Category Archives: Comets

A blue comet

Hi there!

Today we discuss about the comet C/2016 R2. This comet has been discovered in September 2016, i.e. some 20 months before its closest approach to the Sun (passage at perihelion). Meanwhile, it heated more and more, and outgassed. This permitted many observations of the comet and its tails, and it appears that this comet is different from the classical ones. In what way? You have to read until the end.
I present The extraordinary composition of the blue comet C/2016 R2 (PanSTARRS), by N.Biver, D.Bockelée-Morvan, G.Paubert, R.Moreno, J.Crovisier, J.Boissier, E.Bertrand, H.Boussier, F.Kugel, A.McKay, N.Dello Russo and M.A.DiSanti. This study has recently been accepted for publication in Astronomy and Astrophysics.

The comet C/2016 R2

This comet has been discovered on September 7, 2016 at 6.3 astronomical units from the Sun, on the telescope Pan-STARRS (Panoramic Survey Telescope And Rapid Response System), which is located on the Haleakalā (Maui, Hawaii, USA). It was then moving toward its perihelion, i.e. toward the Sun. The passage occurred in May 2018, at a distance of 2.06 AU from the Sun. Since then, it is moving away, and it should come back in about 20,000 years… if it comes back. You can find below its orbital elements, taken from the JPL Small-Body Database Browser. These elements are computed from almost 3,000 observations, over 2 years (remember: the comet has been discovered in September 2016).

Semimajor axis 736.30 AU
Eccentricity 0.996
Inclination 58.22°
Perihelion distance 2.60 AU
Period 19,979.85 yr

This is what we call a long-period comets. You have comets of the Jupiter family, with period of a few years, you have comets with periods close to the century, like the very famous Halley comet, which period is 75 years (next passage in July 2061, be ready), and you have long-period comets like this one.

In fact, we have no proof that this comet already visited us 20,000 years ago, and we cannot be sure it will in 20,000 years. Look at the eccentricity: 0.996 is huge for an elliptic orbit, it actually means that the orbit is almost parabolic. In other words, it is close to never come back. Planetary perturbations or even a star passing by could be strong enough to destabilize the orbit of C/2016 R2, and make it leave our Solar System for ever. Conversely, it could make it more stable, and we can even imagine that this comet has an extrasolar origin. It could have been formed around another star, having been ejected from that system, having visited us, and been stabilized around the Sun. Which does not preclude a future ejection.
In fact, it is difficult to know, since such elongated orbits and such small objects are very sensitive to small planetary perturbations.

You can get clues on the origin of a planetary body by studying its composition. This may be pretty easy (or I should say: not that difficult) because

  1. amateurs are usually enthusiastic with comets,
  2. when a comet approaches the Sun, it creates beautiful tails.

When you observe the tails, you have part of the composition.

Spectroscopy of a comet

As discovered Fred Whipple, a comet is a dirty snowball, which is composed of a nucleus, and tails. This has been confirmed in 1986, when the comet 1P/Halley visited us. The nucleus is composed of water ice and silicates.

Approaching the Sun heats the comet, and sublimates its components. This is how it creates 3 tails:

  1. a dusty tail, which is the visible one. It is curved, and located behind the comet,
  2. an ion tail, which is made of gases. Its direction is opposite to the one of the Sun, because it is strongly affected by the Solar wind,
  3. a weak dusty antitail, which points opposite to the dust tail.

And for guessing the composition of these tails (you can also say the coma), you use spectroscopy. Basically:

  1. the Sun lights the tail,
  2. the Solar light is made of a wide spectrum of radiations. The radiations, which are characteristics of the elements present in the coma, are blocked.
  3. you observe the spectrum of the light crossing the coma. From this spectrum, you know which elements are present.

Of course, this is a little more complicated than that. First, you have to consider that the light you observe crosses the atmosphere, which affects it. So, you have to remove this effect. And then, you also have to consider that a cometary activity might be a weak process (depending on the perihelion distance and on the size of the nucleus), and some elements are sometimes observed, sometimes not. Fortunately, several teams have observed the comet, which secures the results.

For observing these spectra, you need dedicated facilities, which do not necessary observe the visible light. Let us present them now.

Many observation facilities

I here restrict to facilities used by the authors of that specific study, but there are many more.

