Tag Archives: comets

When a comet meets the Solar wind

Hi there! Today, let us talk about the environment of a comet. As you know, a comet is an active body, which emits ionized particles and dust. The Sun itself emits charged particles, which constitute the Solar wind. We discuss today of the interaction between these two emissions. The environment of charged particles around a comet has been measured by the spacecraft Rosetta, and this has motivated modeling these interactions. I present you Solar wind dynamics around a comet: The paradigmatic inverse-square-law model, by M. Saillenfest, B. Tabone, and E. Behar. This study has recently been accepted for publication in Astronomy and Astrophysics.

The spacecraft Rosetta

Let us first speak about the mission Rosetta. Rosetta was a European mission, which orbited the comet 67P/Churyumov–Gerasimenko between 2014 and 2016. It was named after the Rosetta Stone, which permitted the decipherment of Egyptian hieroglyphs. The mission Rosetta was supposed to give us clues on the primordial Solar System, i.e. on our origins, from the study of a comet.

It was launched in March 2004 from Kourou (French Guiana), and then started a 10-years journey, during which it made 3 fly-bys of the Earth and one of Mars. You can say: “why going back to Earth?” The reason is that Rosetta was supposed to orbit 67P/Churyumov–Gerasimenko (spoiler alert: it did it). For this orbital insertion to be possible, it had to arrive slowly enough… but also had to leave Earth fast enough, to get rid off its attraction, and also to shorten the journey. Fly-bys permitted to slow the spacecraft in exchanging energy with the Earth (or Mars).

Rosetta also visited two asteroids: (2867) Šteins, and (21) Lutetia, in September 2008 and July 2010, respectively. It was inserted into orbit around 67P in August 2014, released the lander Philae in November, and the mission ended in September 2016. In particular, Rosetta was present when 67P reached its perihelion in August 2015. At this point, the comet was at its closest distance to the Sun (1.25 astronomical unit, while its mean distance is almost thrice this number), where the cometary activity is maximal.

The asteroids (2867) Šteins (left) and (21) Lutetia (right), seen by Rosetta. © ESA
The asteroids (2867) Šteins (left) and (21) Lutetia (right), seen by Rosetta. © ESA

So, Rosetta consisted of two modules: the orbiter itself, and the lander Philae. The orbiter had 11 instruments on board, and the lander 10. These instruments permitted, inter alia, to map the comet and measure its geometry, to constrain its internal structure and its chemistry, and to characterize its environment.

This environment is strongly affected by the Solar wind, especially in the vicinity of the perihelion, but not only.

The Solar wind

The Solar corona emits a stream of charges particles, which is mainly composed of electrons, protons, and alpha particles (kind of charged helium). This emission is called Solar wind. It is so energetic, that the emitted particles go far beyond the orbit of Pluto, constituting the heliosphere. The heliosphere has the shape of a bubble, and its boundary is called the heliopause. Voyager 1 crossed it in August 2012, at a distance of 121 AU of the Sun. At the heliopause, the pressure of the Solar wind is weak enough, to be balanced by the one of the interstellar medium, i.e. the winds from the surrounding stars. Hence, Voyager 1 is in this interstellar space, but technically still in the Solar System, as under the gravitational attraction of the Sun.

Anyway, our comet 67P/Churyumov-Gerasimenko is much closer than that, and has to deal with the Solar wind. Let us see how.

The physics of the interaction

Imagine you are on the comet, and you look at the Sun… which should make you blind. From that direction comes a stream of these charged particles (the Solar wind), and you can consider that their trajectories are parallel if far enough from the comet. Of course, the Sun does not emit on parallel trajectories, i.e. the trajectories of all these particles converge to the Sun. But from the comet, the incident particles appear to arrive on parallel trajectories.

While a charged particle approaches the comet, it tends to be deflected. Here, the dominating effect is not the gravitation, but the Lorentz force, i.e. the electromagnetic force. This force is proportional to the electric charge of the particle, and also involves its velocity, and the electric and magnetic fields of the comet.

