Tag Archives: Observations

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

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A constantly renewed ring of Saturn

Hi there! The outstanding Cassini mission ended last September with its Grand Finale, and it gave us invaluable data, which will still be studied for many years. Today I present you a study which has recently been published in The Astrophysical Journal: Particles co-orbital to Janus and Epimetheus: A firefly planetary ring, by a Brazilian team composed of Othon C. Winter, Alexandre P.S. Souza, Rafael Sfair, Silvia M. Giuliatti Winter, Daniela C. Mourão, and Dietmar W. Foryta. This study tells us how the authors characterized a dusty ring in the system of Saturn, studied its stability, and investigated its origin.

The rings of Saturn

As you may know, Saturn is the ringed planet, its rings being visible from Earth-based amateur telescopes. Actually, the 4 major planets of our Solar System have rings, and some dwarf planets as well, i.e. Chariklo, Haumea, and possibly Chiron. But Saturn is the only one with so dense rings. I summarize below the main relevant structures and distances, from the center of Saturn:

Distance Structure
60,268 km The atmospheric pressure of Saturn reaches 1 bar.
This is considered as the equatorial radius of Saturn.
66,900 – 74,510 km D Ring
74,658 – 92,000 km C Ring
92,000 – 117,580 km B Ring
117,580 – 122,170 km Cassini Division
122,170 – 136,775 km A Ring
133,589 km Encke Gap
140,180 km F Ring
151,500 km Orbits of Janus and Epimetheus
189,000 km Orbit of Mimas
1,222,000 km Orbit of Titan

The A and B Rings are the densest ones. They are separated by the Cassini Division, which appears as a lack of material. It actually contains some, arranged as ringlets, but they are very faint. The Encke Gap is a depletion of material as well, in which the small satellite Pan confines the boundaries. Here we are interested in a dusty ring enshrouding the orbits of Janus and Epimetheus, i.e. outside the dense rings. The discovery of this ring had been announced in 2006, this study reveals its characteristics.

The rings of Saturn seen by Cassini. From right to left: the A Ring with the Encke Gap, the Cassini Division, the B Ring, the C Ring, and the D Ring. © NASA
The rings of Saturn seen by Cassini. From right to left: the A Ring with the Encke Gap, the Cassini Division, the B Ring, the C Ring, and the D Ring. © NASA

Janus and Epimetheus

The two coorbital satellites Janus and Epimetheus are a unique case in the Solar System, since these are two bodies with roughly the same size (diameters: ~180 and ~120 km, respectively), which share the same orbit around Saturn. More precisely, they both orbit Saturn in 16 hours, i.e. at the same mean orbital frequency. This is a case of 1:1 mean-motion resonance, involving peculiar mutual gravitational interactions, which prevent them from colliding. They swap their orbits every four years, i.e. the innermost of the two satellites becoming the outermost. The amplitudes of these swaps (26 km for Janus and 95 for Epimetheus) have permitted to know accurately the mass ratio between them, which is 3.56, Janus being the heaviest one.

Interestingly, Epimetheus is the first among the satellites of Saturn for which longitudinal librations have been detected. As many natural satellites, Janus and Epimetheus have a synchronous rotation, showing the same face to a fictitious observer at the surface of Saturn. For Epimetheus, large librations have been detected around this direction, which are a consequence of its elongated shape, and could reveal some mass inhomogeneities, maybe due to variations of porosity, and/or to its pretty irregular shape.

Janus and Epimetheus seen by Cassini (mosaic of 2 images). © NASA
Janus and Epimetheus seen by Cassini (mosaic of 2 images). © NASA

Images of a new ring

So, Cassini images have revealed a dusty ring in that zone. To characterize it, the authors have first extracted images likely to contain it. Such images are made publicly available on NASA’s Planetary Data System. Since that ring had been announced to have been observed on Sept 15th 2006 (see the original press release), the authors restricted to 2 days before and after that date. The data they used were acquired by the ISS (Imaging Science Subsystem) instrument of Cassini, more precisely the NAC and WAC (Narrow- and Wide-Angle-Camera). They finally found 17 images showing the ring.

The images are given as raw data. The authors needed to calibrate their luminosity with a tool (a software) provided by the Cassini team, and sometimes to smooth them, to remove cosmic rays. Moreover, they needed to consider the position of the spacecraft, to be able to precisely locate the structures they would see.

