Tag Archives: Astrometry

Satellites of Jupiter

Astrometry from close approaches

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Hi there! Today, we discuss of astrometry of the satellites of Jupiter. It consists in answering the question : where are they? Basically, you look at them, and you see where they are… well, it is more complicated than that, actually.
This is the opportunity to present APPROX – mutual approximations between the Galilean moons: the 2016-2018 observational campaign, by Bruno Morgado and several collaborators. Since this paper presents an observation campaign, involving Brazil and France, there are many observers, hence many co-authors. This study has recently been published in The Monthly Notices of the Royal Astronomical Society.

Outline

Astrometry of the natural satellites
Detecting physical phenomena
Absolute and relative astrometry
Many observational difficulties
The mutual approximation technique
The observation campaign
Very accurate results
The study and its authors

Astrometry of the natural satellites

In fact, you can make astrometry of any celestial object. Star, satellite, galaxy, whatever… It is of course easier when the body looks like a dot.

When we look at the celestial sphere, you look in 2D. The space is in 3D, but technically we see a projection of this volume on a sphere, which is called the celestial sphere. Since it is a 2D space, you can also consider two coordinates. Let us call them right ascension and declination. So, you want to know the motion of these bodies, you observe them, and measure their positions.

The astronomers of the past, I mean in the pre-photographic era, used a filar micrometer. This consisted of a screw and a reticle, made of two threads. When observing two celestial bodies, the observer saw two lines in the field, that (s)he could move thanks to the micrometric screw, one on one body, the other line on the other body, and from the motion of the screw he knew the projected (apparent) distance between the two bodies. So, only one dimension at that time.

And then came the photography: you take a picture of the field (small part of the sky), and you measure the coordinates of the bodies. This way you can get two coordinates. The techniques improved since then, with automatic surveys, taking automatic images of the sky, measuring automatically the coordinates,etc.

But why doing that?

Detecting physical phenomena

It is for understanding the motion of an object. For instance, if you take a natural satellite, you know that it orbits its parent planet, following the classical gravitation law of Newton (improved since Einstein and the discovery of general relativity, but you do not need that refinement to understand the dynamics of the natural satellites). But first you have to make sure it is a satellite of that planet. If it follows it on the sky, then it is fine. And to simulate its motion, you must know some parameters, especially the mass of the planet, of the satellite, the flattening of the planet, the masses of the other satellites (yes, they disturb each other). Well, that becomes tricky. And this is why you need those astrometric observations. Your dynamical model depends on some parameters, you fit them to the observations, and then you have the masses.

This way, you can say: OK, we have everything we need. Why still doing that once we have the masses? You can say that we need more accuracy, but once a spacecraft tours around the planet, it gives you all the data you need, doesn’t it? Including astrometric positions, and direct measurements of the masses.

My answer to that is:

  1. A spacecraft covers a pretty limited time span. You would need observations outside of that range, even less accurate, to detect a drift between the dynamical model and the real motion. And that drift would mean that something significant is not in the model, or maybe it is, but not accurately enough.
  2. And this comes to my second argument: the dissipative processes, especially the tides. During centuries we could consider that celestial mechanics was a conservative discipline, in the sense that there was no dissipation to consider. Of course, we knew there was some dissipation, otherwise it is physically irrelevant, but that dissipation was so small that at that time you could safely neglect it. Not any more.

Let us be more specific about tides. The parent body (Jupiter for the Galilean satellites, Saturn for its satellites, the Sun for the planet) exerted a differential torque on the volume of the small one, which generates stress and strain. In an extreme case, i.e. close enough to the planet (inside what we call the Roche limit), bodies can be destroyed (and this gives you the rings of Saturn).
For the classical satellites of the giant planets, the dissipation appears as volcanoes on Io, fractures on Europa, geysers on Enceladus. And in ephemerides (i.e. orbital motion) the induced energy losses result in secular (i.e. a very slow drift) migration over the ages. And we are now accurate enough to detect this tidal effect on the Moon, and in the systems of Jupiter and Saturn.

