Monthly Archives: October 2017

Interstellar objects

An interstellar asteroid

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Hi there! You may have heard this week of our Solar System visited by an asteroid probably formed in another planetary system. This is why I have decided to speak about it, so this article will not be based on a peer-reviewed scientific publication, but on good science anyway. The name of this visitor is for now A/2017 U1.

Outline

History of the discovery
What are these objects?
Maybe not the first one
Detecting interstellar objects
The press release and its authors

History of the discovery

Discovering a new object usually consists in

  1. Taking a picture of a part of a sky. Usually these are parts of the order of the degree, maybe much less… so, small parts. And this also requires to treat the image, to correct for atmospheric (brightness of the sky, wind,…) and instrumental (dead pixels…) effects,
  2. Comparing in with the objects, which are known to be in that field.

If there is an unexpected object, then it could be a discovery. Here is the history of the discovery of A/2017 U1:

  • Oct. 19, 2017: Robert Weryk, a researcher of the University of Hawaii, discovers a new object while searching for Near-Earth Asteroids with the Pan-STARRS 1 telescope. An examination of images archives revealed that the object had already been photographed the day before.
  • Oct. 25, 2017: The Minor Planet Center (Circular MPEC 2017-U181) gives orbital elements for this new object, from 34 observations over 6 days, from Oct. 18 to 24. Surprisingly, an eccentricity bigger than 1 (1.1897018) is announced, which means that the trajectory follows a hyperbola. This means that if this object would be affected only by the Sun, then it would come from an infinite distance, and would leave us for infinity. In other words, this object would not be fated to remain in our Solar System. That day, the object was thought to be a comet, and named C/2017 U1. 10 observation sites were involved (once an object has been detected and located, it is easier to re-observe it, even with a smaller telescope).
  • Oct. 26, 2017: Update by the Minor Planet Center (Circular MPEC 2017-U185), using 47 observations from Oct. 14 on. The object is renamed A/2017 U1, i.e. from comet “C” to asteroid “A”, since no cometary activity has been detected. Same day: the press release announcing the first confirmed discovery of an interstellar object. New estimation of the eccentricity: e = 1.1937160.
  • Oct. 27, 2017: Update by the Minor Planet Center (Circular MPEC 2017-U234), using 68 observations. New estimation of the eccentricity: e = 1.1978499.

And this is our object! It has an absolute magnitude of 22.2 and a diameter probably smaller than 400 meters. These days, spectroscopic observations have revealed a red object, alike the KBOs (Kuiper Belt Objects). It approached our Earth as close as 15 millions km (0.1 astronomical unit), i.e. one tenth of the Sun-Earth distance.

The trajectory of A/2017 U1.
The trajectory of A/2017 U1.

What are these objects?

The existence of such objects is predicted since more than 40 years, in particular by Fred Whipple and Viktor Safronov. This is how they come to us:

  1. A protoplanetary disk creates a star, planets, and small objects,
  2. The small objects are very sensitive to the gravitational perturbations of the planets. As a consequence, they may be ejected from their planetary system, and become interstellar objects,
  3. They visit us.

Calculations indicate that A/2017 U1 comes roughly from the constellation Lyra, in which the star Vega is (only…) at 25 lightyears from our Sun. It is tempting to assume that A/2017 U1 was formed around Vega, but that would be only speculation, since many perturbations could have altered its trajectory. Several studies will undoubtedly address this problem within next year.

Maybe not the first one

Here we have an eccentricity, which is significantly larger (some 20%) than 1. Moreover, our object has a very inclined orbit, which means that we can neglect the perturbations of its orbit by the giant planets. In other words, it entered the Solar System on the trajectory we see now. But a Solar System object can get a hyperbolic orbit, and eventually be ejected. This means that when we detect an object with a very high eccentricity, like a long-period comet, it does not necessary mean that it is an interstellar object. In the past, some known objects have been proposed to be possible interstellar ones. This is for example the case for the comet C/2007 W1 (Boattini), which eccentricity is estimated to be 1.000191841611794±0.000041198 at the date May 26, 2008. It could be an IC (Interstellar Comet), but could also be an Oort cloud object, put on a hyperbolic orbit by the giant planets.

Detecting interstellar objects

A/2017 U1 object has been detected by the Pan-STARRS (for Panoramic Survey Telescope and Rapid Response System) 1 telescope, which is located at Haleakala Observatory, Hawaii. Pan-STARRS is constituted of two 1.8 m Ritchey–Chrétien telescopes, with a field-of-view of 3°. This is very large compared with classical instruments, and it is suitable for detection of bodies. It operates since 2010.