One of the Cassini images used by the authors. I have added red stars at the location of the ring. © NASA / Ciclops
One of the Cassini images used by the authors. I have added red stars at the location of the ring. © NASA / Ciclops

It appears that the ring presents no longitudinal brightness variation. In other words, not only this is a whole ring and not just an arc, but no density variation is obvious. However, it presents radial brightness variations, over a width of 7,500 km, which is wider than the 5,000 km announced in the 2006 press release.

The next step is to understand the dynamics of this ring, i.e. its stability, its origin, the properties of the particles constituting it… Let us start with the stability.

The ring is removed in a few decades

The authors ran N-body simulations, i.e. numerical integrations of the equations ruling the motion of a ring particle, which would be gravitationally perturbed by the surrounding bodies, i.e. Saturn, and the Janus, Epimetheus, Mimas, Enceladus, Tethys, Dione, and Titan. Moreover, for a reason that I will tell you at the end of this article, the authors knew that the particles were smaller than 13 μm. The motions of such small particles are affected by the radiation pressure of the Sun, in other words the Solar light pushes the particles outward.

The authors simulated 14 times the motion of 18,000 particles equally distributed in the rings. Why 14 times? To consider different particle sizes, i.e. one set with 100 μm-sized particles, and the other sets with sizes varying from 1μm to 13μm.
And it appears that these particles collide with something in a few decades, mostly Janus or Epimetheus. This leaves two possibilities: either we were very lucky to be able to take images of the ring while it existed, or a process constantly feeds the ring. The latter option is the most probable one. Let us now discuss this feeding process.

Renewing the ring

The likeliest sources of material for the rings are ejecta from Janus and Epimetheus. The question is: how were these ejecta produced? By impacts, probably. This study show that Janus and Epimetheus are impacted by the particles constituting the rings, but the impact velocities would not permit to produce ejecta. This is why the authors propose a model, in which interplanetary particles collide with the satellites, generating ejecta.

A firefly behavior

And let us finish with something funny: the ring seems to behave like a firefly, i.e. sometimes bright, and sometimes dark, which means undetectable while present.
To understand what happens, figure out how the light would cross a cloud of particles. If the cloud is dense enough, then it would reflect the light, and not be crossed. But for dust, the light would be refracted, i.e. change its direction. This depends on the incidence angle of the Solar light, i.e. on the geometrical configuration of the Sun-Saturn-ring system. The Solar incidence angle is also called phase. And this phase changes with the orbit of Saturn, which results in huge brightness variations of the ring. Sometimes it can be detected, but most of the time it cannot. This can be explained and numerically estimated by the Mie theory, which gives the diffusion of light by small particles. This theory also explains the creation of rainbows, the Solar light being diffracted by droplets of water.

The study and its authors

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The mountainous equator of Iapetus

Hi there! You may have heard of Iapetus, a large satellite of Saturn which orbits very far. Today I will tell you about its equatorial ridge. This is the opportunity for me to present you Testing models for the formation of the equatorial ridge on Iapetus via crater counting, by Amanda L. Damptz, Andrew J. Dombard & Michelle R. Kirchoff, which has recently been accepted for publication in Icarus.

The satellite Iapetus

The satellite of Saturn Iapetus was discovered by Giovanni Domenico Cassini in October 1671. It orbited then on the western side of Saturn. During many years, i.e. until 1705, he was unable to observe it on the eastern side, since it was two magnitudes fainter. This has two implications:

  1. Iapetus has a two-tone coloration, i.e. a dark and a bright hemisphere,
  2. its rotation is synchronous. Like our Moon, it is locked in the synchronous 1:1 spin-orbit resonance, constantly showing the same face to Saturn.

Beside this, Iapetus is a large body (diameter: 1,470 km), which orbits at 3.5 millions km from Saturn (for comparison, Titan orbits almost thrice closer), with an orbital eccentricity of 0.028, very close to the one of Titan. It has an unexpectedly high orbital inclination, i.e. 15.47° with the equator of Saturn, and 8.13° with the Laplace Plane. We should imagine the proto-Saturn nebula, from which Iapetus has probably been formed. It was pretty much like a disc, but distorted by the Sun if you were far enough from Saturn, which is the case for Iapetus. What I mean is that the gravitational action of the Sun tends to shift the equilibrium orbital plane from the equatorial one, this is why we need to distinguish it, and we call it the Laplace Plane. In that case, the orbital inclination of Iapetus with respect to the Laplace Plane should be very small, but it is not. This probably contains an information on the history of Iapetus, but we do not know which one yet.