Numbers regarding this effect tell us how the planetary material reacts to solicitations, and permits us to extrapolate the migration, i.e. know the past and future of these systems.

Now let us go back to techniques of astrometry.

Absolute and relative astrometry

When you measure the coordinates of a celestial body, you need an origin, i.e. a zero, which is a reference. But the reference is usually not present in your field of view, which is limited (one degree is a huge field).
Fortunately, we can use the background stars as references. The stars are catalogued with their coordinates, and so if you have e.g. 20 catalogued stars in your field, then you have 20 points, which you know the coordinates. And this gives you the coordinates of the other points, i.e. the Solar System bodies.
Making catalogues of stars is not an easy task. ESA’s space observatory Gaia is on it!

Artist's impression of Gaia. © ESA
Artist’s impression of Gaia. © ESA

Differential astrometry is an alternative: for instance if you work on the natural satellites of a given system (let us say the Galilean satellites of Jupiter), it could be enough to know where the satellites are with respect to the other ones and to the parent planet. So, you make differential astrometry: you measure the difference between the coordinates of two given bodies.

As I told you, it is a little more complicated than just taking a picture. You must do it properly. Let us see that now.

Many observational difficulties

Of course, your sky should be clear. But this is not enough. When you take two pictures of an object, which does not move, will they put you the object at the same location? Not necessarily! Because of the seeing, which is due to the wind (the atmosphere), and which kind of noises your images. But this is not enough: you are not sure that all of the pixels of your camera have the same response. Of course they should… if only they could… You always have instrumental problems. You can partially compensate that by making a flat fielding, i.e. you take an image of a starless field (for instance before opening the dome) and this gives you the response of the sensors.
Another problem comes from the fact, that these bodies are not dots. You need to know where the center of mass is… which is not the center of figure, because of a phase effect. When you look at the Moon, it is usually not the full Moon, since part of it is in the umbra… the same happens for the other bodies. Beside this, there are problems due to the anisotropy of the reflection of the light on the surfaces of the satellites.

And let me finally mention timing problems: you do measure a position at a given time. But you need to be exact on that time! If not, then your measurement is inaccurate. In other words, you need to care for errors in positions and in time. If you record your images with a laptop, the internal clock may drift by several seconds. I can tell you that from my experience. So, you have to check it constantly. You can have an accurate clock with GPS systems.

I hope you are now convinced that astrometry is truly a science.

The mutual approximation technique

Other astrometry techniques than taking pictures exist. For instance, you can look for occultations: when an asteroid occultates a star, you do not see the star anymore. Since you know where the star is, you know where the asteroid was during the occultation. This technique also permits to get clues on the shape of the asteroid, with multiple observations, and sometimes even detect asteroids and rings. See here.

In the specific case of this study, the authors developed a technique, which is based on the timing of a minimum of the apparent distance between the two satellites. When two satellites are close to each other in the sky (I mean, in projection onto the celestial sphere), you reach a so-called precision premium, i.e. you optimize the accuracy of the measurements. The reason is that your measurement does not suffer from the field distortion. The two dots are so close that you have the same problems for both. So, the differential measurement is not affected.

Here, the authors measure the timing of the minimum of distance, from which they can determine a position, knowing the relative motion of the satellites from the available orbital theories (the ephemerides). Measuring this instant is not trivial, since actually it has no reason to correspond exactly to a recorded data. So, you take a series of images, on all of them you measure the distance, you plot it with respect to the date. And then you fit a polynomial function to the obtained data; the instant you measure corresponds to the minimum of your polynomial.

The authors published this technique two years ago, and this paper present a campaign of observations.

