APOD: The Comet and the Fireball (2021 Dec 20)

[APOD Robot]

The Comet and the Fireball

Explanation: This picture was supposed to feature a comet. Specifically, a series of images of the brightest comet of 2021 were being captured: Comet Leonard. But the universe had other plans. Within a fraction of a second, a meteor so bright it could be called a fireball streaked through just below the comet. And the meteor's flash was even more green than the comet's coma. The cause of the meteor's green was likely magnesium evaporating from the meteor's pebble-sized core, while the cause of the comet's green was likely diatomic carbon recently ejected from the comet's city-sized nucleus. The images were taken 10 days ago over the Sacramento River and Mt. Lassen in California, USA. The fireball was on the leading edge of this year's Geminid Meteor Shower -- which peaked a few days later. Comet Leonard is now fading after reaching naked-eye visibility last week -- but now is moving into southern skies.

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[Sa Ji Tario]

There is a light between the mountains, similar to that of the stars, it looks like a house or vehicle. In the foreground water there is no reflection of the sky

[neufer]

Explanation: This picture was supposed to feature a comet. Specifically, a series of images of the brightest comet of 2021 were being captured: Comet Leonard. But the universe had other plans. Within a fraction of a second, a meteor so bright it could be called a fireball streaked through just below the comet. And the meteor's flash was even more green than the comet's coma. The cause of the meteor's green was likely magnesium evaporating from the meteor's pebble-sized core, while the cause of the comet's green was likely diatomic carbon recently ejected from the comet's city-sized nucleus.

Olivine (Peridot) from Pakistan

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<<289 Nenetta is a typical A-type asteroid. It was discovered by Auguste Charlois on 10 March 1890 in Nice, France. The spectrum of 289 Nenetta reveals the strong presence of the mineral Olivine, a relative rarity in the asteroid belt. The mineral olivine is a magnesium iron silicate with the chemical formula (Mg²⁺, Fe²⁺)₂SiO₄. The primary component of the Earth's upper mantle, it is a common mineral in Earth's subsurface, but weathers quickly on the surface. For this reason, olivine has been proposed as a good candidate for accelerated weathering to sequester carbon dioxide from the Earth's oceans and atmosphere, as part of climate change mitigation.

Mg-rich olivine has also been discovered in meteorites, on the Moon and Mars, falling into infant stars, as well as on asteroid 25143 Itokawa. Such meteorites include chondrites, collections of debris from the early Solar System; and pallasites, mixes of iron-nickel and olivine. The rare A-type asteroids are suspected to have a surface dominated by olivine. The spectral signature of olivine has been seen in the dust disks around young stars. The tails of comets (which formed from the dust disk around the young Sun) often have the spectral signature of olivine, and the presence of olivine was verified in samples of a comet from the Stardust spacecraft in 2006. Comet-like (magnesium-rich) olivine has also been detected in the planetesimal belt around the star Beta Pictoris.>>

<<Kryptonite is a green, crystalline material originating from Superman's home world of Krypton that emits a unique radiation that weakens Superman. It is generally harmless to humans in the short term, but deadly in the long term. Due to Superman's popularity, kryptonite has become a byword for an extraordinary exploitable weakness, synonymous with "Achilles' heel". Batman and Lex Luthor are two notable individuals who have pieces of kryptonite with them—the former being trusted by Superman to stop him if he goes rogue, and the latter using the mineral to ward off Superman, sometimes incorporating it into weapons to try to kill him.>>

[orin stepanek]

Looks like the fireball stole the Comet Leonard's
Thunder!

[neufer]

Looks like the fireball stole the Comet Leonard's Thunder!

• The Giant Arc stole the fireball's Thunder:

Click to play embedded YouTube video.

<<The Giant Arc is a large-scale structure discovered in June 2021 that spans 3.3 billion light years. The structure of galaxies exceeds the 1.2 billion light year threshold, challenging the cosmological principle that at large enough scales the universe is considered to be the same in every place (homogeneous) and in every direction (isotropic). The Giant Arc consists of galaxies, galactic clusters, as well as gas and dust. It is located 9.2 billion light-years away and stretches across roughly a 15th of the radius of the observable universe. It was discovered using data from the Sloan Digital Sky Survey by the team of Alexia M. Lopez, a doctoral candidate in cosmology at the University of Central Lancashire.

If the Giant Arc were visible in the night sky it would form an arc occupying as much space as 20 full moons, or 10 degrees on the sky.>>

[johnnydeep]

There is a light between the mountains, similar to that of the stars, it looks like a house or vehicle. In the foreground water there is no reflection of the sky

The milkiness of the water seems to me like it's probably mist. You wouldn't see reflections in it.