IRAM

IRAM is a French acronym for Institut de RAdioastronomie Millimétrique, or German for Institut für Radioastronomie im Millimeterbereich, or even Spanish for Instituto de Radioastronomía Milimétrica, i.e. institute of millimetric radioastronomy. What does that mean? It means that this facility observes in wavelengths, which are close to the millimeter. Remember that the visible light lies between 430 (for the red) and 700 (for the violet) nanometers, or 0.0007 millimeters. So, the human eye is definitely not sensitive to millimetric emissions. This part of the spectrum approaches the one of the radio waves.

IRAM is based in Grenoble, France, but Germany and Spain are also involved. Beside this, IRAM has worldwide collaborations.

It has 2 observing facilities:

  1. a 30 m radio telescope located on Pico Veleta (Andalucia, Spain),
  2. an interferometer, located on the Plateau de Bure, in the French Alps.

Both sites are at high altitude, i.e. 2,850 m for the Spanish site, and 2,550 m for the French one. IRAM has been created in 1979, and is still upgrading its facilities. Spain joined it in 1990, and the Plateau de Bure Interferometer, which was originally composed of 6 antennae, has now 10 and should have 12 by 2020, in the framework of the project NOEMA, for NOrthern Extended Millimeter Array. Its deployment, i.e. the inauguration of the seventh antenna, started in September 2014.

IRAM's 30m telescope at Pico Veleta © IRAM
IRAM’s 30m telescope at Pico Veleta © IRAM

The authors used the 30-m telescope based in Spain, to target the carbon monoxide, the hydrogen cyanide HCN, and the hydrogen sulfide H2S, at the frequencies 231, 266 and 169 GHz, respectively. The observations were conducted during two evenings, on 23 and 24 January 2018.

Nançay

They also used the Nançay radio telescope. This facility is based in the center of France, and depends partly on Paris Observatory. The radio telescope observes centimetric waves. For that, it is composed of 2 mirrors, one is planar while the other one is spherical. These are mirrors for centimetric waves, i.e. these are actually railings, in which the metal bars are spaced by less than 1 centimeter. It mainly observes the wavelengths 21, 18 and 9 cm, which correspond to hydrogen, hydroxide OH, and the methylidyne radical CH.

The radio telescope, by <a href="//commons.wikimedia.org/wiki/User:Julien_Descloux" title="User:Julien Descloux">Julien Descloux</a> — <span class="int-own-work" lang="fr">Travail personnel</span>, <a href="https://creativecommons.org/licenses/by-sa/3.0" title="Creative Commons Attribution-Share Alike 3.0">CC BY-SA 3.0</a>, <a href="https://commons.wikimedia.org/w/index.php?curid=8339640">URL</a>
The radio telescope, by Julien DesclouxTravail personnel, CC BY-SA 3.0, URL

Beside this, Nançay has also a decametric interferometer composed of 144 antennae, and an antenna belonging to the Low-Frequency Array (LOFAR) network, which observes metric wavelengths.

The authors used the radio telescope between January and March 2018, for about one hour on average every 2 days.

Amateurs observations

These observations were supplemented by optical observations conducted by amateurs, who co-author the study.
The analysis of visible light could permit to detect, for instance, carbon monoxide CO (to confirm millimetric observations), the cyano radical CN, or the dinitrogen N2.

Detected species

First, the overall emission of the comet constrains the temperature of the gas. For C/2016 R2, it should be close to 23 K (-250°C, or -418°F).

Regarding the species: the authors detected a very large production of carbon monoxide CO. This, combined with a very low dust production (with respect to known comets), makes the coma to be blue, instead of tending to be red or yellow. And relatively to CO, there is a strong depletion of water H20, methanol CH3OH, formaldehyde H2CO, hydrogen cyanide HCN, and hydrogen sulfide H2S.

However, and this is very surprising, the authors detected an excess of dinitrogen N2, with a ratio N2/CO close to 0.08. In this case, N2 dominates the nitrogen budget.

Why so much dinitrogen

We don’t know! And this is why it is interesting.

Such an abundance of dinitrogen is very unusual. The only previous detection of dinitrogen was in situ, by Rosetta on 67P/Churyumov-Gerasimenko. In that case the ratio N2/CO was about 0.006, i.e. 10 times lower than for C/2016 R2. And for the other comets: just no detection.

This means that this comet did not form with the other comets. It formed elsewhere. And this makes this comet unique.

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.