The observation campaign

The authors observed 66 different mutual approximations, from 6 different sites, which are

    • Itajubá (Minas Gerais, Brazil), equipped with a 60-cm telescope,
    • Foz do Iguaçu (Paraná, Brazil), equipped with a 28-cm telescope,
    • Guaratinguetá (São Paulo, Brazil), equipped with a 40-cm telescope,
    • Vitória (Espiríto Santo, Brazil), equipped with a 35-cm telescope,
    • Curitiba (Paraná, Brazil), equipped with a 25-cm telescope,
    • and the Observatoire de Haute-Provence (France), equipped with a 120-cm telescope.

These facilities permitted the determination of 104 central instants, which obviously means that some events, in fact 28 of them, were observed at least twice, i.e. from different sites. All of these telescopes were equipped with a narrow-band filter centered at 889 nanometers. This eliminates the light pollution by Jupiter.

Very accurate results

The authors get a mean accuracy of 11.3 mas (milli-arcseconds), which is ten times more accurate than classical observations. A good way to determine this accuracy is by a statistical comparison between the measurements, and the numbers predicted by the theory.
At this distance, i.e. the distance to Jupiter, 11.3 mas means 40 km, which is much smaller than the radii of these satellites (Io, Europa, Ganymede, and Callisto).

So, you see, this is a very promising technique, and supplementing the database of astrometric observations with such high-quality data can only lead to new discoveries.

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.

Asteroids: Trans-Neptunian Objects

Locating Ultima Thule

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Hi there! First I have to explain the title. Ultima Thule usually stands for any place beyond the known world… Actually, we will not leave the Solar System. Ultima Thule is an unofficial nickname for (486958) 2014 MU69, which is the next target of New Horizons spacecraft. Do you remember this spacecraft, which gave us outstanding images of Pluto and Charon in July 2015? That’s just the same one!
After having left Pluto, New Horizons changed its trajectory to 2014 MU69, which will be reached on January 1st, 2019. 6 months to wait then.
Interestingly, 2014 MU69 was unknown when New Horizons was launched in January 2006. Its primary mission was the binary Pluto-Charon and its satellites, and of course it was worth to extend the mission to another body. But choosing this second target was a difficult task, since the distant Solar System, here the Kuiper Belt, is very difficult to observe, and is pretty sparse. This is why observations programs of the Hubble Space Telescope (HST) were dedicated, and 2014 MU69 has been discovered in 2014.
Discovering an object is one thing, determining accurately its motion in view of a rendezvous with a spacecraft is another thing. This is the topic of the study I present today, High-precision orbit fitting and uncertainty analysis of (486958) 2014 MU69, by Simon Porter et al. This study has recently been published in The Astronomical Journal.

Outline

The New Horizons spacecraft
Ultima Thule
Hubble Space Telescope data
From astrometry to orbit
Predicting stellar occultations
A stable orbit
The study and its authors

The New Horizons spacecraft

New Horizons is the first mission of NASA’s New Frontier program. It was launched in January 2006, and made its closest approach to Pluto in July 2015. Before that, it incidentally encountered the small asteroid (132524) APL at a distance of about 100,000 km in June 2006, and benefited from the gravitational assistance of Jupiter in February 2007.

The asteroid 132524 APL seen by New Horizons. © NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute
The asteroid 132524 APL seen by New Horizons. © NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

It carries seven science instruments:

  • the Long-Range Reconnaissance Imager (LORRI), which images the encountered bodies,
  • the Solar Wind At Pluto (SWAP) instrument, which name is very explicit regarding its goal,
  • the Pluto Energetic Particle Spectrometer Science Investigation (PEPSSI), which supplements SWAP for the detection of high-energy particles,
  • Alice, which is an ultraviolet imaging spectrometer,
  • the Ralph telescope, which is a photographic instrument,
  • the Venetia Burney Student Dust Counter (VBSDC) measures dust. This instrument has been built by students of the University of Colorado,
  • and the Radioscience Experiment (REX), which measured the temperature and the atmospheric pressure of Pluto.