The authors showed in a previous paper that the trajectories of the charged particles could be conveniently described in assuming that the magnetic field obeys an inverse-square law, i.e. its amplitude decreases with the square of the distance to the comet. If you are twice further from the comet, then the magnetic field is four times weaker.

I do not mean that the magnetic field indeed obeys this law. It is in fact more complex. I just mean that if you model it with such an ideal law, you are accurate enough to study the trajectories of the Solar wind particles. And this is what the authors did.

By the way, the authors suggest that any magnetic field following an inverse-power law could work. Of course, the numbers would have been different, but the global picture of the trajectories would be pretty much the same. It seems, at this time, too challenging to determine which of these models is the most accurate one.

Reducing the problem

The authors used analytical calculations, i.e. maths, which are in fact close to the classical ones, you make to show that the gravitation results in elliptic, parabolic, or hyperbolic, trajectories.

A wonderful tool assisting such studies is the First Integrals. A First Integral is a quantity, which remains constant all along a trajectory. For instance, in a gravitational problem where no energy is dissipated, then the total energy (kinetic + potential energies) is conserved. This is a First Integral. Another First Integral in that problem is the norm of the total angular momentum. And the existence of these two quantities helps to understand the shape of the possible orbits.

The authors showed that this is quite similar here. Even if the equations are slightly different (anyway the inverse-square law is a similarity), they showed that the problems has 2 First Integrals. And from these 2 First Integrals, they showed that knowing only 2 parameters is in fact enough to characterize the trajectories of the Solar wind particles. These two parameters are called rC and rE, they have the physical dimension of a distance, and are functions of all the parameters of the problems. rE characterizes the stream, it is related to its velocity, while rC characterizes a given particle. If you know just these 2 parameters, then you can determine the trajectory.

An empty cavity around the comet

The authors give a detailed description of the trajectories. To make things simple: either the particles orbit the comet, or they just pass by. But anyway, there is an empty space around the comet, i.e. a spherical cavity in which no Solar wind particle enters.

To come: comparison with in situ measurements

The journey of Rosetta around 67P crossed the boundary of this empty cavity. In other words, we have measurements of the density of charged particles at different distances from the comet, and also for different distances from the Sun, since the orbital phase of the mission lasted 2 years, during which 67P orbited the Sun. The authors promise us that a study of the comparison between the model and the in situ measurements, i.e. the observations, is to come. We stay tuned!

Rosetta does not operate anymore, and has landed (or crashed…) on 67P in September 2016. It is still there, and has on-board a kind of modern Rosetta stone. This is a micro-etched pure nickel prototype of the Rosetta disc donated by the Long Now Foundation, as part of its Rosetta Project. The disc was inscribed with 6,500 pages of language translations. This is a kind of time capsule, aiming at preserving part of our culture. Maybe someone will one day find it…

The study and its authors

And that’s it for today! Please do not forget to comment. You can also subscribe to the RSS feed, and follow me on Twitter, Facebook, Instagram, and Pinterest.

An active asteroid

Hi there! Today we will detail a recent study by Jessica Agarwal and Michael Mommert, entitled Nucleus of active asteroid 358P/Pan-STARRS (P/2012 T1). This study has recently been accepted for publication in Astronomy and Astrophysics, and consists in increasing our knowledge of a recently discovered object, i.e. P/2012 T1. This object proved to have some activity, like a comet. The authors realized several observations to try to characterize its activity, and infer some physical properties like its size and its rotation.

Comet vs. active asteroid

First of all, I would like to make clear what is a comet, and what is an active asteroid. I am very ambitious here, since these two notions actually overlap. For instance, our object is both an active asteroid, and a main-belt comet.

Let us say that a comet is an active asteroid, while an active asteroid is not necessarily a comet. The difference is in the nature of the activity.

