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Gaia - Surveying a billion stars
Updated: 1 year 10 months ago

Virtual Milky Way

Thu, 2016-11-10 10:57

ESA’s Gaia mission is surveying stars in our Galaxy and local galactic neighbourhood in order to build the most precise 3D map of the Milky Way and answer questions about its structure, origin and evolution.

Launched in 2013, Gaia has already generated its first catalogue of more than a billion stars – the largest all-sky survey of celestial objects to date.

To achieve its scientific aims, the spacecraft operates in an ultra-high-precision pointing mode, and to enable the flight control team to monitor spacecraft performance, Gaia regularly reports to the ground information about its current attitude and the stars that have been observed.

These engineering data have been accumulated over 18 months and combined to create a ‘map’ of the observed star densities, from which a beautiful and ghostly ‘virtual image’ of our magnificent Milky Way galaxy can be discerned, showing the attendant globular clusters and Magellanic clouds.

A ghostly image of our Milky Way galaxy derived from spacecraft orientation data Credit: ESA

The intensity scale of this map represents star density derived from the engineering data representing star density. Where there are more stars, as in the Galactic centre, the map is brighter; where there are fewer, the map is darker. The map includes brightness data corresponding to several million stars.

More information on Gaia operations

Editor’s note: On 21 November, at 16:00 CET, the Gaia mission team will host a live ‘Ask Me Anything’ chat. Details will be posted via ESA social media channels later.

Time-out for blog while catalogue production is underway

Fri, 2014-10-10 10:14

Credit: ESA/ATG medialab; background: ESO/S. Brunier

We started this blog just over one year ago and what a year it has been! We've had the excitement of  the launch, the fabulous first-light image, the challenges of some aspects of commissioning and, more recently, the relief and satisfaction of getting the 'go' for science, and even the first of Gaia's science alerts.

Now that science data have started to flow, the main activity for scientists working on the mission is preparing for the first catalogue release, planned for summer 2016. So while they are busy with that important task, we will take a break on the blog.

But don't worry, this doesn't mean that there will be no news or updates about the Gaia mission. You will be able to keep in touch with the mission via our websites (Space Science Portal,  Science & Technology, and Cosmos), and you can also follow the progress of Gaia via Twitter (@ESAGaia) and using the Gaia Mission app (for iPhones) which can be downloaded from iTunes.

Thanks to all of you for following us through this exciting first year!

The Gaia Team and Blog Editors


Gaia discovers its first supernova

Thu, 2014-09-18 10:16

While scanning the sky to measure the positions and movements of stars in our Galaxy, Gaia has discovered its first stellar explosion in another galaxy far, far away.

This powerful event, now named Gaia14aaa, took place in a distant galaxy some 500 million light-years away, and was revealed via a sudden rise in the galaxy’s brightness between two Gaia observations separated by one month.

Gaia, which began its scientific work on 25 July, repeatedly scans the entire sky, so that each of the roughly one billion stars in the final catalogue will be examined an average of 70 times over the next five years.

“This kind of repeated survey comes in handy for studying the changeable nature of the sky,” comments Simon Hodgkin from the Institute of Astronomy in Cambridge, UK.

Many astronomical sources are variable: some exhibit a regular pattern, with a periodically rising and declining brightness, while others may undergo sudden and dramatic changes.

Light curve

“As Gaia goes back to each patch of the sky over and over, we have a chance to spot thousands of ‘guest stars’ on the celestial tapestry,” notes Dr Hodgkin. “These transient sources can be signposts to some of the most powerful phenomena in the Universe, like this supernova.”

Dr Hodgkin is part of Gaia’s Science Alert Team, which includes astronomers from the Universities of Cambridge, UK, and Warsaw, Poland, who are combing through the scans in search of unexpected changes.

It did not take long until they found the first ‘anomaly’ in the form of a sudden spike in the light coming from a distant galaxy, detected on 30 August. The same galaxy appeared much dimmer when Gaia first looked at it just a month before.