As you can see from some of the names of the instrument, Pluto-Charon was definitely the primary goal of New Horizons. Anyway, Pluto is now behind, and New Horizons is en route to 2014 MU69, also nicknamed Ultima Thule.

Ultima Thule

At this time, our knowledge of Ultima Thule is very limited. This body has been discovered in 2014, from a dedicated observation program on the Hubble Space Telescope, to identify potential targets for New Horizons. Finally, 2014 MU69 has been selected, partly for technical reasons, i.e. it is not so difficult to reach from Pluto.

It was discovered in June 2014, and has an apparent magnitude of nearly 27, and an absolute one of 11. We can guess its size from its magnitude, and its diameter should be smaller than 50 km. So, a very small body. A stellar occultation happening in 2017 has revealed that its diameter should be closer to 25-30 km, and its shape may be bilobal, or it could even be a contact binary.

Observations of its dynamics revealed that it is a cold, classical Kuiper Belt object. Its eccentricity and inclination are limited, since they are not excited by any resonance with the giant planets. So, it belongs to a region of the Solar System, which is quiet from a dynamical point of view.

As I previously said, discovering it is not enough if you want a spacecraft to reach it. You must know its motion accurately, and for that you need more data. And Ultima Thule can be observed only with the Hubble Space Telescope.

Hubble Space Telescope data

The authors disposed of 5 observations of 2014 MU69, by the Wide Field Camera 3 (WFC3) of the HST. Even with the HST, imaging 2014 MU69 requires 6 minutes of exposure, i.e. you need to accumulate photons reflected by 2014 MU69 during 6 minutes to have enough signal.

The detection of 2014 MU<sub data-recalc-dims=
69. © NASA, ESA, SwRI, JHU/APL, and the New Horizons KBO Search Team” class=”size-full wp-image-917″ width=”290″ height=”290″> The detection of 2014 MU<sub>69</sub>. © NASA, ESA, SwRI, JHU/APL, and the New Horizons KBO Search Team

Another difficulty comes from the number of stars in that region of the sky. This is due to the galactic latitude of 2014 MU69, which is close to 0, i.e. close to the Galactic plane. The images have to be treated, to remove their incoming light. This is not just a pixel, but a diffraction spot, which needs to be modeled to be properly removed.

Once you have these data, you can start to determine the orbit of 2014 MU69, i.e. make ephemerides.

From astrometry to orbit

When you catch a body on a 2-dimensional image of the sky, you get two coordinates. Basically, these coordinates translate into a right ascension, and a declination. And to build ephemerides of a body, you need to integrate the equation ruling the orbital motion. This equations is a second-order 3-dimensional ordinary differential equation.

The motion is ruled by the gravitational perturbation of the Sun and the major planets, and for the problem to be solved, you need initial conditions. These are a position and a velocity of 2014 MU69 at a given date, which you derive from your astrometric observations, i.e. the 5 couples (right ascension, declination).

Easy, isn’t it? No, it’s not! Because of the uncertainties on the measurements, your 10 data, i.e. 5 right ascensions and 5 declinations, do not exactly correspond to an initial condition. So, you have to make a fit, i.e. determine the initial condition, which best fits the observations.

There are many potential sources of uncertainties: the accuracy of the positioning of the HST, the accuracy of the coordinates of 2014 MU69 (remember: this is not a pixel, but a diffraction spot), the duration of the exposure… and also the location of the stars surrounding 2014 MU69 in the field of view. To make absolute astrometry, you need to know precisely the location of these stars, and you get their locations from a star catalogue. Currently, the astrometric satellite Gaia is making such a survey, with a never reached accuracy and comprehensiveness. The Gaia Data Release 2 has been released in April 2018, and gives positions and proper motions (i.e. you can now consider that the stars move from date to date) of more than 1 billion stars! The authors had the chance to use that catalogue. This resulted in predictions, which were accurate enough, to predict a stellar occultation, which has been observed from the Earth.