A comet is a dirty snowball, i.e. you have water ice, and some silicates. Its orbit around the Sun is usually pretty eccentric, so that you have large variations of the distance Sun-object. The location of the orbit, at which the distance is the smallest, is called pericentre. When the comet approaches the pericentre, it approaches the Sun, heats, and part of its water ice sublimates. This results in a dusty tail (actually there are two tails, one being composed of ionized particles).

But when you see dust around a small body, i.e. when you see activity, this is not necessarily ice sublimation. There could be for instance rock excavated by an impact, or material expelled by fast rotation. In that case, you still have an active asteroid, but not a comet. One of the goals of this study is to address the cause and nature of P/2012 T1’s activity.

The asteroid P/2012 T1

P/2012 T1, now named 358P, has been discovered in October 2012 by the Pan-STARRS-1 survey. Pan-STARRS stands for Panoramic Survey Telescope and Rapid Response System, it uses dedicated facilities at Haleakala Observatory, Hawaii, USA.

Discovery of P/2012 T1. © Pan-STARRS
Discovery of P/2012 T1. © Pan-STARRS

Its provisional name, P/2012 T1, contains information on the nature of the object, and its discovery. P stands for periodic comet, 2012 is the year of the discovery, and T means that it has been discovered during the first half of October.

Interestingly, this object appeared on images taken in December 2001 at Palomar Observatory in California, while acquiring data for the survey NEAT (Near-Earth Asteroid Tracking).

You can find below its orbital elements, from the Minor Planet Center:

Semi-major axis 3.1504519 AU
Eccentricity 0.2375768
Inclination 11.05645°
Period 5.59 y

From its orbital dynamics, it is a Main-Belt object. As a comet, it is a Main-Belt Comet.

New observations

Once an object is known and we know where it is, it is much easier to reobserve it. The authors conducted observations of 358P from the Southern Astrophysical Research (SOAR) telescope, and the Very Large Telescope.

The SOAR telescope is based on Cerro Pachón, Chile. This is a 4.1-m aperture facility, located at an altitude of 2,700 m. The authors took images with the Goodman High Throughput Spectrograph during one night, from July 27 to July 28, 2017. They wanted to analyze the reflected light by the asteroid at different wavelengths, unfortunately the observational constraints, i.e. cloud coverage, permitted only two hours of observations. Only the observations made with the VR filter, centered at 610 nm, were useful.

These data were supplemented by 77 images taken during 10 hours from August 17 to August 18, 2017, at the Very Large Telescope. This instrument depends on the European Southern Observatory (ESO), and is located on Cerro Paranal, once more in Chile, at an altitude of 2,635 m. The authors used the FOcal Reducer and low dispersion Spectrograph 2 (FORS2), which central wavelength is 655 nm.

The observations give raw images. The authors treated them to get reliable photometric and astrometric measurements of 358P, i.e. they corrected from the variations of the luminosity of the sky, in using reference stars, and from the possible instrumental problems. For that, they recorded the response of the instrument to a surface of uniform brightness, and used the outcome to correct their images.

Let us now address the results.

Measuring its rotation

Such a small (sub-kilometric) body is not spherical. This results in variations of luminosity, which depend on the surface element which is actually facing your telescope. If you acquire data during several spin periods of the asteroid, then you should see some periodicity in the recorded lightcurve.

The best way to extract the periods is to make a Fourier transform. Your input is the time-dependent lightcurve you have recorded, and your output is a frequency-dependent curve, which should emphasize the periods, which are present in the recorded lightcurve. If the signal is truly periodic, then it should exhibit a maximum at its period and its harmonics (i.e. twice the period, thrice the period, etc.), and almost 0 outside (not exactly 0 since you always have some noise).

In the case of 358P, the authors did not identify any clear period. A maximum is present for a rotation period of 8 hours, but the result is too noisy to be conclusive. A possible explanation could be that we have a polar view of the asteroid. Another possibility is that the albedo of the asteroid (the fraction of reflected light) is almost uniform.

