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New Insights into Long-Period Comets

The Voyagers’ continuing interstellar mission reminds us of how little we know about space just outside our own Solar System. We need to learn a great deal more about the interstellar medium before we venture to send fast spacecraft to other stars. And indeed, part of Breakthrough Starshot’s feasibility check re small payloads and sails will be to assess the medium and determine what losses are acceptable for a fleet of such vehicles.

The definitive work on the matter is Bruce Draine’s Physics of the Interstellar and Intergalactic Medium, and thus it’s no surprise that Draine has been involved as a consultant with Starshot. As we saw yesterday, we have only one spacecraft returning data from outside the heliosphere (soon to be joined by Voyager 2), making further precursor missions explicitly designed to study ‘local’ gas and dust conditions a necessity.

Another reminder of the gaps in our knowledge comes from an analysis of WISE data. The Wide-field Infrared Survey Explorer satellite has given us a look at objects perturbed in some fashion within the Oort Cloud and now making occasional forays into nearby space. A distribution of comets and other icy bodies beginning some 300 billion kilometers from the Sun and extending outwards perhaps as far as 200,000 AU, the Oort Cloud’s extent makes it possible that it may extend into similar clouds of icy material around the Alpha Centauri stars.

Image: The fact that this image is logarithmic gives a startlingly clear idea of the extent of the Oort Cloud. The scale bar is in astronomical units, with each set distance beyond 1 AU representing 10 times the previous distance. One AU is the distance from the sun to the Earth, which is about 150 million kilometers. At the outer edge of the Oort Cloud, the gravity of other stars begins to dominate that of the sun. The inner edge of the main part of the Oort Cloud could be as close as 1,000 AU. Voyager 1, our most distant spacecraft, is around 125 AU. It will take about 300 years for Voyager 1 to reach the inner edge of the Oort Cloud and possibly about 30,000 years to fly beyond it. Credit: NASA / JPL-Caltech.

There turns out to be more of such material than we had thought. The outer Oort Cloud is only loosely bound, meaning that gravitational interactions with passing stars or ‘rogue’ planets, not to mention effects from the Milky Way itself, can dislodge comets from their orbits and bring them into the inner system. Such comets may have periods not just in the hundreds but millions of years. The WISE data were gathered during the spacecraft’s primary mission, before its recommissioning as NEOWISE, with the charter of studying near-Earth objects.

Measuring the size of long-period comets is difficult because the cloud of gas and dust around the comet — its coma — makes it difficult to measure the actual cometary nucleus. WISE was able to get around this problem by probing comets in the infrared, subtracting the glow of the coma from the signature of the nucleus. 2010 WISE observations of 95 Jupiter family comets — with periods of 20 years or less — and 56 long-period comets were used in the study.

The result: Comets that move regularly into the inner system are found to be, on average, as much as four times smaller than long-period comets, those moving only rarely near the Sun. Moreover, there are seven times more long-period comets in the size range of one kilometer in diameter and above than had previously been thought. In the eight months of the study period, three to five times more long-period comets were observed moving in the vicinity of the Sun than had been predicted.

“The number of comets speaks to the amount of material left over from the solar system’s formation,” said James Bauer, lead author of the study and now a research professor at the University of Maryland, College Park. “We now know that there are more relatively large chunks of ancient material coming from the Oort Cloud than we thought.”

Image: This illustration shows how scientists used data from NASA’s WISE spacecraft to determine the nucleus sizes of comets. They subtracted a model of how dust and gas behave in comets in order to obtain the core size. Credit: NASA/JPL-Caltech.

The results presumably reflect the fact that, coming closer to the Sun on a much more frequent basis, Jupiter-class short-period comets lose volatiles through sublimation, along with surface materials. An observed clustering in the orbits of long-period comets also suggests that many of these could have been part of larger bodies at some point in the past. The findings may have a bearing on our estimates of water delivery to the early Earth.

Co-author Amy Mainzer (JPL), principal investigator of the NEOWISE mission, points out that, traveling much faster than asteroids, long-period comets like these, many of them quite large, have to be factored into our analyses of impact risk. We’re developing an extensive catalog of near-Earth objects, but a long-period comet dislodged from the Oort Cloud, moving faster than any near-Earth asteroid, poses a risk that is badly in need of assessment.

The paper is Bauer et al., “Debiasing the NEOWISE Cryogenic Mission Comet Populations,” Astronomical Journal Volume 154, Number 2 (14 July 2017) (abstract). This NASA news release is also helpful.


{ 12 comments… add one }
  • Neil S August 3, 2017, 21:32

    The risk of one of these hitting earth is surely quite small but the risk that if it happened it could end human life on earth is pretty high since they are moving fast, tend to be big and we won’t see them orbits ahead. So that’s definitely an argument for getting us established and self sufficient off earth. Global warming is, of course, orders of magnitude more risky to human existence.