“We immediately thought it might be a supernova, but needed more clues to back up our claim,” explains Łukasz Wyrzykowski from the Warsaw University Astronomical Observatory, Poland.

Other powerful cosmic events may resemble a supernova in a distant galaxy, such as outbursts caused by the mass-devouring supermassive black hole at the galaxy centre.

Supernova Gaia14aaa and its host galaxy

However, in Gaia14aaa, the position of the bright spot of light was slightly offset from the galaxy’s core, suggesting that it was unlikely to be related to a central black hole.

So, the astronomers looked for more information in the light of this new source. Besides recording the position and brightness of stars and galaxies, Gaia also splits their light to create a spectrum. In fact, Gaia uses two prisms spanning red and blue wavelength regions to produce a low-resolution spectrum that allows astronomers to seek signatures of the various chemical elements present in the source of that light.

Read the full story on Gaia's first supernova discovery on the ESA Portal.

Gaia in your pocket – mapping the Galaxy with the new Gaia app

Mon, 2014-09-01 13:08

Mapping one billion stars in our Galaxy may seem like an impossible feat, but that’s exactly what ESA’s Gaia mission aims to do, with the ultimate goal of creating the largest, most precise 3D map of our Galaxy ever made. And now you can follow the mission’s progress with a new app created by the University of Barcelona. Being able to track the progress of this groundbreaking mission via your iPhone, iPad or iPod means the stars have never been closer!

Screenshot of Gaia App

The Gaia Mission app is a must-have for space enthusiasts and novices alike. Beautiful images, interactive diagrams, and videos of the satellite explain many aspects of this star-mapping mission, and clear instructions and demos are available for every feature of the app. For more experienced stargazers there is a host of in-depth information available for each section, accessed by simply swiping the page.

Screenshot of Gaia App

The app has interactive diagrams of the spacecraft and payload that can be moved 360˚, letting you explore inside the Gaia satellite, and by tapping on the highlighted regions of the diagrams you’ll find clear information about each component. The trajectory of the satellite and the distance from Earth can be followed via the mission status page, and you can track how much data has been acquired and processed on the mission operations page. Live news updates will ensure that you are among the first to know about any exciting new discoveries!

This is not the first time a satellite has been sent to map the stars. Back in 1989 the ESA Hipparcos mission charted over 120,000 objects, which formed the basis of a huge stellar catalogue. The Gaia mission will greatly improve on this achievement, as it will measure the position and motion of stars with a much higher level of accuracy. The number of stars observed during this five-year mission will increase to over one billion, resulting in the most precise three-dimensional map of our Galaxy ever created. Over the course of the mission, one petabyte (one million gigabytes) of digital information will be sent to the Data Processing and Analysis Consortium for analysis and for cataloguing - that’s enough to fill over 1.5 million CD ROMs!

Screenshot of Gaia App

The Gaia satellite orbits the second Sun-Earth Lagrange point, known as L2. There, at a distance of 1.5 million kilometres from the Earth, Gaia will have excellent views of the Galaxy, free of any eclipses and in stable thermal conditions. Scanning the sky as it rotates on its axis, Gaia will view each star about 70 times, allowing a great deal of information to be collected about each and every one. The Gaia Mission app will give updates on many of Gaia’s activities, from the moment it was launched on 19 December 2013, until the final catalogue is published in 2022.

As Carme Jordi, from the team at the University of Barcelona who developed the app explains: “With Gaia, we will be able to see the entire history of the Milky Way unfolding before our eyes.”

“Gaia has so many interesting aspects – from our view of the Universe, to the life cycles of stars and the detection of exoplanets. With the app you can learn the basics of all of these things and then see how the mission builds up a new picture for us all.”

Screenshot of Gaia App

The team hopes to add more interactive features to the app over the coming months. These could also be interesting for students in second and third level education. “There are different levels within the app,” says Marcial Clotet, the engineer who first came up with the idea for the app. “Those who want to can go through the various levels and find really in-depth information to correspond to their level of interest”.