Dust emission

The authors tried to detect debris around the nucleus of the comet, in widening the aperture over which the photometry was performed. They got no real detection, which tends to rule out the possibility of non-cometary activity.

A 530m-large body

Finally, the magnitude of the asteroid is the one of a sphere of 530 meters in diameter, with an albedo of 6%. This means that a higher albedo would give a smaller size, and conversely. The albedo depends on the composition of the asteroid, which is unknown, and can be only inferred from other asteroids. The authors assumed it to be a carbonaceous asteroid (C-type), as 75% of the asteroids. If it were an S-type (silicateous) body, then it would be brighter. A wide band spectrum of the reflected light would give us this information.

The study and its authors

  • You can find the study here, on Astronomy and Astrophysics’ website. Moreover, the authors uploaded a free version on arXiv, thanks to them for sharing!
  • Here is the webpage of the first author, Jessica Agarwal,
  • and here the website of Michael Mommert.

And that’s it for today! Please do not forget to comment. You can also subscribe to the RSS feed, and follow me on Twitter, Facebook, Instagram, and Pinterest.

The activity of Chiron

Hi there! You may have heard of Chiron, which was he first Centaur discovered, in 1977. This minor planet may have rings, and seems to present some cometary activity, which cause needs to be discussed. This is the topic of the present study, i.e. Activity of (2060) Chiron possibly caused by impacts?, by Stefan Cikota, Estela Fernández-Valenzuela, Jose Luis Ortiz, Nicolás Morales, René Duffard, Jesus Aceituno, Aleksandar Cikota and Pablo Santos-Sanz. This study has recently been accepted for publication in The Monthly Notices of the Royal Astronomical Society.

Chiron’s facts

Chiron was the first discovered Centaur, i.e. the first asteroid / small planet, which orbits between the orbits of Saturn and Uranus. It was discovered in 1977, in the sense that it was identified in 1977. But reexamination of past photographic plates show that it has in fact been observed since 1895. And from the reanalysis of the pre-discovery observations, it was easy to determine an orbit.

Discovery 1977
First observation 1895
Apparent magnitude 19
Absolute magnitude 6
Diameter 220 km
Semimajor axis 13.648 AU
Eccentricity 0.3823
Inclination 6.9497°
Orbital period 50.42 yr
Rotation 5.918 h

The orbital period of Chiron is a slightly longer than 50 years, which means that we dispose of astrometric observations over more than 2 periods. This orbit is highly eccentric, which results in large variations of the distance to the Sun, i.e. between 8.43 AU (astronomical units) at perihelion, and 18.86 AU at aphelion.

A spectral analysis of Chiron reveals a C-type, i.e. a carbonaceous, object. Moreover, it shows large variations of brightness, which are considered to be partly due to cometary activity, and partly due to rings. This cometary activity makes that Chiron, officially the asteroid (2060)Chiron, can also be called the comet 95P/Chiron.

Chiron observed at Kuma Kogen Astronomical Observatory, Japan. © 1997 by Akimasa Nakamura
Chiron observed at Kuma Kogen Astronomical Observatory, Japan. © 1997 by Akimasa Nakamura

The presence of rings around Chiron is not unanimously accepted in the scientific community. Unexpected stellar occultations by something orbiting close to Chiron could be interpreted either as cometary jets, or as rings. But the large variations of brightness and the discoveries of rings around Chariklo and Haumea speak for the presence of rings. The discovery of rings around Chariklo was very surprising, and showed that it is possible. The discovery around Haumea has shown that rings around such bodies were not exceptional. So, why not Chiron? In this study, the authors clearly state that they believe in the presence of rings, and they use it to study the brightness of Chiron. These rings would have a radius of 324 ± 10 km, which is inside the estimated Roche limit of Chiron, i.e. the particles constituting the rings could not accrete into a larger body.