Origin of the ecliptic comets

Hi there! Today we discuss the ecliptic comets. You know the comets, these dirty snowballs which show two tails when they approach the Earth (in fact, they have a tail because they approach the Sun). The study I present today, The contribution of dwarf planets to the origin of low-inclination comets by the replenishment of mean motion resonances in debris disks, by M.A. Muñoz-Gutiérrez, A. Peimbert & B. Pichardo, tells us on the dynamical origin of those of these bodies, which have a low inclination with respect to the orbit of the Earth (the ecliptic). Simulations of their own of the primordial debris disk beyond Neptune show that the presence of dwarf planets, like Eris or Haumea, supplies future ecliptic comets. This study has recently been published in The Astronomical Journal.

The dynamics of comets

As I said, comets are dirty snowballs. They are composed of a nucleus, made of ice and silicates. When the comet approaches the Sun, it becomes hot enough to sublimate the ice. This results in two visible tails: a dusty one, and a tail of ionized particles. Beside this, there is a envelope of hydrogen, and sometimes an antitail, which direction is opposite to the dusty tail.

The comets usually have a highly eccentric orbit. As a consequence, there are huge variations of the distance with the Sun, and this is why their activity is episodic. Their temperature increases with the closeness to the Sun, triggering outgassing.

In fact, a moderately eccentric body may be considered to be a comet, if activity is detected. This is for instance the case of the Centaur Chiron. Chiron was detected as an asteroid, and later, observations permitted to detect a cometary activity, even if it does not approach the Sun that much. But of course, this does not make the kind of beautiful comets that the amateur astronomers love to observe.

Regarding the “classical” comets: they have a high eccentricity. What does raise it? The study addresses this question. But before that, let us talk about the ecliptic comets.

The ecliptic comets

The ecliptic comets are comets with a low inclination with respect to the orbital plane of the Earth. In fact, the detections of comets have shown that they may have any inclination. The ecliptic comets are an interesting case, since they are the likeliest to approach the Earth (don’t worry, I don’t mean collision… just opportunities to observe beautiful tails 😉 ).

These low inclinations could suggest that they do not originate from the Oort cloud, but from a closer belt, i.e. the Kuiper Belt. You know, this belt of small bodies which orbits beyond the orbit of Neptune. The reason is that part of this belt has a low inclination.

It also appears that beyond the orbit of Neptune, you have dwarf planets, i.e. pretty massive objects, which are part of the Trans-Neptunian Objects. The authors emphasize their role in the dynamics of low-inclination comets.

Dwarf planets beyond Neptune

A dwarf planet is a planetary object, which does not orbit another planet (unlike our Moon), and which is large enough, to have a hydrostatic shape, i.e. it is pretty spherical. But, this is not one of the planets of the Solar System… you see it is partly defined by what it is not…

5 Solar System objects are officially classified as dwarf planets. 3 of them are in the Kuiper Belt (Pluto, Haumea and Makemake), while the other two are the Main-Belt asteroid Ceres, and Eris, which is a Trans-Neptunian Object, but belongs to the scattered disc. In other words, it orbits further than the Kuiper Belt. The following table presents some characteristics of the dwarf planets of the Kuiper Belt. I have added 4 bodies, which may one day be classified as dwarf planets. Astronomers have advised the IAU (International Astronomical Union) to do so.

Semi-major axis Eccentricity Inclination Orbital period Diameter
Pluto 39.48 AU 0.249 17.14° 248.09 yr 2,380 km
Haumea 43.13 AU 0.195 28.22° 283.28 yr ≈1,500 km
Makemake 45.79 AU 0.159 28.96° 309.9 yr 1,430 km
Orcus 39.17 AU 0.227 20.57° 245.18 yr 917 km
2002 MS4 41.93 AU 0.141 17.69° 271.53 yr 934 km
Salacia 42.19 AU 0.103 23.94° 274.03 yr 854 km
Quaoar 43.41 AU 0.039 8.00° 285.97 yr 1,110 km

Anyway, the dynamical influence of a planetary object does not depend on whether it is classified or not.

These are objects, which have a significant mass, orbiting in the Kuiper Belt. And they are involved in the study.

The Solar System originates from a disc

The early Solar System was probably made of a disk of small bodies, which formed after the gravitational collapse of a huge molecular cloud. Then the Sun accreted, planets accreted, which destabilized most of the remaining small bodies. Some of them where just ejected, some bombarded the Sun and the planets, some other accreted…

Here the authors work with the Kuiper Belt as a disc. So, they assume the 8 major planets to be formed. Moreover, they already have dwarf planets in the disc. And the small bodies, which are likely to become comets, are under the gravitational influence of all this population of larger bodies.