“Humans have been mapping the stars for centuries, but there is still a great deal to find out with the Gaia mission,” continues Marcial. “Many of the people who made star maps could only dream of being able to observe from space; now, not only can we do it, we can share the adventure in real time.”

The app was created by the University of Barcelona and is available for free from the iTunes store in English, Spanish and Catalan. The University of Barcelona team is now working on the Android version, which will be available later this year.

This project was co-financed by the Spanish Foundation for Science and Technology - Ministry of Economy and Competitiveness.


‘Go’ for science

Tue, 2014-07-29 13:42

Following the extensive in-orbit commissioning review and after encountering the unexpected challenges highlighted previously on the blog, Gaia is now ready to begin its science mission.

Read the announcement published today on the ESA Portal: Gaia: 'Go' for science

And for a full quantitative analysis of Gaia’s expected science performance based on the results of commissioning, see: Commissioning review: Gaia ready to start routine operations

Asteroids at the “photo finish”

Thu, 2014-07-03 07:00

Today's Gaia blog post is contributed by Paolo Tanga, Associate Astronomer at the Observatoire de la Côte d’Azur, Nice (France).

We tend to think that a still picture, shot with an ordinary camera, represents a subject at a given time. But this is not always the case. In some situations, a picture can show the evolution in time of the depicted subject. This is the case, for example, of the well-known “photo finish” technique widely used in athletics to record the competing athletes as they cross the arrival line at the end of the race.

How does it work? Simply, the camera aims only at a vertical strip containing the finish line and repeatedly photographs it at high speed. By putting all the strips together side-by-side, one can obtain the evolution of the image of the finish line as a function of time. As weird as it may sound, the CCD camera onboard Gaia works exactly the same way – by transforming the recorded star positions into times, the finish line being a thin strip of pixels on the edge of the detector.

Let’s imagine that we are looking at a number of athletes all running at the same speed on a straight track, but each of them having started the race at a different time: in this analogy, these are the stars, which drift across the Gaia telescopes all at the same velocity – given by the constant rotation of the satellite. If Gaia observes them several times, they will always appear spaced by the same delays.

Now, let’s add to these well-behaved competitors a different type of athlete, a rebel one who's not playing by the rules, always running either much faster or much slower than the others, and not following the direction of the track lanes but drifting as he/she pleases. Each time this eccentric athlete crosses the finish line, it will be in a different position relative to the competing runners. This is how an asteroid appears to Gaia, as its motion relative to stars makes it appear always in a different position, as a function of the time at which it is observed.

This unorthodox behaviour opens up a specific category of problems when dealing with asteroid observations. The first one is predicting when – and where – Gaia will observe a given object. In practice, it’s like predicting in advance the delays of the eccentric athlete relative to the others, when on the finish line. To perform this computation, we need to have an exact knowledge of its trajectory (the orbit of the asteroid), along with the precise speed of the “ordinary” competitors (the stars). In the case of Gaia, all these pieces of information are known, but the complexity of the scanning law, which displaces the “arrival line” in non-trivial patterns, makes the task extremely delicate. Besides, there are several “finish lines” on the Gaia focal plane (at least one per CCD), so the whole geometry of the system plays a role.

The second type of problem concerns the processing of asteroid observations, especially in the case of newly detected asteroids or of asteroids whose orbit is not yet known to great precision. In fact, each time the asteroid crosses the “finish line” it will be in a different region of the sky. Only observations that are close in time can be easily linked together, as the asteroid displacement relative to its background will be small. If the observations are performed over longer time spans, the presence of several such “rebel runners” can make things extremely complex.

