pasturage. Tunguska of course happened in midsummer. So, not until months later did the tribe reassemble and count noses, and even then it would have been impossible to say for sure whether someone was killed in the incident, had a heart attack alone in the forest, fell down a hole, or just went off with another tribe for the winter.

        That said, the confident assertions that “nobody was killed by Tunguska” are not well based. It’s hard to prove a negative, but if you dig a little you’ll see that most of these aren’t even trying. Tunguska hit right in the middle of Evenk country. The Evenk grazed their reindeer in the forest; they spread out thinly but broadly, and the “impassable” wilderness was their home. You’d reasonably expect a few of them to be within the blast radius.

        The Soviets denigrated their testimony — formally because it was 20 years later, but Soviet officialdom often had a low opinion of the “pre-feudal” and “backwards” Siberian tribes. And AFAIK, to this day the full Soviet report, with appendices, hasn’t been translated. But yes, apparently the Evenk said something like “yup that was Alice and Bob’s grazing area, they went up there every summer, and nobody ever saw them after that.”

        Doug M.

  • ljk June 10, 2019, 9:08
  • Harry R Ray June 11, 2019, 9:52

    Because of a now DECLASSIFIED error bar, the probability of CNEOS 2014-01-08 being of solar system origin is now only one in one hundred thousand. Thus Siraj and Loeb’s case for it to be of interstellar origin is now at 99.999%. Not quite as good as the confidence level of the Higgs Boson, but good enough, in my opinion.

    • ljk June 11, 2019, 11:35
      • Harry R Ray June 11, 2019, 18:01

        No and(maybe)yes(if you add an asterisk). As for CNEOS-2014-01-08 and any of its decadal ilk(i.e. for 1 cubic meter interstellar asteroids based on Siraj’s and Loeb’s PREDICTION), with that high of an entry velocity, unless there is substantial TUNGSTEN in them, they would almost certainly vaporize completely. You would need a Chelyabinsk-event sized interstellar asteroid to have any chance of anything making it to the ground! That would most likely happen once every hundred or so centuries. However, there is one bona-fied interstellar object that DID make it to Earth! It’s called the Hypatia Stone. The only problem with it is that it almost certainly did MOT form around another star, but, instead; formed in the interstellar medium from a cloud of gas and dust that was far too small to form a star even as cool as a small brown dwarf, but still with enough self -gravity to at least partially condense and form the Hypatia Stone PROGENATOR at near absolute zero. This has yet to be proven, but it is most likely the case. But, even if it originated in another part of the galaxy from the gas and dust cloud that produced our Sun, it would tell us very little about the composition of other solar systems.