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A New Trio of ‘Twilight’ Asteroids

I have further thoughts on ‘Stapledon thinking,’ as discussed in the last post, but my second piece on the topic isn’t ready just yet, and in any case I want to give a quick nod to a topic we looked at a few months back, the discovery and analysis of Near Earth Objects that orbit between the Sun and the orbit of Earth. So far we haven’t found many of these ‘twilight objects,’ but the attempts to find them continue.

As witness current work with an exceptional instrument. The Dark Energy Camera is a wide-field CCD imager, mounted on a 4-meter telescope at Cerro Tololo (Chile), that was designed for the Dark Energy Survey. The latter mapped hundreds of millions of galaxies to look for insights into the structure of the cosmos. The DES ended in 2019, but DECam continues to produce data that have helped us find fascinating objects like 2015 TG387, a dwarf planet on an extreme orbit that takes it to aphelion at 1000 AU, with a closest solar approach of 65 AU. DECam has also found 12 new moons of Jupiter and a number of ‘stellar streams’ produced when small galaxies interact with the Milky Way. Now we learn that it has flagged three Near Earth Objects on inner system orbits.

These NEOs stand out beyond the fact that they are part of that small population of asteroids found inside the orbits of Earth and Venus. One of them, the 1.5-kilometer-wide 2022 AP7, merits more than a passing glance, as there is a faint chance it could one day intersect with Earth’s orbit. That would be problematic because the size of this asteroid takes it into the category that Scott S. Sheppard (Carnegie Institution for Science) calls ‘planet killers.’ 2022 AP7 is the second asteroid of this size found in the ongoing survey. Another, designated 2021 PH27, is the closest known asteroid to the Sun, and has the largest effects from general relativity in the system.

So should we worry about the 1.5-kilometer 2022 AP7? Only in the sense that we should keep an eye on it. Unfortunately, its designation as a Potentially Hazardous Asteroid (PHA) is going to generate scare stories, so let’s untangle the definition of a PHA.

What we know about 2022 AP7 is that it crosses Earth’s orbit with a perihelion near 0.83 AU and an aphelion near the orbit of Jupiter. Asteroids that cross Earth’s orbit are known as Apollos, and this one is the largest asteroid designated as a PHA that has been found in the last eight years. According to the paper on these observations, the Minimum Orbit Intersection Distance (MOID) with Earth is 0.0475 AU. That’s close enough to declare it a PHA, which basically tells us that this is an object that merits continued observation rather than one posing imminent danger.

The Center for Near Earth Object Studies, which computes asteroid and comet orbits, defines a Potentially Hazardous Asteroid this way:

Potentially Hazardous Asteroids (PHAs) are currently defined based on parameters that measure the asteroid’s potential to make threatening close approaches to the Earth. Specifically, all asteroids with a minimum orbit intersection distance (MOID) of 0.05 au or less and an absolute magnitude (H) of 22.0 or less are considered PHAs.

So asteroids that fail to approach closer to the Earth than 0.05 au (this works out to about 7,480,000 km) – or are smaller than 140 meters in diameter – do not fit the definition of PHAs. The concern is that over timespans of hundreds of years or more, such an object’s orbit may take it closer still to Earth, making identification of PHAs a necessary part of our planetary defense. Much can happen over extended periods of time to nudge the orbit of an NEO. Consider another PHA, the now well-known and visited Bennu, studied up close by the OSIRIS-REx mission.

The CNEOS monitors PHA orbits and updates them as new data become available. Let me quote NASA’s Planetary Defense Coordination Office about Bennu:

CNEOS predicts that the next time Bennu will pass Earth within the Moon’s orbit will be in 2135. This particularly close approach will change Bennu’s orbit by a small amount, which is uncertain at this time and which may lead to a potential impact on Earth sometime between 2175 and 2199. CNEOS has calculated that the cumulative risk of impact by Bennu during this 24-year period is 0.037 percent or a 1 in 2,700 chance. That means there is a 99.963 percent probability that Bennu will not impact Earth during this quarter-century period.