Detections could be expected from the future Large Synoptic Survey Telescope (LSST), which should operate from 2022 on. This facility will be a 8.4-meter telescope based in Chile, and will conduct surveys with a field-of-view of 3.5°. A recent study by Nathaniel Cook et al. suggests that LSST could detect between 0.001 and 10 interstellar comets during its nominal 10 year lifetime. Of course, 0.001 detection should be understood as the result of a formula. The authors give a range of 4 orders of magnitude in their estimation, which reflects how barely constrained the theoretical models are. This also means that we could be just lucky to have detected one.

What Pan-STARRS can do, LSST should be able to do. In a few years, i.e. in the late 2020s, the number or absence of new discoveries will tell us something on the efficiency of creation of interstellar objects in the nearby stars. Meanwhile, let us enjoy this exciting discovery!

The press release and its authors

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.

Satellites of Jupiter

The composition of Himalia, Elara, and Carme

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Hi there! Today I tell you on 3 irregular satellites of Jupiter, you know, these small bodies which orbit very far from the planet. Himalia, Elara and Carme have been observed in the Near-InfraRed (NIR), and this gave Composition of Jupiter irregular satellites sheds light on their origin, by M. Bhatt et al., which has been recently accepted for publication in Astronomy and Astrophysics.

Outline

The irregular satellites of Jupiter
Himalia, Elara, and Carme
Spectroscopic observations
Detection of minerals
Himalia and Elara are alike, Carme is different
The study and the authors

The irregular satellites of Jupiter

Jupiter has 69 known satellites, which we can divide into 3 groups:

  1. The 4 Galilean satellites Io, Europa, Ganymede and Callisto. These are large bodies, discovered in 1610 by Galileo Galilei,
  2. The 4 inner satellites Amalthea, Metis, Adrastea, and Thebe. These are small bodies, orbiting inside the orbit of Io,
  3. The irregular satellites, which orbit very far from Jupiter. These are small bodies as well, which are usually thought to have been captured, i.e. they probably not formed in the protojovian nebula.

Contrary to the inner and the Galilean satellites, the irregular satellites have pretty eccentric and inclined orbits. Their eccentricities may exceed 0.4, and most of them are retrograde, i.e. with an inclination larger than 90°. In fact, plotting their inclination vs. their semimajor axes reveals clustering.

Semimajor axes and inclinations of the irregular satellites of Jupiter. The inclinations are given with respect to the ecliptic.
Semimajor axes and inclinations of the irregular satellites of Jupiter. The inclinations are given with respect to the ecliptic.

At least 4 dynamical groups have been defined, all of them being named after the largest of their members:

  1. The Himalia group is made of prograde bodies, with inclinations between 26.6° and 28.3°, eccentricities between 0.11 and 0.25, and semimajor axes between 159 and 176 Jupiter radii (while Callisto orbits at 27 Jupiter radii),
  2. The Ananke group is composed of bodies with inclinations between 145.7° and 154.8°, eccentricities between 0.02 and 0.28, and semimajor axes between 250 and 305 Jupiter radii,
  3. The Pasiphase group is made of bodies with inclinations between 144.5° and 158.3°, eccentricities between 0.25 and 0.43, and semimajor axes between 320 and 350 Jupiter radii,
  4. The Carme group is made of bodies with inclinations between 164.9° and 165.5°, eccentricities between 0.23 and 0.27, and semimajor axes between 329 and 338 Jupiter radii

The clustering among these bodies suggests a common origin, i.e. a group of objects would have a unique progenitor. It is also interesting to notice that some groups are more dispersed than others. In particular, the dispersion of the Carme group is very limited. This could tell us something on the date of the disruption of the progenitor. Another clue regarding a common origin is the composition of these bodies.

Before addressing our 3 objects of interest, i.e. Himalia, Elara (member of the Himalia group), and Carme, I would like to mention Themisto and Carpo, which seem to be pretty isolated, and so would not share a common origin with the other bodies. Their dynamics might be affected by the Kozai-Lidov mechanism, which induces a correlated periodic evolution of their eccentrities and inclinations.

Himalia, Elara, and Carme

These 3 bodies are the ones addressed in this study. You can find below their relevant characteristics.

Semimajor axis Eccentricity Inclination Discovery Radius Albedo
Himalia 163.9 Rj 0.16 27.50° 1904 70-80 km 0.04
Elara 167.9 Rj 0.22 26.63° 1905 43 km 0.04
Carme 334.7 Rj 0.25 164.91° 1938 23 km 0.04

These were among the first known irregular moons of Jupiter. The inclinations are given with respect to the ecliptic, i.e. the orbital plane of the Earth. As a member of the Himalia group, Elara has similar dynamical properties with Himalia. We can also notice the small albedo of these bodies, i.e. of the order of 4%, which means that only 4% of the incident Solar light is reflected by the surface! In other words, these bodies are very dark, which itself suggests a carbonaceous composition. Spectroscopic observations permit to be more accurate.