Predicting stellar occultations

When a Solar System body occultates a star, you can indirectly observe it. You observe the star with your telescope, and during a few seconds, the star disappears, and then reappears, because of this object, which light is too faint for you. Multiple observations of a stellar occultation give information on the motion of the object, and on its dimensions. The rings around Chariklo and Haumea have been discovered that way.

For 2014 MU69 (or Ultima Thule), an occultation has been successfully predicted. It has been observed 5 times on 2017 July 17, in Argentina, giving 5 solid-body chords. This permitted us to infer that 2014 MU69 could be bilobal, or even a contact binary.

A stable orbit

And from these astrometric data, the authors propagated the orbit of 2014 MU69 over 100 million years, in considering the uncertainties on the initial positions. The outcomes of the simulations safely state that 2014 MU69 is on a very stable orbit, with a mean semimajor axis of 44.23 astronomical units (39.48 for Pluto), and an orbital eccentricity smaller than 0.04. This results in an orbital period of 294 years, during which the distance to the Sun barely varies.

We are looking forward for the encounter in 6 months!

The study and its authors

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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

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Satellites of Neptune

Where is Triton?

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Hi there! Today’s post is on the location of Triton. Triton is the largest satellite of Neptune and, of course, we know where it is. The paper I present you, entitled Precise CCD positions of Triton in 2014-2016 using the newest Gaia DR1 star catalog, aims at assisting an accurate modeling of its motion. This is a Chinese study, by Na Wang, Qing-Yu Peng, Huan-Wen Peng, H.J. Xie, S. Ma and Q.F. Zhang, which presents observations made at the Yunnan Observatory. This study has recently been published in The Monthly Notices of the Royal Astronomical Society.

Triton’s facts

Triton is by far the largest satellite of Neptune, with a mean radius of 1,350 km. It was discovered almost simultaneously with Neptune, i.e. 17 days later, in 1846. It orbits Neptune in a little less than 6 days, and rotates synchronously.

Surprisingly, its orbit is very inclined with respect to the equator of Neptune (some 24°), and it is retrograde… which means that the inclination should not be given as 24° but as 180-24 = 156°. Usually the satellites have a very small inclination, since they are supposed to have been formed in a nebula which gave birth to the planet. Such a large inclination means that Triton has probably not been formed in situ but was an asteroid, which has been trapped by Neptune. Since then, it loses some orbital energy, which has resulted in a circularization of its orbit and a fall on Neptune.

Triton has been visited by the Voyager 2 spacecraft in 1989, which covered about 40% of its surface. It revealed an atmosphere of nitrogen and evidences of melting, which indicate a geological activity, probably resulting in a differentiated body. It could even harbor or have harbored a subsurface ocean.

Astrometry in the Solar System

The goal of the paper I present today is to give accurate positions of Triton. This is called astrometry. The idea is this: measuring accurately the position of a body at a given date requires to take a picture of the satellite. Stars must be present in the field since the satellite will be positioned according to them. The result will then be compared to the predictions given by dynamical models, called ephemerides. Since we observe only in 2 dimensions, i.e. on the celestial sphere, which is a surface, then an astrometric position consists of two coordinates: the right ascension and the declination.

The positions of the stars surrounding the satellite, actually its image projected on the celestial sphere, since the stars are much further, are known thanks to systematic surveys. The satellite Gaia is currently conducting the most accurate of such surveys ever performed, and the first data were released in 2016, in the Gaia Data Release 1 (DR1, see this post, from October 2016). The accuracy of Gaia lets us hope very accurate future astrometry, and even past, since old observations could be retreated in using Gaia’s catalog.

But why making astrometry? For improving the ephemerides, which would give us a better knowledge of the orbital motion of the satellite. And why improving the ephemerides? I see at least two reasons:

  1. To help future space missions,
  2. To have a better knowledge of the physical properties of the bodies. Some of these properties, like the mass and the energy dissipation (tides), affect the orbital motion.