Note that “1 in 2,700” statement. CNEOS has been clear that this is not an impact probability for a single year, but the cumulative probability of impact over all years between 2175 and 2199. The risk of an impact in 2175, for example, is listed as 1 in 24,000. And the figures adjust as observations accumulate. A close approach to the Moon and Earth will itself tweak Bennu’s orbit by an amount that we can only estimate. Thus new data are constantly sought to tighten the window of uncertainty. One of the many good reasons for the OSIRIS-REx mission was to do just this as we revise the future possibilities for an impact. Remember, we’re looking a century out.

I feel comfortable with this arrangement. We’re doing an excellent job of identifying NEOs, we’re learning about asteroids through observation and sample return missions, and we’re investigating means of altering the trajectories of any objects that one day may threaten us. Which is to say I’m not losing any sleep about NEOs, as long as this level of vigilance and investigation continues, as it surely must in the name of planetary defense.

For more on CNEOS, see its Sentry: Earth Impact Monitoring page, which provides extensive background information.

Image: Artist’s impression of an asteroid that orbits closer to the Sun than Earth’s orbit. Credit: NSF NOIRLab.

The aforementioned Scott Sheppard is lead author of the paper on the three new NEAs. He, notes that the survey has found one other PHA thus far:

“Our twilight survey is scouring the area within the orbits of Earth and Venus for asteroids. So far we have found two large near-Earth asteroids that are about 1 kilometer across, a size that we call planet killers. There are likely only a few NEAs with similar sizes left to find, and these large undiscovered asteroids likely have orbits that keep them interior to the orbits of Earth and Venus most of the time. Only about 25 asteroids with orbits completely within Earth’s orbit have been discovered to date because of the difficulty of observing near the glare of the Sun.”

We looked at asteroids on orbits interior to Earth’s last summer (see The Challenge of ‘Twilight Asteroids’), so I’ll just remind readers that asteroids on orbits entirely within the orbit of Earth are known as Atiras, while those within the orbit of Venus are Vatiras (only one of these is currency known). If there are any asteroids entirely within the orbit of Mercury, they would be known as Vulcanoids, but none have been found yet. I turn to the paper for a note about the frequency of the Atira asteroids:

There are likely several more 1 km sized Atira-type asteroids left to find, which probably have low semimajor axes and high inclinations, like 2021 PH27, making them hard to find for most asteroid surveys. The DECam twilight survey is covering sky geometries and areas that most other surveys do not cover to depths not usually obtained, filling an important niche in the survey for the last few remaining relatively large unknown NEOs.

Noteworthy is the apparent lack of smaller asteroids in the survey:

…the twilight survey has discovered more larger asteroids (≳1 km) than smaller ones even though the survey is sensitive to smaller asteroids. This might suggest the smaller asteroids are dynamically less stable and/or more susceptible to break-up from the extreme thermal and gravitational environment near the Sun, though additional discoveries of asteroids with orbits near the Sun must be made to determine statistically if the smaller asteroids are under-abundant since in general they are also harder to detect.

The paper is Sheppard et al., “A Deep and Wide Twilight Survey for Asteroids Interior to Earth and Venus,” Astronomical Journal Vol. 164, No. 4 (29 September 2022), 168 (abstract). Note as well Shappard’s “In the Glare of the Sun,” Science Vol. 377, Issue 6604 (21 July 2022), 366-367 (abstract).

{ 14 comments… add one }
  • Alex Tolley November 1, 2022, 14:32

    I hope these small hard-to-find twilight asteroids are indeed rare.

    Clarke’s novel Rendezvous with Rama opens with the impact of a mere 1000-ton asteroid at 50 km/s wiping out Padua, Italy. So a city killer, rather than a planet killer.
    If I have done my calculations correctly, a metal and stone asteroid of that mass is less than 10 meters in diameter, ~1/20th the diameter of a 140m planet-killer.

    Such an impact, on or near a major city would be devastating. Just the flooding from an offshore impact would wreck any major city like London, New York, Tokyo, Shanghai, or Lagos, with a global economic impact, let alone the deaths of perhaps millions of people if the impact was unforeseen.

    If the cumulative probability of an impact increases with time, we need to think in longer terms than just a century. A major impact in the next 10,000 years would be appalling even if humanity has sunk back to an almost pre-technological state.

    Suppose an asteroid could be almost certain to reset evolution to remove any [pre-]technological species on its surface every 1 million years. That could be a serious filter to get through for ETI. The hominid lineage would have been removed at that time, forestalling the emergence of Homo sapiens.