Spectroscopic observations

These bodies were observed in the near infrared, at wavelengths between 0.8 and 5.5 μm. The observations were made at the IRTF (InfraRed Telescope Facility), located on the Mauna Kea (Hawai’i), with the SpeX spectrograph, during 4 nights, in 2012 and 2013. In measuring the light flux over a specific range of the spectrum, one can infer the presence of some material, which would absorb the light at a given wavelength. For that, we need to be accurate in the measurements, while the atmospheric conditions might alter them. This difficulty is by-passed by the presence of a star in the field, which serves as a reference for the measured light flux.

Detection of minerals

Once a spectrum reflectance vs. wavelength is obtained, it needs to be interpreted. In this study, the authors assumed that the observed spectra were a mixture of the spectra given by different minerals, which have been obtained in laboratories. They disposed of a database of 30 minerals, and fitted mixtures involving 4 of them, to the obtained spectra. This is an optimization algorithm, here named Spectral Mixture Analysis, which fits the relative proportion of the minerals. 4 minerals is actually the best they could obtain, i.e. they failed to produce a significantly better fit in adding a 5th mineral.

In other words, from the absorption spectrum of such a body, you can guess its 4 main components… at least of the surface.

Himalia and Elara are alike, Carme is different

Well, the title contains the conclusion. This is not very surprising, since Himalia and Elara belong to the same group. We can say that the composition confirms the guess that they should have a common origin. Previous studies gave the same conclusions.

In this specific case, Himalia and Elara have a peak of absorption centered around 1.2 μm, and their spectra are similar to C-type, i.e. carbonaceous, asteroids (52) Europa and (24) Themis, of the outer asteroid belt. The best match for Himalia is obtained with a mixture of magnetite and ilmenite, both being iron oxides, with minnesotaite, which is a ferric phyllosilicate. Elara seems to have a similar composition, but the match is not that good. In particular, the spectrum is more dispersed than for Himalia, and a little redder.

Carme has a different spectrum, with a peak of absorption centered around 1.6 μm, and is probably composed of black carbon, minnesotaite, and ilmenite. Another study has proposed that Carme could have a low-level cometary activity, but that would require to observe it at shorter wavelengths. Out of the scope of this study.

The study and the authors

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.

Asteroids: Near-Earth Objects

Indirect measurement of an asteroid’s pole

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Hi there! Today, another paper on the Yarkovsky effect. You know, this non-gravitational force which acts on the asteroid, especially if it is close enough to the Sun. After reading this post, you will know how it can reveal us the obliquity of an asteroid. I present you Constraints on the near-Earth asteroid obliquity distribution from the Yarkovsky effect, by C. Tardioli, D. Farnocchia, B. Rozitis, D. Cotto-Figueiroa, S.R. Chesley, T.S. Statler & M. Vasile. This paper has recently been accepted for publication in Astronomy and Astronomy.

Outline

The way it works
The rotation of an asteroid
Yarkovsky: A thermal effect
Measuring Yarkovsky
So many retrograde asteroids
The study and the authors

The way it works

Imagine you want to know the rotation of an asteroid… but you cannot measure it directly. However, you can measure the orbital motion of the asteroid, with enough accuracy to detect an effect (here Yarkovsky), which itself depends on the rotation… measuring Yarkovsky is measuring the rotation! Easy, isn’t it?

The rotation of an asteroid

As any planetary body, an asteroid has a rotational motion, which consists in spinning around one axis (actually 3, but you can safely neglect this fact), at a given rate. We can consider that we know its rotation when

  1. We know its spin rate, or its rotational period (let us assume it is constant),
  2. We know the orientation of its spin pole. We will call it the obliquity.

Usually the asteroids spin in a few hours, which is very fast since they need at least several months to complete one revolution around the Sun. The obliquity is between 0° and 180°. 0° means that the spin axis is orthogonal to the orbital plane, and that the rotation is prograde. However, 180° is the other extreme case, the spin axis is orthogonal, but with a retrograde rotation.

A direct measurement of these two quantities would consist in following the surface of the asteroid, to observe the rotation. Usually we cannot observe the surface, but sometimes we can measure the variations of the magnitude of the asteroid over time. This is directly due to the Solar light flux, which is reflected by the surface of the asteroid. Because the topography is irregular, the rotation of the asteroid induces variations of this reflection, and by analyzing the resulting lightcurve we can retrieve the rotational quantities.