For them to become comets, their eccentricities have to be raised. And an efficient mechanism for that is resonant excitation.

Eccentricity excitation by Mean-Motion Resonances (MMR)

A mean-motion resonance (MMR) between two bodies happens when their orbital periods are commensurate. In the present case, the authors considered the 2:3 and 1:2 MMR with Neptune. The 2:3 resonance goes like this: when Neptune makes 3 orbital revolutions around the Sun, the small object makes exactly 2. And when an object makes one revolution while Neptune makes 2, then this object is at the 1:2 MMR. These two resonances are in the Kuiper Belt disc considered by the authors.

Such period ratios imply that the small bodies orbit much further than Neptune. Neptune orbits at 30.1 AU (astronomical units) of the Sun, so the 2:3 MMR is at 39.4 AU (where is Pluto), and the 2:1 MMR is at 47.7 AU.

When a small body is trapped into a MMR with a very massive one, the gravitational perturbation accumulates because of the resonant configuration. And this interaction is the strongest when the two bodies are the closest, i.e. when the small body reaches its perihelion… which periodically meets the perihelion of the massive perturber, since it s resonant. So, the accumulation of the perturbation distorts the orbit, raises its eccentricity… and you have a comet!

But the issue is: in raising the eccentricities, you empty the resonance… So, either you replenish it, or one day you have no comet anymore… Fortunately, the authors found a way to replenish it.

Numerical simulations

The authors ran different intensive numerical simulations of multiple disc particles, which are perturbed by Neptune and dwarf planets. These dwarf planets are randomly located. They challenged different disc masses, the masses of the dwarf planets being proportional to the total mass of the disc.

And now, the results!

Replenishment of the 2:1 Mean-Motion Resonance (MMR)

The authors found nothing interesting for the 3:2 MMR. However, they found that the presence of the dwarf planets replenishes the 2:1 MMR. So here is the process:

  1. When a particle (a km-size body) is trapped into the 2:1 MMR, its eccentricity is raised
  2. It becomes a comet and may be destabilized. It could also become a Jupiter-family comet, i.e. a comet which period is close to the one of Jupiter. This happens after a close encounter with Jupiter.
  3. Other particles arrive in the resonances, and become comets themselves.

One tenth of the ecliptic comets

The authors also estimated the cometary flux, which this process should create. The authors estimate that it can give up to 8 Jupiter-family comets in 10,000 years, while the observations suggest a ten times larger number.
So, this is a mechanism, but probably not the only one.

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.

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.

Some comets are not randomly distributed

Hi there! Everybody knows the comets, which may show us impressive tails, when they approach our Sun. This is due to what we call cometary activity. You can find comets in many places in the Solar System. Today we will focus on the 9 ones, which are located in the Main Asteroid Belt, i.e. between the orbits of Mars and Jupiter. This is the opportunity to present you Orbital alignment of Main-belt comets, by Yoonyoung Kim, Yougmin JeongAhn, and Henry H. Hsieh. This study has recently been published in The Astronomical Journal.

Comets in the Solar System

A comet is a small body, which presents some activity. This activity manifests as 2 tails, which are a gas tail and a dust tail. These two tails have different directions because the dust is heavier than the gas, and so is differently affected by the Sun. The Sun is actually responsible for at least part of this activity: if the body has water ice at its surface, the proximity of the Sun heats it enough to sublimate it.

We distinguish different classes of comets, from their orbital motion. The short-period comets have a period below 200 years, i.e. they make a close approach to the Sun periodically, with less than 200 years between two approaches. This is for instance the case of the famous Halley comet, or 1P/Halley, which period is 75 years. The comets with a period smaller than 20 years are called Jupiter-family comets, their orbits are strongly affected by the gravitational perturbation of Jupiter.
And we also have long-period comets, with periods larger than 200 years, up to several thousands of years, or even more… The extreme case is the one of the parabolic and hyperbolic comets, which eccentricities are close to or larger than 1. In such a case, we just see the comet once.

The Jupiter-family comets should originate from the Kuiper Belt, and have been so strongly perturbed by Jupiter that their semimajor axes became much smaller, reducing their orbital periods. However, we attribute the origin of the longer periods comets to the Oort cloud, which is thought to lie between 50,000 and 200,000 astronomical units (remember: the Sun-Earth distance is 1 AU). The comets we are interested in today are much closer, in the Main-Belt of asteroids.