    Dark Forest speculation. What if ETI has placed propulsion units on PAH asteroids with the intention of either keeping them from impacting Earth to keep us safe, or conversely to kill us off if we become perceived as dangerous? No fleets of starships need to arrive, nor send near c velocity impactors. Just a signal to activate the propulsion unit to gently shift the asteroid’s orbit so that an impact was assured in some period of time. Unless the device was discovered before impact, there would be no trace of ETI interference. Perhaps we might add such a long-shot possibility to any Lurker search just to rule out any such device, not to mention capturing the technology should such a device be found.

    • Jeff Wright November 2, 2022, 7:04

      2021 PH 27 interests me. I wonder if a reflector could be put there to direct sunbeams. Phys.org wrote of a carbide that can stir molten steel and resist 7,000 degree temps.

    • ljk November 2, 2022, 10:51

      Alex, you are quite correct about the possibility of a malevolent ETI using a planetoid or comet to take out humanity without making a radioactive mess in the process. I wrote about that and all the other possible ways aliens could get rid of us here:


      In the SF novel Footfall, invading aliens tried to bombard humanity into submission with well-aimed planetoids.


      Unfortunately I would be more immediately concerned about a rogue nation or organization from Earth dropping a space rock on places vital to their enemy. As you said, even a lesser space-capable power could attach a motor to a planetoid and guide it towards a target. This much at least is no longer science fiction.

      Carl Sagan co-authored a letter to Nature in 1994 about how planetoid deflection technology could also be used against us, so it is not a new concept:



      • Alex Tolley November 2, 2022, 20:00

        I hope I wasn’t trying to claim that asteroids as weapons was a new idea. The Zagons were using flaming meteors as bombs against Metalluna in This Island Earth. The Trisolarians were doing something similar with their ships in Liu’s The Dark Forest. The Earthshock episode of Dr. Who had the cybermen directing the KT event asteroid. A stealthy, long-term attack is somewhat more subtle and may be used to provide “plausible deniability” of malfeasance.

        In practice, I think it represents a very crude, blunt instrument. Surely a biological weapon would be cheaper and more targettable? A gene drive could be used to eliminate humanity, not just mosquitoes. It wouldn’t surprise me at all if work isn’t already being done to use some sort of biological attack to hugely reduce a population with a particular genetic type. Clarke wrote a shaggy dog story (Reunion) where the returning galactic humans could reverse the genetic trait that resulted in “whites” and restore the dark brown color. Today things look more sinister. A stealth approach would be to target nations with an attack that reduces fertility, reducing the population, a subtle attack that might not be detected for a long time, and with no obvious way to determine the instigator. [ Why else would aliens be abducting people to take samples rather than wiping us out with more direct weapons? *smile* ].

        Sun Tsu said that the best way to defeat an enemy was to convince them they were beaten before any fighting started. In essence that is what propaganda and disinformation are for, now even better mediated through social media. It also helps to wage asymmetric warfare, such as using cheap drones to attack expensive armaments like warships, as well as destroying civilian infrastructure to push the population to demand a settlement, even in the enemy’s favor. ET civilizations probably know that direct physical warfare need not have the desired outcome even when overwhelming force is applied. Subtlety is probably a better way to go, as Earth’s history shows that even the greatest ancient civilizations can collapse given a series of natural disasters. As our technological civilization becomes ever more complex, the number of such attack surfaces increases, making us ever more vulnerable.

      • charlie November 4, 2022, 16:19

        “Carl Sagan co-authored a letter to Nature in 1994 about how planetoid deflection technology could also be used against us, so it is not a new concept:”

        To be honest Carl Sagan was more of liberal nut-chaser and money seeker than true scientist. He was more of a showman.

  • Martin November 1, 2022, 18:04

    Have you ever looked at this website: https://impact.ese.ic.ac.uk/ImpactEarth/ImpactEffects/
    The select a density from a list option doesn’t work, but you can type in a density (I used 8000 kg/m3 for iron)

    • Alex Tolley November 1, 2022, 21:51

      Interesting site. Either Clarke or I have done the wrong calculations based on this site’s estimate of damage.