EDIT: although I do see reflections of vegetation close to the farther shore - perhaps there's no mist there.

[drjhammond]

It's impossible to tell if that light would reflect from the water at all because of the growth on the side of the river. A bit of choppiness on the river surface could obscure any reflection and perhaps the light was not on all images that comprised the composite. Look up "Cory Poole Photography" on Facebook. I think he will explain the processing that went into this image. I think the green of the meteor is being scattered by thin clouds and being attenuated rather than saturating the sensor in the brightest part of the meteor track. I am still thinking about this.

[mbw]

Is the meteor something that broke off from the comet or just something completely separate, or we don't know?

Doesn't it seem likely to be a comet fragment since it is so aligned with the meteor's path?

[johnnydeep]

Is the meteor something that broke off from the comet or just something completely separate, or we don't know?

Doesn't it seem likely to be a comet fragment since it is so aligned with the meteor's path?

As the text says, the meteor is a Geminid, probably determined from following it's path back to the well-known radiant of all Geminids. The Geminids are caused by Earth's passage through the debris left by comet 3200 Phaethon, whereas comet Leonard is a separate comet following a different path.

[Chris Peterson]

Is the meteor something that broke off from the comet or just something completely separate, or we don't know?

Doesn't it seem likely to be a comet fragment since it is so aligned with the meteor's path?

Different orbits. Interestingly, the comet's orbit intersects the orbit of Venus, so it may have created a new meteor shower on that planet.

[neufer]

Interestingly, the comet's orbit intersects the orbit of Venus, so it may have created a new meteor shower on that planet.

• Unfortunately, Venusians can't experience meteor showers (or even acid rain showers, for that matter).

<<Venusian clouds are thick and are composed mainly (75–96%) of sulfuric acid droplets.These clouds obscure the surface of Venus from optical imaging, and reflect about 75% of the sunlight that falls on them. The density of the clouds is highly variable with the densest layer at about 48.5 km, reaching 0.1 g/m³ similar to the lower range of cumulonimbus storm clouds on Earth. The cloud cover is such that typical surface light levels are similar to a partly cloudy day on Earth. The equivalent visibility is about three kilometers, but this will likely vary with the wind conditions.

Venus's sulfuric acid rain never reaches the ground, but is evaporated by the heat before reaching the surface in a phenomenon known as virga. It is theorized that early volcanic activity released sulfur into the atmosphere and the high temperatures prevented it from being trapped into solid compounds on the surface as it was on the Earth.>>

[MarkBour]

Looks like the fireball stole the Comet Leonard's Thunder!

• The Giant Arc stole the fireball's Thunder:

https://www.youtube.com/watch?v=DXtmRRXTiSw

<<The Giant Arc is a large-scale structure discovered in June 2021 that spans 3.3 billion light years. The structure of galaxies exceeds the 1.2 billion light year threshold, challenging the cosmological principle that at large enough scales the universe is considered to be the same in every place (homogeneous) and in every direction (isotropic). The Giant Arc consists of galaxies, galactic clusters, as well as gas and dust. It is located 9.2 billion light-years away and stretches across roughly a 15th of the radius of the observable universe. It was discovered using data from the Sloan Digital Sky Survey by the team of Alexia M. Lopez, a doctoral candidate in cosmology at the University of Central Lancashire.

If the Giant Arc were visible in the night sky it would form an arc occupying as much space as 20 full moons, or 10 degrees on the sky.>>

Thanks for sharing the talk about the Giant Arc, oh Great Art.

Why would cosmological models expect bland homogeneity on any scale?
For instance, I expect that a similar analysis of: https://apod.nasa.gov/apod/ap191013.html would also find such structures.

I think it is a misinterpretation of the principle to assume that random fluctuation is gone above a specific size threshold. We need to have a little more respect for the way randomness can be very creative -- it must always be capable of surprising us. We will therefore always find "structures" whose existence is extremely unlikely, everywhere, on every scale, whether they were really created by an underlying forcible cause, or not.

Sure, we search for such unexpectedness, and then search for a possible cause underlying it, but I'm pretty sure we will often find nothing in particular that drives the very particular.

[Chris Peterson]

Interestingly, the comet's orbit intersects the orbit of Venus, so it may have created a new meteor shower on that planet.

• Unfortunately, Venusians can't experience meteor showers (or even acid rain showers, for that matter).

