Monthly Archives: July 2018

Asteroids: Near-Earth Objects

The Earth will encounter Apophis

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Hi there! You may have heard of (99942) Apophis. As 2004 MN4, this Near-Earth Asteroid was considered to be a potential hazard. Don’t worry, it is not anymore. Anyway, it will have some close approaches with the Earth, the next one occurring in 2029. This makes it an interesting object, and the paper I present today deals with the way this close encounter will affect the rotation of Apophis. This study, Changes of spin axis and rate of the asteroid (99942) Apophis during the 2029 close encounter with Earth: a constrained model, led by Jean Souchay, has recently been accepted for publication in Astronomy and Astrophysics.

Outline

The asteroid (99942) Apophis
Potentially Hazardous Asteroids
A close encounter changes the dynamics
The rotation is critical
A numerical study
Different obliquity, same spin rate
The study and its authors

The asteroid (99942) Apophis

The asteroid (99942) Apophis has been discovered on June 19, 2004, and re-observed the day after (I should say the night, actually), at Kitt Peak Observatory in Arizona. It was then re-discovered six months later from Siding Spring Observatory, New South Wales, Australia, on December 18, and very soon confirmed that it was the same body. On December 27, it was realized that this object had actually already been observed in March. This precovery revealed to be very useful to determine its orbit. You can find below some of its characteristics:

Semi-major axis 0.9225867 AU
Eccentricity 0.1914717
Inclination 3.33687°
Period 323.5 d
Diameter ~350 m

Its orbital dynamics makes it a member of the group Aten. You can see that its orbital period is pretty close to the one of the Earth, i.e. close to one year. This raises the question: could it collide with our Earth? I answer NOT AT ALL, but the question was raised.

Potentially Hazardous Asteroids (PHAs)

Several programs, like NEODys in Italy, or the CNEOS in America, follow Near-Earth Asteroids which could possibly hit the Earth. Up to now, the identified PHAs have been proved to actually present no risk. The Torino scale categorizes the impact hazard associated with near-Earth objects, on a scale from 0 to 10. The risks of collisions and the energies involved are considered. 0 means no risk of impact, 5 means serious threat, 8 means certain collision… and 10 is the worst case, of course, which would characterize the Chicxulub impact, believed by most scientists to be a significant factor in the extinction of the dinosaurs. In such a case, the very existence of the human kind would be jeopardized.

The Minor Planet Center maintains a list of Potentially Hazardous Asteroids, i.e. worthwhile to be scrutinized. I currently count 1,923 of them, but this list is not static.

The observations of Apophis in December 2004 rated it at the level 4, which is a record since the creation of the Torino scale in 1999. Level 4 means that a collision with regional devastation has a probability of at least 1%. On December 27, 2004, the precovery images of Apophis dating from March have ruled out this possibility, and we now know that Apophis will not collide our Earth… or at least not before centuries. The next close approach will occur on April 13, 2029, at a distance of 38,400 km, which is about one tenth of the Earth-Moon distance. Such accurate numbers have been obtained after almost 15 years of astrometric observations of Apophis, which permitted to refine the dynamical models, i.e. fit the ephemerides.

A close encounter changes the dynamics

The mass ratio between the Earth and Apophis implies that, at such a small distance, Apophis will suffer from a huge kick of the Earth. This will drastically affect its dynamics, and would have significant implications for further predictions of its orbit. An accurate determination of the orbital changes requires to consider the non-sphericity of the Earth, the influence of the Sun and the Moon, and also non-gravitational forces, like the Yarkovsky effect. This is a thermal effect, due to the proximity of the Sun. It is barely constrained since it depends on the surface properties and the rotation of the body.
Of course, the future close approaches depend on the next ones. Another one will occur in 2036, its prediction will be much more accurate after the one of 2029.

A study by the same authors anticipate that the 2029 close encounter will affect the orbit of Apophis in such a way that it will move from the dynamical group of Aten to the one of Apollo. In particular, its semimajor axis will be close to 1.1 AU. As a consequence, its orbital period will lengthen from 324 to 422 days.

The rotation is critical

As I said, the Yarkovsky effect depends on the rotational state of Apophis. And this is probably why the study we discuss today deals with the rotation.
The rotation of Apophis has actually been studied in a recent past, from lightcurves. This is something I already discussed on this blog: in recording the Solar light, which is reflected by the surface of the body, you see variations, which are signatures of the rotational motion.