But the central point is the cometary activity, i.e. evidence for cometary jets is reported.

Triggering a cometary activity

Classical comets behave this way: these are dirty snowballs, i.e. made of ice, dust, and some other elements. When approaching the Sun, the comet gets so warm that the ice is sublimated. But a Centaur with cometary activity is different, since it does not get closer to the Sun. Moreover, Chiron is essentially carbonaceous. So, another cause has to be found. And in such a case, it is often tempting to invoke impacts.

A problem is that impacts are not that frequent in that region of the Solar System. First because the gravitational action of the Sun tends to focus the orbits of the potential impactors, i.e. they will be more inclined to get closer to the Sun, and second because, the more distant from the Sun you are, the emptier the space appears, this is just a geometrical effect.
The consequences of these effects is that a collision of a 1km-radius comet is expected on a body like Chiron every 60 Gyr… while the age of the Solar System is 4.5 Gyr… quite unlikely.

Photometric observations

Anyway, Chiron is known to have some cometary activity, and the author tracked it from Calar Alto Observatory (CAHA) in Almeria, Spain, during 3 observation campaigns, between 2014 and 2016. The first campaign was primarily devoted to the study of the rotation of Chiron, and consisted of 3 runs in 2014, using the 3.5 and the 1.23 m telescopes. The second campaign was conducted in September 2015 on the 2.2 m telescope, with the CAFOS instrument (Calar Alto Faint Object Spectrograph), and looked for rotation, absolute magnitude, and cometary activity. The third campaign took place on 2016, September 2, to get a better constraint on Chiron’s absolute magnitude, once again with CAFOS.

The authors were particularly interested in the photometry, since cometary jets translate into variations of brightness. For that, they had to correct the variations due to observational constraints, and to the orientation of Chiron.

The 3.5m telescope at Calar Alto Observatory (CAHA). © Alfredo Madrigal
The 3.5m telescope at Calar Alto Observatory (CAHA). © Alfredo Madrigal

Observational constraints are likely to give artificial variations of photometry, since

  • the height of Chiron on the horizon varies, which means that the thickness of the atmosphere varies,
  • the wind might result in unstable images (seeing),
  • the detectors are different, even on the same instrument,etc.

To try to make things as proper as possible, the authors corrected the images from flat fielding, i.e. from the variations of the response of the CCD chip, and they observed a large enough field (at least 16 arcmin), to have the same stars as photometric references.

Regarding the orientation of Chiron, variations of brightness can reveal:

  • the rotation of Chiron, which would present different surface elements to the observer,
  • the orientation of the rings.

These two effects were modeled, to be removed from the photometric measurements. And the result is…

Impacts from the rings

The authors do observe a small cometary activity on Chiron, which is pretty faint. It has actually been stronger in the past, a measurement in 1973 showed a peak with respect to another measurement in 1970, and since then the coma is monotonously decreasing. The authors interpret that as a possible small impact having occurred between 1970 and 1973, the associated coma tail having almost disappeared. This activity appears to be supplemented by a continuous micro-activity, which could be due to impacts by small particles falling from the rings.

The study and its authors

And that’s it for today! Please do not forget to comment. You can also subscribe to the RSS feed, and follow me on Twitter, Facebook, Instagram, and Pinterest.

Breaking an asteroid

Hi there! Asteroids, these small bodies in the Solar System, are fascinating by the diversity of their shapes. This is a consequence of their small sizes. Another consequence is their weakness, which itself helps to split some of them into different parts, sometimes creating binary objects, asteroids families… The study I present you today, Internal gravity, self-energy, and disruption of comets and asteroids, by Anthony R. Dobrovolskis and Donald G. Korycansky, proposes an accurate computation of the required energy to provoke this break-up, at any place of the asteroid, i.e. you are more efficient when you hit at a given location. This study has recently been accepted for publication in Icarus.