The Main-Belt Comets

Main Belt Comets (MBCs) are comets, which are located in the asteroid belt. As such, they present some cometary activity. It appears that there is no general agreement on the way to identify them. Some asteroids present an activity, which is mainly driven by dust, and not by sublimation of water ice, so it could be relevant to call them active asteroids instead of comets. But they may have some sublimation driven activity as well.

The first identified MBC is 133P/Elst-Pizarro, which has been discovered in 1979 and is since then identified as an asteroid… and also as a comet since 1996. I mean, this is officially both a comet and an asteroid. The authors considered 9 MBC, there could be a little more of them, since classifying them is not that easy.

The comet Elst-Pizarro seen at La Silla Observatory. © ESO
The comet Elst-Pizarro seen at La Silla Observatory. © ESO

The MBC should originate from the Main-Belt. In this study, we are interested in the orbital dynamics. Let us talk about orbital elements.

Proper, forced, and osculating elements

As I have already told you in a previous post, we usually describe the orbit of a planetary body with 6 orbital elements, which characterize the ellipse drawn by the trajectory.

These orbital elements are

  • the semi-major axes (which would be the distance to the Sun, if the orbit were circular… this remains almost true for slightly eccentric orbits),
  • the mean longitude,
  • the eccentricity of the trajectory (0 means circular, the eccentricity must be smaller than 1 for the orbit to be elliptic),
  • the pericentre, i.e. location of the point of the trajectory, where the distance to the Sun is minimal,
  • the inclination, with respect to a given reference plane,
  • the ascending node, which locates the intersection between the reference plane and the orbit.

We call them osculating elements. These are the elements that the orbit would have at a given time, if it were exactly an ellipse. The real trajectory is very close to an ellipse, actually.

We will just keep in mind the two couples (eccentricity, pericentre), and (inclination, ascending node). Because these variables are coupled: without eccentricity, the pericentre is irrelevant, since the distance Sun-body is constant. And without inclination, the ascending node is irrelevant, since the whole trajectory is in the reference plane.

And these variables are the sums of a proper and a forced component. Imagine you are a MBC. You want to have your own motion around the Sun. This gives you the proper (or free) component, which is actually ruled by your initial conditions, and the interaction with the Sun (what we call the 2-body, or Kepler, problem). Unfortunately for you, there is this big guy perturbing your motion (Jupiter is his name). He is heavy enough to force your motion to follow his. This gives you a forced motion, and the actual motion is the sum of the proper and the forced ones. The forced motion tends to align your pericentre and your ascending node with the ones of Jupiter. The authors studied these motions.

The Main-Belt Comets are clustered

And their conclusions is that the MBC are clustered, in particular the pericentres. They tend to be aligned with the one of Jupiter. This could have been anticipated, but the authors found something more: the MBC are more clustered than the others asteroids, which lie in that region of the outer main-belt.

For quantifying this more clustered, they ran several statistical tests, which I do not want to detail (the Kolmogorov-Smirnov test, the F-test, and the Watson’s U2 test). These tests show that this excess of clustering happened very unlikely by chance. In other words, there is something. And this is more obvious for comets, for which the sublimation activity is overwhelming. This permits the authors to make a link between this activity, i.e. the presence of water ice, and this clustering. And to suggest favorable conditions for the detection of cometary activity for Main-Belt objects.

Where are the other MBCs?

Based on the result that the eccentricities of MBCs are secularly excited by Jupiter, the authors suggest to look for them in the fall night sky, when Jupiter’s perihelion is at opposition.
We would not necessarily be looking for new bodies, but also for cometary activity of already known bodies. Because of the variations of the distance with the Sun, the sublimation of water ice is not a permanent phenomenon. Remember that Elst-Pizarro has been classified as a comet 17 years after its discovery.

Clustering of TNOs suggests the existence of the Planet Nine

I would like to finish with a reminder that the Planet Nine was hinted that way, in 2016. A clustering among the orbits of Trans-Neptunian Objects was statistically proven. Since then, the Planet Nine has not been detected (yet), but other clues have suggested its presence, like the obliquity of the Sun.

More generally, I would say that big objects strongly affect the orbits of small ones, and in observing the small ones, then you can deduce something on the big ones!

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.