The lightcurves of Apophis revealed two main periods, at 27.38 h and 30.51 hours. The authors of that study (or here) interpreted these two periods as a combination between a fast precessional motion of the rotation axis, with a period of 27.38 h, and a slow and retrograde rotation, with a period of 263 h. This means that the rotation itself is slow and retrograde, but meanwhile the orientation of the North Pole of the body is moving some 10 times faster. Moreover, the authors discovered that the rotation axis was very close to the smallest figure axis. This is called Short-Axis Mode (SAM), and this means that the rotational energy is close to a minimum. In other words, some of it has been dissipated over the ages.

This is the currently observed rotation, but what will it be after the encounter?

A numerical study

The authors performed intensive numerical tests to answer this question. For that, they started from a set of 10,000 model-Apophis, all consistent with our current knowledge of the rotation of Apophis. In other words, these model-Apophis were oriented consistently with the uncertainties of the observations. They also considered the shape, which is itself derived from the lightcurves by Pravec et al. (2014).
Then, they propagated the rotation in using famous equations of the rigid rotation due to Kinoshita (1977), supplemented by a model of the tidal deformation of Apophis by the Earth, during its close approach. Then, the authors deduced the results from the statistics of the outcomes of their 10,000 numerical simulations.

Different obliquity, same spin rate

And here is the results: the authors find that the close encounter with the Earth should not significantly affect the spin rate of Apophis. However, the orientation of its spin axis will tend to align with the one of the Earth, affecting its obliquity.

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.

Comets

An active asteroid

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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.

Outline

Comet vs. active asteroid
The asteroid P/2012 T1
New observations
Measuring its rotation
Dust emission
A 530m-large body
The study and its authors

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.

Satellites of Uranus

Uranus’s satellites are red

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Hi there! Today, we speak about the satellites of Uranus. They have been visited only once by a spacecraft, i.e. Voyager 2 in 1986, but we dispose of Earth-based facilities, which are able to give us some clues. The study I present today, Red material on the large moons of Uranus: Dust from irregular satellites? discusses the fact that the main moons appear to be redder than the large moons of Jupiter and Saturn.

Let me define redder first: the surface of these bodies reflects the Solar incident light. A spectral decomposition of the reflected light tells us something on the material coating the surface. And the relative response of the surface in the different wavelengths is higher in the infrared for the large moons of Uranus, than it is for the moons of Jupiter and Saturn.

This study, by Richard Cartwright et al., has recently been published in Icarus.

Outline

The satellites of Uranus
Voyager 2 at Uranus
Observations at IRTF
Geometrical constraints
A red leading side
Pollution by the irregular moons
The study and its authors

The satellites of Uranus

First: Uranus. This is the seventh planet of our Solar System, which orbits in 84 years, and which seems to roll on its orbit. Actually, its rotation axis is tilted by nearly 90° (actually 97.8°), and its main satellites and rings orbit close to its equatorial plane. Their orbits are tilted as well.

The satellites of Uranus, all named after Shakespeare’s characters, can be categorized into 3 groups:

  1. The 13 small, inner satellites, which are embedded into the rings,
  2. the 5 main ones,
  3. and the 9 irregular satellites, which orbit much further from Uranus, and which orbits may be tilted. Contrary to the other two groups, they have probably not been formed in the proto-Uranus nebula, but were former asteroids, which have been trapped by Uranus.

You can find below some properties and orbital characteristics of the main satellites. All of these bodies have been discovered from the Earth. These are the targets of the study I present.

Discovery Semimajor axis Eccentricity Inclination Orbital period Mean diameter
Miranda 1948 129,900 km 0.0013 4.338° 1.413 d 471.6 km
Ariel 1851 190,900 km 0.0012 0.041° 2.520 d 1,157.8 km
Umbriel 1851 266,000 km 0.0039 0.128° 4.144 d 1,169.4 km
Titania 1787 436,300 km 0.0011 0.079° 8.706 d 1,577.8 km
Oberon 1787 583,500 km 0.0014 0.068° 13.46 d 1,522.8 km

You can see that they have limited eccentricities and inclinations, except for the inclination of Miranda, which probably results from a past resonant forcing by Umbriel. In the past, the orbital period of Umbriel was exactly thrice the one of Miranda, and this has forced its inclination, which was thus initially very small. Anyway, it remains close to the equatorial plane for Uranus.

You can see below that things are different for the small satellites.