Shapes of asteroids

Please allow me, in this context, to call asteroid a comet, a comet being a small body, i.e. like an asteroid, but with a cometary activity. The important thing is that the involved bodies are small enough.

Beyond a given size, i.e. a diameter of ~400 km, a planetary body is roughly spheroidal, i.e. it is an ellipsoid with it two equatorial axes almost equal and the polar one smaller, because of its rotation. For a tidally despun body, like the Moon, or a satellite of a giant planet, the shape is more triaxial, since the tidal (gravitational) action of the parent planet tends to elongate the equatorial plane. The same phenomenon affects Mercury.

However, for smaller bodies, the self-gravitation is not strong enough to make the body look more or less like a sphere. As a consequence, you can have almost any shape, some bodies are bilobate, some are contact binaries, i.e. two bodies which permanently touch together, some others are rubble piles, i.e. are weak aggregates of rocks, with many voids.

These configurations make these bodies likely to undergo or have undergone break-up. This can be quantified by the required energy to extract some material from the asteroid.

The energies involved

For that, an energy budget must be performed. The relevant energies to consider are:

  • The impact disruption energy: the minimum kinetic energy of an impactor, to shatter the asteroid and remove at least half of its mass,
  • The shattering energy: the minimum energy needed to shatter the asteroid into many small pieces. It is part of the impact disruption energy. This energy is roughly proportional to the mass of the asteroid. It represents the cohesion between the adjacent pieces.
  • The binding energy: this energy binds the pieces constituting the asteroid. In other words, once you have broken an asteroid (don’t try this at home!), you have to make sure the pieces will not re-aggregate… because of the binding energy. For that, you have to bring enough energy to disperse the fragments.
  • The self-gravitational energy: due to the mutual gravitational interaction between the blocks constituting the asteroids. Bodies smaller than 1 km are strength-dominated, i.e. they exist thanks to the cohesion between the blocks, which is the shatter energy. However, larger bodies are gravity-dominated.
  • The kinetic energy of rotation: the spin of these bodies tends to enlarge the equatorial section. In that sense, it assists the break-up process.

This study addresses bodies, which are far enough from the Sun. This is the reason why I do not mention its influences, i.e. the tides and the thermic effects, which could be relevant for Near-Earth Objects. In particular, the YORP effect is responsible for the fission of some of them. I do not mention the orbital kinetic energy of the asteroid either. Actually the orbital motion is part of the input energy brought by an impact, since the relative velocity of the impactor with respect to the target is relevant in this calculation.

I now focus on the two cases studied by the authors to illustrate their theory: the asteroid Kleopatra and the comet 67P/Churyumov-Gerasimenko.

2 peculiar cases: Kleopatra and Churyumov-Gerasimenko

216 Kleopatra is a Main-Belt asteroid. Adaptive optics observations have shown that is is constituted of two masses bound by material, giving a ham-bone shaped. As such, it can be considered as a contact binary. It is probably a rubble pile. Interestingly, observations have also shown that Kleopatra has 2 small satellites, Alexhelios and Cleoselene, which were discovered in 2008.

Reconstruction of the shape of Kleopatra. © NASA
Reconstruction of the shape of Kleopatra. © NASA

However, 67P Churyumov-Gerasimenko is a Jupiter-family comet, i.e. its aphelion is close to the orbit of Jupiter, while its perihelion is close to the one of the Earth. It has an orbital period of 6.45 years, and was the target of the Rosetta mission, which consisted of an orbiter and a lander, Philae. Rosetta orbited Churyumov-Gerasimenko between 2014 and 2016. The shape of this comet is sometimes described as rubber ducky, with two dominant masses, a torso and a head, bound together by some material, i.e. a neck.

