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The Challenge of ‘Twilight Asteroids’

We have the Zwicky Transient Facility at Palomar Observatory to thank for the detection of the strikingly named ’Ayló’chaxnim (2020 AV2). This is a large near-Earth asteroid with a claim to distinction, being the first NEO found to orbit inside the orbit of Venus. I love to explore the naming of things, and now that we have ’Ayló’chaxnim (2020 AV2), we have to name the category, at least provisionally. The chosen name is Vatira, which in turn is a nod to Atira, a class of asteroids that orbit entirely inside Earth’s orbit. Thus Vatira refers to an Atira NEO with orbit interior to Venus.

As to the ’Ayló’chaxnim, it’s a word from indigenous peoples whose ancestral lands took in the mountainous region where the Palomar Observatory is located. I’m told by the good people at Caltech that the word means something like ‘Venus Girl.’ On June 7, people of Pauma descent gathered for a ceremony at the observatory, having been asked by the team manning the Zwicky Transient Facility to choose a local name.

I couldn’t tell you how ’Ayló’chaxnim is pronounced, but with the ZTF on watch, it’s possible we’ll find more Vatiras, or at least Atiras, which seem to be more numerous, so we may have more Pauma names to come and perhaps we’ll learn. 2020 AV2 is 1 to 3 kilometers in size and has an orbit tilted about 15 degrees from the plane of the Solar System. On its 151 day orbit, it stays interior to Venus and comes close to the orbit of Mercury. Postdoc Bryce Bolan at Caltech flagged it as a candidate in early 2020.

The ZTF itself is a survey camera mounted on the Samuel Oschin Telescope at Palomar, conducting a wide-field survey making rapid scans of the sky. 2020 AV2, says Caltech’s George Helou, who is a ZTF co-investigator, is on an interesting orbit, surely the result of migration from further out in the system:

“Getting past the orbit of Venus must have been challenging. The only way it will ever get out of its orbit is if it gets flung out via a gravitational encounter with Mercury or Venus, but more likely it will end up crashing on one of those two planets.”

Image: The Zwicky Transient Facility field of view. The ZTF Observing System delivers efficient, high-cadence, wide-field-of-view, multi-band optical imagery for time-domain astrophysics analysis. The camera utilizes the entire focal plane of 47 square degree of the 48-inch Samuel Oschin Schmidt telescope, providing the largest instantaneous field-of-view of any camera on a telescope of aperture greater than 0.5 m: each image will cover 235 times the area of the full moon. Credit: Zwicky Transient Facility.

This close to the Sun, Vatiras are only going to be visible at dusk or dawn. As the University of Hawaii’s Scott Sheppard points out in a recent issue of Science, our asteroid surveys mostly take place with a dark night sky, which implies that small objects orbiting between the Earth and the Sun are not likely to be found. Modeling of the NEO population predicts that objects as large as 2020 AV2 are unlikely among Vatiras but smaller objects could be plentiful. Asteroid surveys interior to Venus’ orbit are few, so there is work here for facilities like the ZTF, or the NSF’s Blanco 4-meter telescope in Chile with the Dark Energy Camera (DECam) to fill out this population. Both have fields of view sufficient to carry out this kind of survey.

So let’s get down to the asteroid mitigation question. Sheppard points out that what with current NEO surveys coupled with formation models for these objects, more than 90 percent of what he calls ‘planet killer’ NEOs have probably already been found – these would be objects larger than 1 kilometer, and he’s talking here about the entire range of NEOs, not just those interior to the orbits of Earth or Venus. He writes:

The last few unknown 1-km NEOs likely have orbits close to the Sun or high inclinations, which keep them away from the fields of the main NEO surveys. The 48-inch Zwicky Transient Facility telescope has found one Vatira and several Atira asteroids, making it one of the most prolific asteroid hunters interior to Earth. To combat twilight to find smaller asteroids, one can use a bigger telescope. Large telescopes usually do not have big fields of view to efficiently survey. The National Science Foundation’s Blanco 4-meter telescope in Chile with the Dark Energy Camera (DECam) is an exception. A new search for asteroids hidden in plain twilight with DECam has found a few Atira asteroids, including 2021 PH27.

Sheppard’s also describes a category he calls ‘city killers,’ which takes in NEOs larger than 140 meters; of these, he believes we have found about half. The progress in tracking NEOs has been heartening as we learn about potentially dangerous trajectories, and turning to twilight surveys like these will help us learn more about NEOs hidden in the glare of the Sun.

It turns out that the Zwicky team recently found the asteroid with the smallest known semimajor axis (0.46 AU). This is 2021 PH27, an object with high eccentricity whose orbit crosses the orbit of Mercury as well as Venus. Thus, given our categorization, PH27 is an Atira rather than a Vatira. With a perihelion of 0.13 AU, this NEO shows 1 arc minute of precession per century, the highest of any object in the Solar System including Mercury. This is another large NEO at about 1 kilometer in size. Although as Sheppard notes:

…because the diameter of these interior asteroids is calculated with an assumed albedo and solar phase function, the actual diameters for both of these discoveries could be under 1 km. This would put them in a more-expected population and make them less of a statistical fluke.

Image: 2020 AV2 orbits entirely within the orbit of Venus. Credit: Bryce Bolin/Caltech

Clearly we have much to do to build our catalog of objects close to the Sun. We can extend the catalog of exotic names as well. Asteroids called Amors are those that approach the Earth but do not cross its orbit. Apollos do cross the orbit of the Earth but have semimajor axes greater than Earth’s. Atens, in turn, cross Earth’s orbit but have semimajor axes less than that of the Earth. Sheppard points out that NEOs have dynamically unstable orbits, and speculates that a reservoir that replenishes their numbers must exist because the overall count seems to be in a steady state.