Discovery Semimajor axis Eccentricity Orbital period Mean diameter
Francisco 2001 4,282,900 km 0.13 267 d ≈22 km
Caliban 1997 7,231,100 km 0.18 580 d ≈72 km
Stephano 1999 8,007,400 km 0.22 677 d ≈32 km
Trinculo 2001 8,505,200 km 0.22 749 d ≈18 km
Sycorax 1997 12,179,400 km 0.52 1,288 d ≈150 km
Margaret 2003 14,146,700 km 0.68 1,661 d ≈20 km
Prospero 1999 16,276,800 km 0.44 1,978 d ≈50 km
Setebos 1999 17,420,400 km 0.59 2,225 d ≈48 km
Ferdinand 2001 20,430,000 km 0.40 2,790 d ≈20 km

These are very small bodies, which orbit very far from Uranus, on eccentric orbits. Besides, their orbital planes have just nothing to do with the equatorial plane of Uranus. This is why we believe they are former asteroids. Beside Margaret, they all orbit on retrograde orbits, while all the regular moons are prograde. Discovering them required to use telescopes of a least 5 m, the satellites discovered in 2003 having been discovered during a systematic survey with the Subaru telescope at Mauna Kea, Hawaii, over a field of 3.5 degrees. They all have apparent magnitudes larger than 20.

Only one space mission visited them: Voyager 2, in January 1986.

Voyager 2 at Uranus

The spacecraft Voyager 2 was launched from Cape Canaveral, Florida, in August 1977. It benefited from a favorable geometrical configuration of the 4 giant planets to visit all of them. Unfortunately, this required the spacecraft to travel too fast to permit an orbital insertion. So, contrary to Cassini which toured around Saturn during 13 years, Voyager 2 just passed by.

Its closest approach to Uranus was on January 24, 1986, at a distance of 81,500 km from the planet’s cloud tops. It permitted the discovery of 11 inner satellites, and partly imaged the large ones. It revealed in particular geological features on Miranda, and analyzed the light reflected by the surface of these bodies. The study we discuss today supplements these measurements.

Miranda seen by Voyager 2. © NASA/JPL/USGS
Miranda seen by Voyager 2. © NASA/JPL/USGS

Observations at IRTF

The authors used NASA’s InfraRed Telescope Facility (IRTF). This is a 3-meter telescope, optimized for infrared astronomy. It is located at the Mauna Kea Observatory (altitude: 4,200 m) in Hawaii (USA), and 50% of the observation time is devoted to planetary observation.
Several instruments are available, the authors used the spectrograph-imager SpeX, which decomposes the incident light between 0.8 and 5.4 µm. In that study, the authors limited to 4.2 µm.

NASA's InfraRed Telescope Facility at Maune Kea, Hawaii. © Afshin Darian
NASA’s InfraRed Telescope Facility at Maune Kea, Hawaii. © Afshin Darian

The outcome of such observations is a plot amplitude vs. wavelength of a given surface element of a satellite. It is interesting to keep in mind that the regular moons rotate synchronously, permanently showing the same face to Uranus. The consequence is that they have a leading and a trailing hemisphere. During their orbital motion, the same hemisphere always leads. And this has implications for the surface composition, because the leading hemisphere can be polluted by the dusty environment. In other words: when you observe something on the leading hemisphere, which is not present on the trailing one, this is probably pollution.

When you observe, you actually observe the surface element which faces you. And this depends on the dynamics of the planet.

Geometrical constraints

As you know, Uranus rolls on its orbit, while the satellites have an equatorial orbit. As a consequence, during a 84-y orbit of Uranus around the Sun, the Earth crosses twice the orbital plane, and two periods are favorable for the observation of the poles of Uranus and the satellites. The northern hemispheres of these bodies face us during half the orbit (42 years), while the southern ones face us during the other half.

The last transition happened in 2007. Since then, the northern hemispheres of the satellites face us. And part of the visible face belongs to the leading hemisphere.

A red leading side

The results show that for Ariel and Umbriel, and even more for Titania and Oberon, the leading hemisphere is significantly redder than the trailing one, while it is not the case for the major satellites of Jupiter and Saturn. Titania and Oberon are the outermost of the satellites of Uranus, and the largest ones as well.

To understand the chemical nature of this reddening, previous studies have conducted lab experiments, consisting in reproducing the spectrum of mixtures of chemical elements, which could be found on the natural satellites of the outer planets. Of course, the conditions of temperature and pressure are considered. Then the spectrums are compared to the actually observed ones. And it appears that the reddening agents should be tholins and pyroxene.

Titania seems to have a red spot on its surface, which makes it the redder of the main Uranian satellites. Contrariwise, Miranda does not present this reddening. Latitudinal variations of color are not obvious, while they are in longitude, since they depend on the leading / trailing effect.

Now, the question is: how did these agents reach the satellites? They are probably not endogenous, since similar satellites around Jupiter and Saturn do not have them.

Pollution by the irregular moons

The smoking gun is the irregular moons: they are pretty red. And numerical simulations of the motion of dust expelled from these satellites by impacts show how they are likely to coat the leading sides of Oberon, Titania, Ariel and Umbriel.
And this is what we observe!

Of course, a space mission to Uranus would be very helpful… but this is another story.

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.