Churyumov-Gerasimenko seen by Rosetta. © ESA
Churyumov-Gerasimenko seen by Rosetta. © ESA
216 Kleopatra 67P/Churyumov-Gerasimenko
Semimajor axis 2.794 AU 3.465 AU
Eccentricity 0.251 0.641
Inclination 13.11° 7.04°
Spin period 5.385 h 12.761 h
Mean radius 62 km 2.2 km
Magnitude 7.30 11.30
Discovery 1880 1969

The irregular shapes of these two bodies make them interesting targets for a study addressing the gravitation of any object. Let us see now how the authors addressed the problem.

Numerical modeling

Several models exist in the literature to address the gravity field of planetary bodies. The first approximation is to consider them as spheres, then you can refine in seeing them as triaxial ellipsoids. For highly irregular bodies you can try to model them as cuboids, and then as polyhedrons. Another way is to see them as duplexes, this allows to consider the inhomogeneities dues to the two masses constituting bilobate objects. The existence of previous studies allow a validation of the model proposed by the authors.

And their model is a finite-element numerical modeling. The idea is to split the surface of the asteroid into small triangular planar facets, which should be very close to the actual surface. The model is all the more accurate with many small facets, but this has the drawback of a longer computation time. The facets delimit the volume over which the equations are integrated, these equations giving the local self-gravitational and the impact disruption energies. The authors also introduce the energy rebate, which is a residual energy, due to the fact that you can remove material without removing half of it. This means that the impact disruption energy, as it is defined in the literature, is probably a too strong condition to have extrusion of material.
The useful physical quantities, which are the gravitational potential, the attraction, and the surface slope, are propagated all along the body thanks to a numerical scheme, which accuracy is characterized by an order. This order quantifies the numerical approximation which is made at each integration step. A higher order is more accurate, but is computationally more expensive.

Once the code has been run on test cases, the authors applied it on Kleopatra and Churyumov-Gerasimenko, for which the shape is pretty well known. They used meshes of 4,094 and 5,786 faces, respectively.

Results

The validation phase is successful. The authors show that with a 3rd order numerical scheme, they recover the results present in the literature for the test cases with an accuracy of ~0.1%, which is much better than the accuracy of the shape models for the real asteroids. Regarding Kleopatra and Churyumov-Gerasimenko, they get the gravity field at any location, showing in particular excesses of gravity at the two lobes.

Such a study is particularly interesting for further missions, which would determine the gravity field of asteroids, which would then be compared with the theoretical determination by this code. Other applications are envisaged, the authors mentioning asteroid mining.

The study and its authors

And that’s it for today! Please do not forget to comment. You can also subscribe to the RSS feed, and follow me on Twitter and Facebook.

And Merry Christmas!

A quest for sources of meteor showers

Hi there! Today I will present you a study entitled Dynamical modeling validation of parent bodies associated with newly discovered CMN meteor showers, by D. Šegon, J. Vaubaillon, P. Gural, D. Vida, Ž. Andreić, K. Korlević & I. Skokić, which has recently been accepted for publication in Astronomy and Astrophysics. It addresses the following question: when you see meteors, where do they come from?

The meteor showers

Imagine you have a comet, i.e. a small body, which wanders in the Solar System with a large eccentricity. This means that it orbits around the Sun, but with large variations of its distance with the Sun. The consequence is that it experiences large variations of temperature during its journey. In particular, when it reaches the perihelion, i.e. when its distance to the Sun is the smallest, the temperature is so hot that it outgasses. The result is the ejection of a cloud of small particles, which itself wanders in the Solar System, on its own orbit.

When the Earth meets it, then these particles are burnt in our atmosphere. This results in meteor showers. Such showers can be sporadic, or happen every year if the cloud is pretty static with respect to the orbit of the Earth. The body from which the particles originate is called the parent body. The study I present you today aims at identifying the parent body of some of these meteor showers.

How to observe them

Understanding the meteor showers is an issue for the safety of the Earth environment, particularly our artificial satellites. Some meteors can even impact the surface of the Earth. This is why numerous observation programs exist, and for that amateurs are very helpful!