<<Venusian clouds are thick and are composed mainly (75–96%) of sulfuric acid droplets.These clouds obscure the surface of Venus from optical imaging, and reflect about 75% of the sunlight that falls on them. The density of the clouds is highly variable with the densest layer at about 48.5 km, reaching 0.1 g/m³ similar to the lower range of cumulonimbus storm clouds on Earth. The cloud cover is such that typical surface light levels are similar to a partly cloudy day on Earth. The equivalent visibility is about three kilometers, but this will likely vary with the wind conditions.

Venus's sulfuric acid rain never reaches the ground, but is evaporated by the heat before reaching the surface in a phenomenon known as virga. It is theorized that early volcanic activity released sulfur into the atmosphere and the high temperatures prevented it from being trapped into solid compounds on the surface as it was on the Earth.>>

99.9999% of the observed meteors on Earth are recorded with radar or by looking at radio signatures. So we just require Venusians with sufficient technology, or with senses in an unusual part of the EM spectrum.

[neufer]

Interestingly, the comet's orbit intersects the orbit of Venus, so it may have created a new meteor shower on that planet.

Unfortunately, Venusians can't experience meteor showers (or even acid rain showers, for that matter).>>

99.9999% of the observed meteors on Earth are recorded with radar or by looking at radio signatures. So we just require Venusians with sufficient technology, or with senses in an unusual part of the EM spectrum.

<<Venus has an extended ionosphere located at altitudes 120–300 km. The ionosphere almost coincides with the thermosphere. The high levels of the ionization are maintained only over the dayside of the planet. Over the nightside the concentration of the electrons is almost zero. The ionosphere of Venus consists of three layers: v1 between 120 and 130 km, v2 between 140 and 160 km and v3 between 200 and 250 km. The maximum electron volume density (number of electrons in a unit of volume) of 3×10⁵ cm⁻³ [electron plasma critical frequency ~5 MHZ] is reached in the v2 layer near the subsolar point. The upper boundary of the ionosphere (the ionopause) is located at altitudes 220–375 km and separates the plasma of the planetary origin from that of the induced magnetosphere. The main ionic species in the v1 and v2 layers is O₂⁺ ion, whereas the v3 layer consists of O⁺ ions. The ionospheric plasma is observed to be in motion; solar photoionization on the dayside and ion recombination on the nightside are the processes mainly responsible for accelerating the plasma to the observed velocities. The plasma flow appears to be sufficient to maintain the nightside ionosphere at or near the observed median level of ion densities.>>

Up, up, up, up to the Heaviside Layer

---

The Heaviside layer is the E region

---

<<The ionosphere is the ionized part of Earth's upper atmosphere, from about 48 km to 965 km altitude, a region that includes the thermosphere and parts of the mesosphere and exosphere. The ionosphere is ionized by solar radiation. At night the F layer is the only layer of significant ionization present, while the ionization in the E and D layers is extremely low. During the day, the D and E layers become much more heavily ionized, as does the F layer, which develops an additional, weaker region of ionisation known as the F1 layer. The F2 layer persists by day and night and is the main region responsible for the refraction and reflection of radio waves.

As early as 1839, the German mathematician and physicist Carl Friedrich Gauss postulated that an electrically conducting region of the atmosphere could account for observed variations of Earth's magnetic field. Sixty years later, Guglielmo Marconi received the first trans-Atlantic radio signal on December 12, 1901, in St. John's, Newfoundland using a 500 ft kite-supported antenna for reception. The transmitting station in Poldhu, Cornwall, used a spark-gap transmitter to produce a signal with a frequency of approximately 500 kHz [requiring ionospheric electron plasma densities > ~3,000 cm⁻³] and a power of 100 times more than any radio signal previously produced. The message received was three dits, the Morse code for the letter S. To reach Newfoundland the signal would have to bounce off the ionosphere twice. Marconi achieved transatlantic wireless communications in Glace Bay, Nova Scotia, one year later.

In 1902, Oliver Heaviside proposed the existence of the Kennelly–Heaviside layer of the ionosphere which bears his name. Heaviside's proposal included means by which radio signals are transmitted around the Earth's curvature. Heaviside's proposal, coupled with Planck's law of black-body radiation, may have hampered the growth of radio astronomy for the detection of electromagnetic waves from celestial bodies until 1932 (and the development of high-frequency radio transceivers). Also in 1902, Arthur Edwin Kennelly discovered some of the ionosphere's radio-electrical properties:

The critical frequency is the limiting frequency at or below which a radio wave is reflected by an ionospheric layer at vertical incidence. If the transmitted frequency is higher than the plasma frequency of the ionosphere, then the electrons cannot respond fast enough, and they are not able to re-radiate the signal. It is calculated as shown below:

• Electron plasma critical frequency = sqrt{N} x 9kHZ (where N = electron density per cm³).