Let me go back to the rotation. Iapetus is so far from Saturn that it needs almost 80 days to complete one revolution, and 80 days for a rotation, since it is synchronous. This is the largest known spin period for a natural satellite in the Solar System.

As most of the satellites of Saturn, our knowledge of Iapetus made invaluable progress since 2004 thanks to the Cassini spacecraft, which imaged it. It confirmed the two-tone coloration, and detected a mountainous equatorial ridge.

Iapetus seen by the Cassini spacecraft. © NASA
Iapetus seen by the Cassini spacecraft. © NASA

The equatorial ridge

The Cassini images showed a 20 km-wide mountainous ridge, which is very close to the equator. So close that it is hard to believe it appeared there by chance. It is present on the dark hemisphere, while isolated equatorial mountains can be seen on the bright side. Some peaks reach 20 km.

Since its discovery late 2004, this ridge is a matter of investigation, and several competing explanations can be found in the literature:

  1. A signature of its past, fast rotation (the measured oblateness of Iapetus is consistent with a rotation period of 16 hours)
  2. A signature of a past critical spin state, i.e. close to provoke disruption of Iapetus,
  3. Upwarping of the lithosphere from below,
  4. Cryovolcanism,
  5. Planetary contraction,
  6. Material from an ancient ring system,
  7. Material from impact generated debris.

We can see that some of these scenarios propose an inner (endogenic) cause, while others propose an outer (exogenic) one. Almost all of them suggest an early formation of the ridge, except the last one.
One way to date a geological feature is to count its craters, and this is where this study intervenes. Its first product is a database of 7,748 craters ranging from 0.83 to 591 km in diameter.

Counting the craters

When an impactor reaches a planetary surface, it creates a crater. If one day geological processes are strong enough to create a tectonic feature, then it may at least alter the crater, or even hide it. If we see an uncraterized geological feature, that means that it is pretty young. We could even try to give it an age in estimating the evolution of the cratering rates over the evolution of the Solar System. By the way, the early Solar System was very intensively bombarded, with an episode of Late Heavy Bombardment occurring between 4.1 and 3.8 billions years ago. Bombardments still happen nowadays, but are much less frequent.

In this study the authors worked from Cassini and Voyager images of the surface of Iapetus, and considered different zones: central ridge, peripheral ridge, and off ridge. Moreover they classified the craters following their diameters, so as to estimate a distribution law: number of craters vs. size. They also catalogued the orientation of the deformations of the craters, since it could tell us something on the geological evolution of Iapetus (how did it alter the surface?)

This systematic search for craters was assisted by the commercial software Esri’s ArcGIS, supplemented by the dedicated add-on Crater Helper Tools.


The first result is a database of 7,748 craters. But the main question is: what can we say about the ridge? The authors observe a depletion of large craters, i.e. with a diameter bigger than 16 km, in the ridge, which would be consistent with a pretty recent formation, and thus would favor the scenario of a ridge created by the debris of an impact. Nevertheless, the authors are prudent with this conclusion, they seem to suggest that the resolution of the images and the risk of saturation of small craters (when you are heavily bombarded, new craters destroyed ancient ones, and the overall number does not increase) do not permit to discard a scenario of early formation of the ridge. Further studies will probably be needed to reach an agreement on the origin of this mountainous equator.

The study and its authors

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Rotation and activity of a comet

Hi there! We, Earthians, are regularly visited by periodic comets, the most famous one being probably 1P/Halley, which will visit us in 2061. Since we cannot wait, we study others of that kind. Today I tell you about 49P / Arend-Rigaux. This is the opportunity for me to present you The rotation and other properties of Comet 49P/Arend-Rigaux, 1984 – 2012, by Nora Eisner, Matthew M. Knight and David G. Schleicher. This study has recently been published in The Astronomical Journal.

The comet 49P / Arend-Rigaux

The comet 49P / Arend-Rigaux has been discovered in February 1951 at the Royal Observatory of Belgium, by Sylvain Arend and Fernand Rigaux. It is a periodic comet of the Jupiter family, i.e. with a period smaller than 20 years. Its period is actually 6.71 years, its semimajor axis 3.55 AU (astronomical units, 1 AU being 150 millions km, i.e. the Sun-Earth distance), its eccentricity 0.6, and its orbital inclination 19°, with respect to the ecliptic. These numbers are extracted from the JPL Small-Body Database Browser, and are calculated at the date Apr 6, 2010. I have plotted below the distances Sun-comet and Earth-comet.

Distance to the Sun.
Distance to the Sun.
Distance to the Earth.
Distance to the Earth.