Potential difficulties

This study presents Earth-based observations, which are affected by:

  1. Diffraction on the CCD chip. When you observe a point as a light-source, you actually see a diffraction disk, and you have to decide which point on the disk is the position of your object. You can partly limit the size of the diffraction disk in limiting the exposure time, this prevents the chip from being saturated. If this results in a too faint object, then you can add several images.
  2. The anisotropy of the light scattering by the surface of the body (here Triton). When we see Triton, we actually see the Solar light, which is reflected by the surface of Triton. Since the surface could be pretty rough, since the limb is darker than the center because of a different incidence angle, and since Triton is not seen as a whole disk (remember the lunar crescent), then the center of the diffraction disk, i.e. the photocentre, is not exactly the location of the body.
  3. The refraction by the atmosphere. The atmosphere distorts the image, which makes the satellite-based images more accurate than the Earth-based ones. This distortion depends on the location of the observatory, and the weather. Some systems of adaptive optics exist, which partly overcome this problem.
  4. The aberration. The relative velocity of the observed object with the observer (the Earth is moving, remember?) alters the apparent position of the objects.
  5. The seeing. When you have some wind, the locations of the stars present some erratic variations.
  6. The inhomogeneity of the CCD chip. An homogeneous lightning will not result in a homogeneous response, because of the positions of the pixel on the image (you have a better sensitivity in the center than close to an edge), and some technical differences between the pixels. A way to overcome this problem is to normalize the light measurement by a flat image, which is the response to a homogeneous lightning. This is usually obtained in taking a picture in the dome, or from the averaging of many images.

This paper

This paper presents 775 new observations, i.e. 775 new positions of Triton at given dates, between 2014 and 2016. The images were taken with a 1-meter refractor at the Yunnan National Observatory, Kunming, Yunnan, China. The residuals, i.e. observed minus predicted positions, are obtained from the ephemerides made by the Jet Propulsion Laboratory in California, USA. The authors obtain mean residuals of a few tens of milli-arcsec, i.e. some thousands of kilometers. Something interesting is the dispersion of these residuals: the authors show that when the stars are positioned with Gaia DR1, the residuals are much less dispersed than with an older catalog. The authors used the catalog UCAC4, released in August 2012 by the US Naval Observatory, for comparison.

These new observations will enrich the databases and permit the future improvements of ephemerides.

Some links

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Uncategorized

The first release of Gaia astrometric data

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Hi there! Today is a little bit different, since I will tell you about positions of stars in the sky. WTF??? No Solar System today? Well, actually this is very useful for studying the Solar System. This deals with astrometry, which tells you where your object is.
Another difference with the usual business is that I do not present you a paper, but a series of paper. I have counted 6 papers related to this first release of Gaia data, i.e. the Gaia DR1, for Gaia Data Release 1. They will be published soon in Astronomy and Astrophysics, and some of them are freely available on arXiv. The Gaia Data Release 1 was made available online on Sept, 14th.

Why astrometry?

When you want to study Solar System objects, you need to know where they are, especially if you study their orbital motion, but not only. For that, you use stars as fixed enough reference points, with respect to which you will locate your planetary object of interest. Actually, the stars have some motion with respect to the observer. They have their proper motion, since our galaxy is moving, and a parallax effect, which is a consequence of the motion of the Earth. If you observe something that does not move while you are moving, you will see an apparent motion. This motion will be all the more significant that the object is closer. These problems motivate the use of even further objects, the quasars, with respect to which the stars will be located. These quasars, for quasi-stellar radio-sources, are actually galaxies with an active nucleus. As galaxies, they are further from us than the observed stars, which belong to our galaxy. Moreover, they are brighter, which make them ideal reference points for defining reference frames, in which the stars will be positioned.

One of the goals of the Gaia mission is to elaborate the most accurate and exhaustive catalog giving the positions of stars.