Objects in the Solar System that have appreciable atmospheres (i.e., all of the major planets and many of the larger natural satellites) generally produce ionospheres. Planets known to have ionospheres include Venus, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto. The atmosphere of Titan includes an ionosphere that ranges from about 880 km to 1,300 km in altitude and contains carbon compounds. Ionospheres have also been observed at Io, Europa, Ganymede, and Triton.>>

<<The earliest direct observation of interaction between meteors and radio propagation was reported in 1929 by Hantaro Nagaoka of Japan. In 1931, Greenleaf Pickard noticed that bursts of long-distance propagation occurred at times of major meteor showers. At the same time, Bell Labs researcher A. M. Skellett was studying ways to improve night-time radio propagation, and suggested that the oddities that many researchers were seeing were due to meteors. The next year Schafer and Goodall noted that the atmosphere was disturbed during that year's Leonid meteor shower, prompting Skellett to postulate that the mechanism was reflection or scattering from electrons in meteor trails. In 1944, while researching a radar system that was "pointed up" to detect the V-2 missiles falling on London, James Stanley Hey confirmed that the meteor trails were in fact reflecting radio signals. In 1946 the US Federal Communications Commission (FCC) found a direct correlation between enhancements in VHF (30 to 300 MHz) radio signals and individual meteors.

The first serious effort to utilize this technique was carried out by the Canadian Defence Research Board in the early 1950s. Their project, "JANET" (named for Janus, who looked both ways), sent bursts of data pre-recorded on magnetic tape from their radar research station in Prince Albert, Saskatchewan to Toronto, a distance exceeding 2,000 km. A 90 MHz "carrier" signal was monitored for sudden increases in signal strength, signalling a meteor, which triggered a burst of data. The system was used operationally starting in 1952, and provided useful communications until the radar project was shut down around 1960

As the Earth moves along its orbital path, millions of particles known as meteoroids enter the Earth's atmosphere every day, a small fraction of which have properties useful for point-to-point communication. When these meteoroids begin to burn up, they create a glowing trail of ionized particles (called a meteor) in the E layer of the atmosphere that can persist for up to several seconds. The ionization trails can be very dense and thus used to reflect VHF (30 to 300 MHz) radio waves. The frequencies that can be reflected by any particular ion trail are determined by the intensity of the ionization created by the meteor, often a function of the initial size of the particle, and are generally between 30 MHz and 50 MHz [electron densities: 11x10⁶ to 31x10⁶ cm⁻³].

The distance over which communications can be established is determined by the altitude at which the ionization is created, the location over the surface of the Earth where the meteoroid is falling, the angle of entry into the atmosphere, and the relative locations of the stations attempting to establish communications. Because these ionization trails only exist for fractions of a second to as long as a few seconds, they create only brief windows of opportunity for communications.>>

• The Department of Physics and Astronomy: Meteor Physics

<<Radio waves can reflect off ionized trails left behind as meteoroids ablate in the atmosphere. As the meteoroid moves through the atmosphere collisions with air molecules produce ions and electrons along the trail. The electrons are small enough to respond to the incident radio waves by vibrating themselves as dipole radiators. Provided the trail is small compared to the radio wave, the electrons will tend to reflect back to the radar in phase and produce a strong specular signal. This specular reflection implies that only that portion of the trail at right angles to the local apparent meteor radiant will contribute to the returned signal.

This specular scattering condition implies that meteors coming from a particular direction (a single radiant) will only be detected by the radar if they occur in a plane that has the radiant direction as the normal to the plane. When this echo plane constraint is combined with the ablation height of typical meteors (80 - 110 km), their intersection of the two produces an echo surface, where all radar meteor echoes from one radiant must occur as seen from the main radar site.

As seen from the main radar site, all meteors from a single meteor shower (having one radiant) will lie on this great circle. An example, shown below, for the Geminids, shows all echos from the shower detected over a one hour interval as a function of echo arrival azimuth and zenith distance. Here black dots show measured echo locations and open circles are theoretical echo line based on assumed radiant location and single height of ablation. Details of how this specular condition can be used to compute effective radar collecting areas can be found here.

The Western meteor physics group operates a triple-frequency, meteor orbital radar 100 km from London (near Tavistock, Ontario) where we record ~2500 meteoroid orbits per day. CMOR is a multi-frequency HF/ VHF radar used to detect the ionized trails associated with ablating meteoroids. It has been in single-station operation (echoes) since 1999 and multi-station (orbits) since January of 2002. The radar produces data on the range, angle of arrival, and velocity/orbit in some instances. To the end of 2009 we have measured 4 million individual orbits.>>