  • Michael Fidler August 4, 2017, 2:48

    Especially if it comes in from the far side of the Sun – lost in the solar glare and matching the movement of the Sun. This could be very deadly since the time necessary to complete any mission to deflect the comet could be as low as a few weeks!!!

  • James Stilwell August 4, 2017, 8:23

    Elon Musk is working as fast as anybody can to get to the planets…one little baby step at a time…meanwhile the earth’s population in 2050 will be 9.7 billion and in 2100 11.3 billion…surely a political drag on resources devoted to space flight…maybe population pressure will be what gets us to the Moon and Mars anyway…space tourism too…

  • ljk August 4, 2017, 10:04

    Scientists Just Discovered the Oldest Asteroid Family Ever:


    “We used a tremendous amount of physical information about asteroids,” Delbo’ told Gizmodo. “But we [ourselves] did not use a telescope. A bit sadly, as I really love telescopes.”

  • Alex Tolley August 4, 2017, 10:51

    Long period comets offer us a convenient way to acquire primordial matter that condensed out in the Oort cloud, bringing us material from a currently unattainable 1000s of AU out to within reach in the inner solar system. The problem for us is that to plan for a sample mission, we must know the precise orbit of such a comet long before it becomes obvious. This means a rendezvous with a comet that has been in the inner system before and is no longer pristine, except perhaps deep below its surface.

    When we have fast propulsion systems it will then be possible to do such a rendezvous with a comet that has perhaps never been in the inner system before, allowing us unprecedented opportunities to analyze the material and start a statistical analysis of cometary material. We may even be able to detect that some comets originated around another star.

    I’m optimistic that beamed energy propulsion approaches will prove the best way to do this. Whether by a small sail and chip like Starshot, a larger sail and science payload, or an electric drive, we should be able to detect and send out a probe that can quickly rendezvous with a comet while still in the outer system and analyze its composition. Build standard systems using mass production to minimize hardware costs and we could really start to gather data out to the farthest reaches of our system.

  • Thomas Shearer August 7, 2017, 0:42

    quote: “as much as four times smaller than …”

    I see you are falling into the “X times smaller than” (or “less than”) trap. When X is larger than 1, the result is negative, which is rarely the desired result.

  • J. Jason Wentworth August 7, 2017, 5:27

    Because the warning time would be so short in the case of a long-period comet impact, I have long thought that a thermonuclear comet destruction capability should be developed and maintained. This need not require keeping a force of such vehicles on alert, as we do with ICBMs and ABMs (Anti-Ballistic Missiles). Instead, a U.S. “earmark” plan could make use of existing launch vehicle, spacecraft, and ICBM warhead (or free-fall nuclear bomb) hardware, and it could work like this:

    Two or three current satellite (probably geosynchronous orbit satellite) buses could be kept in storage, either by their manufacturer or by the agency in charge of the anti-comet defense program; these buses could be “rotated” as needed, whenever the satellite manufacturer got an order for a commercial satellite, and:

    These spacecraft would, if needed for destroying a comet, be fitted with existing thermonuclear devices (minus their re-entry vehicle heat shields or aerodynamic bomb casings) that would be pulled from ICBMs or free-fall nuclear bombs in the stockpile. The launch vehicles for the spacecraft would be currently-used U.S. ones, which would be pulled from the commercial (or military) launch queue (at the present time, these would be the Atlas V, Delta IV, and Falcon 9). Also:

    The only new hardware that would definitely be needed (which the anti-comet defense program’s responsible agency could keep in storage) would be a physical interface containing the necessary arming and detonation circuitry for the thermonuclear device. These units (several of which would be kept in storage, one for each satellite) would be used to mount the thermonuclear devices on the spacecraft.
    Depending on the distance of the comet interception point from the Earth, a high-gain antenna might not be needed on the spacecraft, but such hardware (particularly if body-mounted and electronically-steered, as was the MESSENGER Mercury orbiter’s high-gain antenna) would not be difficult to add if it proved necessary. In addition:

    A test flight–using a simulated thermonuclear device (the old nuclear-armed Nike-Zeus ABMs were fitted with these for test interceptions of ICBM warheads, producing brilliant flashes at their burst points)–could be flown to qualify the interface hardware & circuitry (as well as the integrated spacecraft for this role), using an NEA (a Near-Earth Asteroid) as a test target. (The asteroid’s orbit would not be modified noticeably, and it would not be destroyed. The simulated thermonuclear warhead’s burst point [which could be pin-pointed by means of observations from Earth and from space telescopes, and from IKAROS solar sail-type wireless cameras, ejected from the spacecraft shortly before the burst] would enable the spacecraft’s–and the flight controllers’–navigation accuracy to be determined.) As well:

    Over time, as new and/or updated spacecraft buses and launch vehicles became standard hardware, the comet interception-specific hardware and software could also be modified and updated periodically, and be replaced with new hardware and software when needed. This would be cheap insurance against the least likely–but the most destructive–catastrophe that could befall our world within timescales that are relevant to mortal human beings.