Very well, but sometimes the photometric observations are not accurate enough to get these quantities. And other times, the measured rotational quantities present an ambiguity, i.e. 2 solutions, which would need an independent measurement to discriminate them, i.e. determine which of the two possible results is the right one.

It appears that the Yarkovsky effect, which is an alteration of the orbital motion of the body due to the inhomogeneity of its temperature, itself due to the Solar incident flux and the orientation of the body, i.e. its rotation, can sometimes be measured. When you know Yarkovsky, you know the obliquity. Well, it is a little more complicated than that.

Yarkovsky: A thermal effect

Since I have already presented you Yarkovsky with words, I give you now a formula.

The Yarkovsky effect, i.e. the thermal heating of the asteroid, induced a non-gravitational acceleration of its orbital motion. This acceleration reads A2/r2, where r is the distance to the Sun (remember that the asteroid orbits the Sun), and

A2 = 4/9(1-A)Φ(αf(θs)cos(ε)-f(θo)sin2(ε)),

where

  • A: albedo of the asteroid, i.e. quantity of the reflected light wrt the incident one,
  • Φ: Solar radiation,
  • α: an enhancement factor. This is a parameter…
  • ε: the obliquity (which the authors determined),
  • θs / θo: thermal parameters which depend on the spin period (s), and the orbital one (o), respectively.

If you know Yarkovsky, you know A2, since you know the distance r (you actually know where the asteroid is). If you know all the parameters except ε, then A2 gives you ε. In fact, some of the other parameters need to be estimated.

Measuring Yarkovsky

As you can see, this study is possible only for asteroids, for which you can know the Yarkovsky acceleration. Since it is a thermal effect, you can do it only for Near-Earth Asteroids, which are closer to the Sun than the Main Belt. And to measure Yarkovsky, you must simulate the orbital motion of the asteroid, which is perturbed by the main planets and Yarkovsky, with the Yarkovsky acceleration as a free parameter. A fit of the simulations to the actual astrometric observations of the asteroid gives you a number for the Yarkovsky acceleration, with a numerical uncertainty. If your number is larger than the uncertainty, then you have detected Yarkovsky. And this uncertainty mainly depends on the accuracy of your astrometric observations. It could also depend on the validity of the dynamical model, i.e. on the consideration of the forces perturbing the orbital motion, but usually the dynamical model is very accurate, since the masses and motions of the disturbing planets are very well known.
The first detection of the Yarkovsky acceleration was in 2003, when a drift of 15 km over 12 years was announced for the asteroid 6489 Golevka.

So, you have now a list of asteroids, with their Yarkovsky accelerations. The authors worked with a final dataset of 125 asteroids.

So many retrograde asteroids

The authors tried to fit a distribution of the obliquities of these asteroids. The best fit, i.e. which reduces the distance between the resulting obliquities and the Yarkovsky acceleration that they would have produced, is obtained from a quadratic model, i.e. 1.12 cos2(ε)-0.32 cos(ε)+0.13, which is represented below.

Distribution of the asteroids with respect to their obliquity.
Distribution of the asteroids with respect to their obliquity.

What you see is the number of asteroids with respect to their obliquity. The 2 maxima at 0° and 180° mean that most of the asteroids spin about an axis, which is almost orthogonal to their orbital plane. From their relative heights, it appears that there about twice more retrograde asteroids than prograde ones. This is consistent with previous studies, these obliquities actually being a consequence of the YORP effect, which is the influence of Yarkovsky on the rotation.

The study and its authors

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.

Asteroids: Near-Earth Objects

Equatorial cavities due to fissions

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Hi there! Today I present you a theoretical study, which explains why some asteroids present cavities in their equatorial plane. The related paper, Equatorial cavities on asteroids, an evidence of fission events, by Simon Tardivel, Paul Sánchez & Daniel J. Scheeres, has recently been accepted for publication in Icarus.

When you see a cavity, i.e. a hole at the surface of a planetary, you… OK, I usually assume it is due to an impact. Here we have another explanation, which is that it spun so fast that it ejected some material. These cavities have been observed on the two NEOs (Near-Earth Objects) 2008 EV5 and 2000 DP107 α,for which the authors describe the mechanism.