The first space experiment devoted to high precision astrometry was Hipparcos, for High precision parallax collecting satellite. It was made by the European Space Agency (ESA), launched in 1989, and has operated until 1993. It could detect light sources until the magnitude 12.5. It resulted in 3 catalogs: Hipparcos, Tycho-1 and Tycho-2.

The Hipparcos catalog was constituted of 118,218 entries, giving astrometric and photometric data for almost all of them. The astrometric data were composed of 6 elements: right ascension and declination, which locate the object on the sky, the parallax, which is related to its distance, the proper motion in right ascension and declination, and its radial velocity, i.e. the time variation of its distance.

A more extensive analysis of the stars detected by Hipparcos resulted in 2 more comprehensive catalogs, Tycho-1 and Tycho-2, constituted of respectively 1,058,332 and 2,539,913 entries. Tycho-2 was the most accurate catalog we disposed on until this first release of Gaia data. It gives astrometric data at the mean date J1991.25.

Gaia is an astrometric satellite made by ESA and launched in December 2013. It orbits close to the Lagrange point L2 of the Sun-Earth system. This means that it lies between the Sun and the Earth, at a distance of 1.5 millions km from the Earth, and that its orbit is very stable, since the gravitational attraction of the Earth balances the one of the Sun, at that place. This pretty limited distance from the Earth allows a high rate of data transmission (40 Gbyte / day). From that place, Gaia makes systematic scans of the sky during its 5-years operational phase, which has started on July 25th 2014. It is composed of 2 telescopes with a very stable angle between them, and the whole sky shall be observed 70 times during the 5-years nominal mission.

Gaia can detect light sources up to the magnitude 20. This will permit the discovery of unknown Solar System objects, like asteroids or comets, but also of exoplanets. The discovery of a supernova, named Gaia14aaa, has been announced in September 2014. Moreover, the accurate determination of the proper motion of the stars shall give us an accurate picture of the motion of our galaxy, and permit a better knowledge of the position of the stars in the past and in the future. This shall help to redetermine astrometric position of Solar System objects on old astrometric planets, and so refine their orbital ephemerides, as proposed by the NAROO (New Astrometric Reduction of Old Observations) project.

The Data Release 1

The Data Release 1 has been released on Sep, 14th 2016. It contains positions of more than one billion of stars brighter than magnitude 20.7, and proper motion and parallaxes of about 2 millions of stars, which are the Tycho-2 objects. These numbers are given at the date J2015.0. The data are based on the first 14 months of the operational phase, and they should be seen as very preliminary results.

This release is of high importance, since it represents a major improvement with respect to the catalog Tycho-2, and shows the efficiency of Gaia. We could thus be very confident in the accuracy of the future releases.

In the future

This Data Release 1 is just the first release. Others will come, in which the astrometric data will be accompanied by photometric data. The Data Release 2 is planned for summer 2017, the releases 3 and 4 for 2018 and 2019 respectively, while the final one should come in 2022. This final release shall also include discoveries of Jupiter-like planets out of our Solar System.

At the end, Gaia shall have an astrometric accuracy of 25 micro-arcseconds at the magnitude 15, while Hipparcos reached 1 milli-arcsecond. Reaching such an accuracy is a challenge. For that, the timing must be extremely precise, and second-order relativistic effect of the deviation of the light by the Earth and other object must be considered.

Regarding the parallaxes, i.e. the distance: Hipparcos has given us the parallaxes of 60,000 objects with an accuracy of 20%, while the Gaia Data Release 1 gives us the same information, with the same accuracy, for 1 million objects. The Final Release shall give us 10 millions of parallaxes with an accuracy of 1%, 150 millions of them with an accuracy of 10%, 280 millions of them with an accuracy of 20%. Knowing the distances of stars with such a precision will permit major improvements in the understanding of star clusters and in the structure of the Milky Way.

 

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