    • Michael Fidler August 12, 2017, 2:02

      Yes, but we are dealing with something on cosmic scales and of unknown composition. A nuclear device could make for a bigger problem. Comets are the hammer of the gods and deflecting there wrath would be the order of the day.

      • J. Jason Wentworth August 14, 2017, 23:58

        Better possible death–or even probable death–than certain death, which is what we’d face if a long-period comet (especially a large one) hit the Earth in one piece. It would punch through our atmosphere almost as if it weren’t there, and its explosion (whether it occurred on impact or above the ground) would blow off a significant portion of the atmosphere, which would be lost to space. (On Mars, explosions above a certain size [I forget the TNT equivalence] blast away all of the air above a plane that’s tangent to the planet’s surface at the impact point–that’s a lot of atmosphere!) The suborbital “rain” of molten (terrestrial) rock and excavated dirt would also–shades of the dinosaur killer that blasted the Chicxulub crater–ignite every forest (and city) on Earth within less than an hour, and would freeze the Earth’s surface by blotting out the Sun for weeks or months, and:

        A thermonuclear warhead-fragmented comet nucleus would be more like buckshot than a single shotgun slug, but each smaller fragment would be more affected by its entry into our atmosphere (breaking up and vaporizing more readily). Even if the fragments produced Tunguska-type air bursts (if the fragments were large enough), I’d accept the odds of most of them occurring over oceans or uninhabited land (the fragments could be tracked, making evacuations possible if any were falling toward cities). Also:

        While the comet fragments would likely cause rain, cooler temperatures, and spectacularly colorful sunrises and sunsets for a while (due to their evaporated ices and dust in the air), we could deal with that. Even “warhead-implanted” radiation from the nucleus fragments probably wouldn’t be much of a problem, due to the huge amount of comet nucleus mass versus the very small mass of the bomb debris (much of the bomb-vaporized portion of the comet nucleus would have been scattered far, wide, and very thinly in space by the solar wind). Increasing our intake of anti-oxidants, such as blueberry and bilberry extracts, juices, jams, and jellies, would help us ward off the effects of residual radiation.

      • J. Jason Wentworth August 16, 2017, 3:44

        Michael, I wholeheartedly agree that deflecting a long-period comet on a collision course toward Earth would be the best of all options. At our current level of space technology, however, I doubt if we could detect and observe long-period comets far enough from the Sun to have enough warning time to deflect them as intact objects. Even if we could, there would remain two huge problems:

        [1] The sheer volume of space that would have to be “patrolled”–from the outer Oort Cloud to the Kuiper Belt–is daunting, because such comets can come from any direction on the celestial sphere, and:

        [2] Sending fast spacecraft (such as gravity tractor ships, space tugs, etc.) out there in time would require high-speed, high-acceleration propulsion systems, which would likely have to be nuclear. Both this option and the alternative–keeping spacecraft stationed in the outer solar system so that they’d be closer to the comets (space telescopes might have to be way out there, too, to see the comets in the very dim light)–would be prohibitively expensive, and a logistical nightmare. But:

        This statistically unlikely (in any given human lifetime) but almost-too-horrible-to-contemplate celestial catastrophe could, especially if we ever experience a near-hit by a long-period comet (which I’m *not* hoping for!), be a strong incentive to develop a human deep-space interplanetary travel capability. This would also make all of the cometary and centaur (“ice-teroid”) material out there accessible, for supporting space colonies out there.

  • ljk August 17, 2017, 9:39

    Will we see a bright comet during the August 21 total solar eclipse? Probably not, but hey, we will be looking that way anyway, so…


  • ljk August 24, 2017, 10:28

    Have astronomers detected exocomets crossing in front of alien suns?


    The paper online here:


    To quote:

    The authors of the new paper also make an interesting comparison to the star KIC 8462852 (aka ‘Boyajian’s Star’) which has attracted a lot of interest from astronomers lately for its weird and still-unexplained dips in brightness which don’t resemble any kind of transits or similar phenomena ever seen before. Some early theories suggested massive “comets” as one possible explanation, but the new paper seems to suggest that is unlikely:

    “Finally, the deep dips in the flux of KIC 8462852 (aka ‘Boyajian’s Star’; Boyajian et al. 2016) are worth trying to relate to what is observed in KIC 3542116. By contrast, the largest flux dips in the former star reach 22% which is more than two orders of magnitude greater than the transits we see in KIC 3542116.

    Furthermore, the dips in KIC 8462852 can last for between 5 and 50 days, depending on how the beginning and end points of the dip are defined. These are one to two orders of magnitude longer than for the transits in KIC 3542116. Finally, we note that none of the dips in KIC 8462852 has a particularly comet-shaped profile.

    There have been a number of speculations about the origin of the dips in KIC 8462852, including material resulting from collisions of large bodies and moving in quasi-regular orbits (Boyajian et al. 2016); swarms of very large comets (Boyajian et al. 2016); and even a ring of dusty debris in the outer Solar System (Katz 2017). However, there is currently no compelling evidence for any of these scenarios.”

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