Among possible reservoirs are those that may exist in long-term resonances with Venus or Mercury, and there may conceivably be a population of asteroids not yet observed, the so-called Vulcanoids, that could have orbits entirely within the orbit of Mercury. Sheppard’s excellent article makes the point that Vulcanoids would be at the mercy of many factors, including Yarkovsky drift, collisions and thermal fracturing from proximity to the sun, so they’re likely uncommon. We do know that spacecraft observations of the region near the Sun seem to rule out Vulcanoids larger than 5 kilometers, but stable reservoirs for smaller objects may exist. Remember, too, that we have found numerous exoplanets closer to their host stars than the Vulcanoid region in our Solar System.

Overall, NEOs in the Sun’s glare should not be too prolific:

Fewer Atiras should exist than the more-distant NEOs, and even fewer Vatiras, because it becomes harder and harder for an object to move inward past Earth’s and then Venus’ orbit. Random walks of a NEO’s orbit through planetary gravitational interactions can make an Aten into an Atira and/or Vatira orbit and vice versa. Atiras should make up some 1.2% and Vatiras only 0.3% of the total NEO population coming from the main belt of asteroids (4). 2020 AV2 itself will spend only a few million years in a Vatira orbit before crossing Venus’ orbit. Eventually, 2020 AV2 will either collide with or be tidally disrupted by one of the planets, disintegrate near the Sun, or be ejected from the inner Solar System.

Scott Sheppard’s article is “In the Glare of the Sun,” Vol. 377 Issue 6604 (21 July 2022), pp. 366-367 (full text). For more on the Zwicky Transient Facility, see Graham et al., “The Zwicky Transient Facility: Science Objectives,” Publications of the Astronomical Society of the Pacific Vol. 131, No. 1001 (22 May 2019). Full text.

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{ 14 comments… add one }
  • Alex Tolley July 29, 2022, 15:14

    While the Chelyabinsk meteorite is likely an Apollo class asteroid, it had presumably been undetected in the past and its possible approach was during its orbit inside earth’s orbit [please correct me if I am wrong.]. It remains the most devasting impact in recent memory.

    While the asteroids orbiting inside the Earth’s orbit are relatively few, and those inside Venus’ orbit even fewer, purely from a planetary defense posture, it would make sense to have a program of detection that is not hampered by the limitations of daylight. Given the relatively small aperture mirror telescope needed for a sky survey inside the Earth’s orbit, wouldn’t it make sense to have one or more space-based telescopes to do due the survey? A small version of Gaia (or even Gaia itself?) seems like the approach to use to try to catalog these asteroids and to ensure that any PHAs are known and can be intercepted. We were relatively lucky at Chelyabinsk, but what if the asteroid had the energy released by the Tunguska asteroid/comet (~ 12 megatons TNT vs ~ 0.5 megatons for Chelyabinsk)?

    • Brett Bellmore July 30, 2022, 17:43

      Indeed, I was going to suggest that: Given the importance of the mission, and the optics, the obvious thing is to put a dedicated telescope into an orbit closer to the Sun than Earth. Maybe one of Venus’ Lagrange points?

      That way any asteroid that was a threat to Earth would be optimally illuminated at least part of the time, and at least moderately illuminated most of the time.

      • Alex Tolley July 31, 2022, 3:41

        If the asteroids are tumbling, would an IR telescope, rather than visible light, be the best option for looking more toward the sun? Wouldn’t they stand out as “hot spots” moving against the background stars?

        • Brett Bellmore July 31, 2022, 10:16

          Certainly would. But for a given sized telescope, the resolution increases with decreasing wavelength, so an optical scope will always have better resolution.

          You’d expect a body about Venus’ orbit to have about a 260K thermal equilibrium, the peak wavelength would be around 11um. Visible wavelengths are 15-20 times shorter than that, so you get a much higher resolution for the same sized scope.

          Also depends on how fast it’s tumbling; If slowly, only a small portion beyond the terminator will still be hot enough to radiate effectively, even in IR the brightest part of the asteroid will be the part facing the Sun.

    • Hop David July 30, 2022, 23:48

      Amy Mainzer’s telescope will be for hunting asteroids. It will be parked at the Sun Earth L1.

    • Antonio August 1, 2022, 4:32

      There is NEO Surveyor, scheduled to launch in 2026.

    • Geoffrey Hillend August 1, 2022, 16:18

      The meteor that made meteor crater was 50m (160) feet had a yield of 10 megatons according to Wikipedia. NEO of 140m must have a higher yield. I like the idea of a space telescope used for the purpose of early warning. High powered ground based radar might also work

  • Michael Fidler July 29, 2022, 19:20

    What has been the studies of comets in orbits that are blocked by the sun’s glare? The number of close encounters to the sun of comets is much higher then NEO’s so what are the odds of a comet being in the sun’s glare before an orbit can be ploted out?

  • Robin Datta July 30, 2022, 18:19

    Meteorite strikes on earth are routine; the questions are when a big one will strike, and how big. The further out and the smaller, the easier to deflect. In setting up a defence system, the limits that will be handled without additdional assistance could be defined. That would be the first step towards the assembly of appropriate infrastructure.

  • Thomas Mazanec August 1, 2022, 16:04

    I wonder if there are Matiras (orbit all inside Mercury’s).

  • Ljk August 2, 2022, 7:54
  • ljk September 16, 2022, 12:54

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