Outline

The 2 asteroids involved
Yarkovsky and YORP
Asteroid fission
Ejecting a protrusion
The kinetic sieving
Summary
The study and the authors

The 2 asteroids involved

The following table gives you orbital and physical data relevant to these two bodies:

2008 EV5 2000 DP107 α
Semimajor axis 0.958 AU 1.365 AU
Eccentricity 0.083 0.377
Inclination 7.437° 8.672°
Orbital period 343 d 583 d
Spin period 3.725 h 2.775 h
Diameter 450 m 950 m

And you can see the shape model of 2008 EV5 on this video, from James Richardson:

They both are small bodies, which orbit in the vicinity of the Earth, and they spin fast. You cannot see that 2000 DP 107 α has a small companion, so this is the largest component (the primary) of a binary asteroid. Their proximity to the Earth made possible the acquisition of enough radar data to model their shapes. We know that they are top-shaped asteroid, i.e. they can be seen as two cones joined by their base, giving an equatorial ridge. Moreover, they both have an equatorial cavity, of diameters 160 and 400 m, and depths 20 and 60 meters, respectively. The authors estimate that given the numbers of potential projectiles in the NEO population, the odds are very small, i.e. one chance over 600, that these two craters are both consequences of impacts. Such an impact should have occurred during the last millions of years, otherwise the craters would have relaxed. This is why it must be the signature of another mechanism, here fission is proposed.

To have fission, you must spin fast enough, and this fast spin cannot be primordial, otherwise the asteroid would not have formed. So, something has accelerated the spin. This something is YORP, for Yarkovsky-O’Keefe-Radzievskii-Paddack.

Yarkovsky and YORP

When you are close enough to the Sun, the side facing the Sun warms, and then radiates in cooling. This is the Yarkovsky effect, which is a non-gravitational force, which affect the orbit of a small body. When you have an irregular shape, which is common among asteroids (you need to reach a critical size > 100 km to be pretty spherical), your response to the Sun light may be the one of a windmill to the wind. And your spin accelerates. This is the YORP effect.

These Yarkovsky and YORP effects have actually been measured in the NEO population.

Asteroid fission

When you spin fast enough, you just split. This is easy to figure out: the shape of a planetary body is a balance between its own gravity, its spin, and if applicable the tidal action of a large perturber. For our asteroids, we can neglect this last effect. So, we have a balance between the own gravity, which tends to preserve the asteroid, and the centrifugal force, which tends to destroy it. When you accelerate the rotation, you endanger the body. But it actually does not explode, since once some material is ejected, enough angular momentum is lost, and the two newly created bodies may survive. This process of fission is assumed to be the main cause of the formation of binaries in the NEO population.
2000 DP107 α belongs to a binary, while 2008 EV5 does not. But that does not mean that it did not experience fission, since the ejecta may not have aggregated, or the formed binary may not have survived as a binary.

Now, let us see how this process created an equatorial cavity.

Ejecting a protrusion

The author imagined that there was initially a mass filling the cavity. This mass would have had the same density as the remaining body, and they considered its size to be a free parameter. They assumed the smallest possible mass to exactly fill the cavity, the other options creating protrusion. As a consequence, the radius of the asteroid would have been larger at that very place, while it is smaller now. And this is where it is getting very interesting.

In accelerating the rotation of the asteroid, you move the surface limit, which would correspond to the balance between gravitation and spin. More exactly, you diminish its radius, until it reaches the surface of the asteroid… the first contact being at the protrusion. The balance being different whether you are inside or outside the asteroid, this limit surface would go deeper at the location of the protrusion, permitting the ejection of the mass which lies outside, and thus creating an equatorial cavity. Easy, isn’t it?

But this raises another question: this would mean that the cohesion at the equatorial plane is not very strong, and weaker than expected for an asteroid. How to solve this paradox? Thanks to kinetic sieving!

The kinetic sieving

The authors simulated a phenomenon that is known by geologist as reverse grading. In granular avalanches, the separation of particles occurs according to size, involving that the largest particles are expelled where the spin is faster, i.e. at the equator, which would result in a lowest tensile strength, which would itself facilitate the ejection of the mass, and create an equatorial cavity. This phenomenon has been simulated, but not observed yet. So, this is a prediction which should be tested by future space missions.

By the way, the size of the companion of 2000 DP107 α is consistent with a protruder of height 60m.

Summary

  1. Initial state: a Near-Earth Object, with irregular shape. Probably spins fast enough to be top-shaped, i.e. having an equatorial ridge,
  2. YORP accelerates the rotation, favoring the accumulation of large particles at the equator, while tropics are more sandy,
  3. A mass is ejected at the equator, leaving a cavity,
  4. You get a binary, which may survive or not.

More will be known in the next future, thanks to the space mission Osiris-REx, which will visit the asteroid (101955) Bennu in 2018 and return samples to the Earth in 2023. Does it have sandy tropics?

The Near-Earth Asteroid Bennu. © NASA.
The Near-Earth Asteroid Bennu. © NASA.

The study and the authors

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.