The first way to observe meteor showers is visually. When you know that meteor showers are likely to happen, you look at the sky and take note of the meteors you see: when you saw it, from where, where it came from, its magnitude (~its brightness), etc. The point from where the meteor seems to come is called the radiant. It is written as a set of two angles α and δ, i.e. right ascension and declination, which localize it on the celestial sphere.

For unpredicted showers, we can use cameras, which continually observe and record the sky. Then, algorithms of image processing can detect the meteor. Meteors can also be detected in the radio wavelengths.

Dynamical modeling

If you want to simulate the orbit of a particle, you have to consider:

  • the location of the parent body when the particle was ejected (initial position),
  • the ejection velocity,
  • the ejection time, likely when the parent body was close to its perihelion. The question how close? cannot be accurately answered,
  • the gravitational action of the Sun and the planets of the Solar System,
  • the non-gravitational forces, which might have a strong effect on such small bodies.

These non-gravitational forces, here the Poynting-Robertson drag, are due to the Solar radiation, which causes a loss of angular momentum of the particle during its orbital journey around the Sun. It is significant for particles smaller than the centimeter, which is often the case for such ejecta.

You cannot simulate the orbit of a specific particle that you would have identified before, just because they are too small to be observed as individuals. However, you can simulate a cloud, composed of a synthetic population of fictitious particles, with various sizes, ejection times, initial velocities… in such a way that your resulting cloud will have global properties which are close to the real cloud of ejecta. Then you can simulate the evolution of the cloud with time, and in particular determine the time, duration, direction, and intensity of a meteor shower.

Simulating such a cloud reveals interesting dynamical features. It presents an initial size, because of the variations in the ejection times of the particles. But it also widens with time, since the particles present different ejection velocities. This usually (but not always!) results in a kind of a tire which enshrouds the whole orbit of the parent body. Unfortunately, it can be observed only when the Earth crosses it. So, simulating the behavior of the cloud will tell you when the Earth crosses it, how long the crossing lasts, and the density of the cloud during the crossing.
It should be kept in mind that a cloud is composed of a hyue number of particles. For this reason, dedicated computation means are required.

This study

This study aims at identifying the parent body of meteor showers, which were detected by the Croatian Meteor Network (CMN in the title). For that, the first step is to make sure that a shower is a shower.
The detected meteors should resemble enough, which can be measured with the D-criteria, that are a measurement of a distance, in a given space, between the orbits of two objects. Once a meteor shower is identified, the same D-criteria can be used to try to identify the parent body, from its orbit. The parent bodies are comets or asteroids, they are usually known enough for candidates to be determined. And once candidates are identified, then their outgassing is simulated, to predict the meteor showers associated. If a calculated meteor shower is close enough to an observed one, then it is considered that the parent body has been successfully identified. This last close enough is related to the time and duration of the showers, and the location of the radiants.

The authors analyzed 13 meteor showers, and successfully identified the parent body for 7 of them. Here is their list, the showers are identified under their IAU denominations:

  • #549FAN – 49 Andromedids comes from the comet 2001 W2 Batters,
  • #533 JXA – July ξ Arietids comes from the comet 1964 N1 Ikeya,
  • #539 ACP – α Cepheids comes from the comet 255P Levy,
  • #541 SSD – 66 Draconids comes from the asteroid 2001 XQ,
  • #751 KCE – κ Cepheids comes from the asteroid 2009 SG18,
  • #753 NED – November Draconids comes from the asteroid 2009 WN25,
  • #754 POD – ψ Draconids comes from the asteroid 2008 GV.

For this last stream, the authors acknowledge that another candidate parent body has not been investigated: the asteroid 2015 FA118.

For the 6 other cases, either the identification of a parent body is speculated but not assessed enough, or just no candidate has been hinted, possibly because it is an asteroid or a comet which has not discovered yet, and / or because data are missing on the meteor shower.

Some links

That’s it for today! As usual, I accept any comment, feel free to post!