The distance to the Sun clearly shows the periodic variations. The orbit of the Earth is at 1 AU, the one of Mars at 1.5 AU, and the one of Jupiter at 5.2 AU. Every 6.71 years, the comet reaches its perihelion, i.e. minimizes its distance to the Sun. This proximity warms the comet and provokes an excess of cometary activity, i.e. sublimation of dirty ice. At these occasions, the distance with the Earth is minimized, but with variations due to the orbital motion of the Earth. We can see for instance that the comet gets pretty close to the Earth in 1951 (when it was discovered), in 1984, and in early 2032. These are favorable moments to observe it. The paper I present you today is mainly (but not only) based on photometric observations made between January and May 2012, at Lowell Observatory.

Observations at Lowell Observatory

Lowell Observatory is located close to Flagstaff, AZ (USA). It was founded by the famous Percival Lowell in 1894, and is the place where Clyde Tombaugh discovered Pluto, in 1930. Among its facilities is the 4.28 m Discovery Channel Telescope, but most of the data used in this study were acquired with the 1.1 m Hall telescope, which is devoted to the study of comets, asteroids, and Sun-like stars. The authors also used a 79 cm telescope. The observations were made in the R(ed) band.

The data

Besides these 33 observation nights during the first half of 2012, the authors used data acquired close to the 1984 and 2005 perihelion passages, even if the 2005 ones revealed unusable. The observations consists to measure the magnitude (somehow, the luminosity) of the comet, in correcting for atmospheric problems, so as to be able to detect the variations of this magnitude. You can find below an example of data:

Magnitude of 49P / Arend-Rigaux measured in April 2012.
Magnitude of 49P / Arend-Rigaux measured in April 2012.

Of course, the data have holes, since you cannot observe during the day. Moreover, the comet needs to be visible from Arizona, otherwise it was just impossible to observe it and make any measurements.

We can see a kind of periodicity in the magnitude, this is a signature of the rotation of the comet.

Measuring the rotation

Most of the planetary bodies are kinds of triaxial ellipsoids. Imagine we are in the equatorial plane of one of them. We see an alternation of the long and short axes of its equatorial section. If the albedo of the surface element we face depends mainly on its curvature (it depends on it, but mainly may be an overstatement), then we should see two peaks during a period. As a consequence, the period of the lightcurve we observe should be half the rotation period of the comet.

In combining all the measurements, the authors managed to derive a rotation period of 13.45 ± 0.01 hour. For that, they used two different algorithms, which gave very close results, giving the authors confidence in their conclusions. The first one, Phase Dispersion Minimization (PDM), consists to assume a given period, split the measurements into time intervals of this period, and overlap them. The resulting period gives to the best overlap. The other algorithm is named Lomb-Scargle, following its authors. It is a kind of Discrete Fourier Transform, but with the advantage of not requiring uniformly sampled data.

In addition to this rotation period, the authors detected an increasing trend in the 2012 data, as if the spin of the comet accelerated. This is in agreement with an alteration of the measured rotation from the Earth, which moves, and reveals a retrograde rotation, i.e. an obliquity close to 180°. In other words, this is an illusion due to the motion of the observer, but this illusion reveals the obliquity.

Moreover, in comparing the 2012 data with the ones of 1984, the authors managed to detect a variation in the rotation period, not larger than 54 seconds. This is possible regarding the fact that the comet is altered by each perihelion passage, since it outgasses. In this case, that would imply a change of at the most 14 seconds of the rotation period between two passages. Such variations have also been detected for at least 4 other comets (2P/Encke, 9P/Tempel 1, 10P/Tempel 2, and 103P/Hartley 2, see Samarinha and Mueller (2013)).

Comet Period (h) Variation (s)
2P/Encke 11 240
9P/Tempel 1 41 -840
10P/Tempel 2 9 16.2
103P/Hartley 2 18 7200
49P/Arend-Rigaux 13.45 -(>14)

Finally, since the lightcurve is a signature of the shape as well, the authors deduced from the amplitude of variation that the axial ratio of the nucleus, i.e. long axis / short axis, should be between 1.38 and 1.63, while an independent, previous study found 1.6.

Cometary activity

49P / Arend-Rigaux has a low activity. Anyway, the authors detected an event of impulse-type outburst, which lasted less than 2 hours. The analysis of the coma revealed an excess of cyanides with respect to the 1984 passage. Moreover, 49P / Arend-Rigaux is the first comet to show hydroxyde.

The study and its authors

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An interstellar asteroid

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.

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

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