  • Antonio November 1, 2022, 18:57

    If it reachs the orbit of Jupiter, why it was not discovered before?

  • Alex Tolley November 2, 2022, 12:26

    And this is why CD is the place to read about such events, and not the media.

    Even the normally sober Guardian has gone ‘clickbaity’ over this asteroid, and even the text makes no mention of the probability of an impact, leaving the reader to guess whether to buy property on Mars.
    Huge ‘planet killer’ asteroid discovered – and it’s heading our way

    Even CNN managed to do better.

  • Robin Datta November 3, 2022, 6:36

    Sooner or later, if we have the means, it will be imperative to detect and deflect a Near Earth Object. An oversized object or the loss of scientific and technical know-how would eliminate the imperative. Would in that case a suborder of mammals go on to become a dominant lineage, as have aves from a sliver of the saurischian dinosaurs?

    We would be getting close to the end of the biosphere in about a 1.0 +/- 0.2 Gy by then.

    • Alex Tolley November 4, 2022, 11:23

      So a Gy from now is 3x as long as the period from the Cambrian “explosion” of life forms until now. Plenty of time for new species to come and go. However, we are the only “highly intelligent” species on the planet, and how we got to this state seems highly contingent. There is no guarantee that intelligence would similarly evolve again, coupled with dexterous hands to manipulate objects. I suspect that the birds are a dead end in this regard. Rodents, especially rats might be the lineage to evolve the needed intelligence and manipulative skills to develop a civilization after we are long gone. Or maybe intelligence is not a survival trait and no species evolves down what may be a dead end for humanity.

      My personal bet is that machine intelligences are likely to be our descendants once they can do without us.

  • Mike Serfas November 3, 2022, 19:20

    I’d suspect there are more important ulterior motives at work behind the search for these near-Earth asteroids. They are, after all, *near Earth*, and the sum of our fears is that a tiny delta vee can get material from their orbit to ours. But this should be even more true for refined metals or luxury “genuine asteroid” floor tiles shot from a launcher than it is for the whole asteroid! I expect someone is about to show the world that there is a great deal of money to be made up there – and the implication of a private confiscation of these once-public assets (not to mention a small risk of being hit by a floor tile) hints that many of us will not be happy about it.

    • Alex Tolley November 3, 2022, 21:01

      Even NEOs have non-trivial delta)Vs that make asteroid mining very expensive. In the case of 2022 AP7, just knowing that it has an aphelion near Jupiter tells you that when it nears Earth there will be a considerable delta_V to be met. In addition, most of the time, the point at which it crosses Earth’s orbit will be nowhere near us.

      One of the cost drivers is propellant cost. Cheaper launch costs and more efficient engines are ways to reduce the cost, as is amortizing it over a larger cargo. Ultimately, if one can tolerate the time to chase, mine, and return the mined cargo, solar sails might be one way to go. In the case of 2022 AP7, the mining craft would have to launch and potentially chase the asteroid in its orbit, and ride it on its journey all the way to Jupiter and back to Earth, before decelerating to deliver the cargo to earth’s orbit. Maybe just let the cargo be decelerated by aerobraking rather than using propellant. Typically, when you run the numbers, even returning platinum group elements is iffy, even assuming mining is nothing harder than just collecting nuggets of the surface.

      The only business scenario that works at all, AFAIK, is using the mined materials in space. Maybe there is a market for floor tiles in a billionaire’s lunar chateau, or orbiting hotel.

      [I love the idea of asteroid tiles, it is so different from the boring precious metals or volatiles that are usually the target of asteroid mining.]

      • Mike Serfas November 4, 2022, 8:48

        You’re right that I abused the notion of delta vee, especially for the asteroid mentioned in this article (though I really had Bennu and Ryugu in mind). Aerobraking can be used to slow packages in an initial pass through the atmosphere. But NASA is actually making plans to crash land probes directly onto Mars! ( https://phys.org/news/2022-10-nasa-mars.html ) Also, with the world presumably racing to build tens of thousands of hypersonic nuclear missiles, there might be some spin-off technology that can move directly from a fast orbital approach into a long hypersonic glide, fireball hot but never actually breaking up on reentry, followed by an automated landing. I think a plan could shake out to bring shock-insensitive materials to Earth’s surface fairly cheaply.

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