These various aspects are illustrated in the following pictures. The first one (right) is a test image of the asteroid (54) Alexandra, a bright moving target. It was obtained by programming Gaia in a special imaging mode. As described before, this is a “photo finish” image. It was reconstructed by moving along the horizontal axis, which is equivalent to the observer moving in time: each pixel column represents the signal present on the “finish line” (in practice: the edge of the CCD) at a given moment. In the image, the time delay between the arrival at the finish line of the bright star and the asteroid is about 1.26 seconds. A very accurate timing of each source “arrival” is the basis of the extraordinary astrometric capabilities of Gaia.

More important, however, is the fact that in this image the predicted position of the asteroid is very close to the observed one, only a few pixels away. Given the computational difficulties involved in this process, this is an achievement with important consequences, such as the possibility to predict well enough very close “encounters” between a star and an asteroid on the plane of the sky – these are potential sources of confusion while searching for other types of anomalies (when monitoring the brightness of a star, for example). Many astronomers want to be alerted when an interesting change occurs, not when an asteroid is just passing by!

On the other hand, other astronomers (planetary scientists!) are interested in the asteroids themselves. In fact, Gaia will observe 350,000 asteroids, providing the richest sample of precise orbits and physical properties that we could dream of. Those rebel runners, containing clues about the Solar System's formation, are really interesting, and come in large quantities. Our capability to track their position is essential in the identification process.

The case of the asteroid (4997) Ksana (above) is more difficult, and showcases the capabilities of Gaia in detecting and identifying asteroids. Because it is very faint, it may have been confused with several stars – some not even present in current catalogues – making its identification more ambiguous. The presence of a source very close to the position where the asteroid was predicted to be is very encouraging, but only a comparison of data acquired over time can provide a confirmation.

The result is shown in (left), which represents an intermediate product of the processing itself: the preliminary positions of the sources seen by Gaia, as determined by the “Initial Data Treatment”. In these images, each point is a source and the point size is proportional to the source's brightness. Different colours represent the stars observed during five different sweeps of the same sky region, each lasting 6 seconds, by a single CCD.

The asteroid (4997) Ksana is now clearly seen moving from one sweep to the next (as indicated by the arrows). Checking the presence and motion of the object at the corresponding epoch provides a secure confirmation of its nature. A final remark: the observations are not equally spaced in time, and the closer couple of detections correspond to the source passing through the two telescopes (106 minutes apart) while the satellite rotates. A full rotation of the satellite (every 6 hours) separates the two detections in each pair.

Gaia asteroid observations will be processed using the software pipeline designed and implemented by Coordination Unit 4 of the DPAC, running at the CNES processing centre (Toulouse, France).

The data presented here are extracted from the results obtained by the Initial Data Treatment (IDT) pipeline, which was largely developed at the University of Barcelona and runs at the Data Processing Centre at ESAC.

Commissioning the Radial Velocity Spectrometer

Fri, 2014-06-27 12:27

Guest blog post by George Seabroke, RVS Payload Expert, on behalf of the team commissioning the RVS instrument.

The Radial Velocity Spectrometer (RVS) is one of three instruments onboard Gaia (see Figure 1). It is designed to measure the line-of-sight velocity component of Gaia stars (radial velocity, RV) to complement Gaia astrometry, which measures the transverse velocity component (parallax converts proper motions to transverse velocity). Combining the radial and transverse velocities gives the 3D space velocity of Gaia stars, allowing Gaia to produce not only a map of where Gaia stars are but how they are moving.

Figure 1: Photographs of RVS components (insets) overlaid on a photograph of the Gaia Payload Module at Astrium in Toulouse. Credits: Astrium, except top left (ESA/Gaia/DPAC/Airbus DS), bottom left (ESA) and top right (Selex Galileo, Italy). Composite designed by George Seabroke, MSSL.

The accuracy that Gaia astrometry is aiming for can only be achieved above the Earth’s atmosphere. The RV accuracy that RVS is aiming for (a few km/s for bright stars) can be achieved from the ground: however, the ground-based RV follow-up of the 118,200 Hipparcos stars managed about 20,000 stars in 15 years (a remarkable effort at the time!). Therefore it was decided to include an RV instrument onboard Gaia from the beginning and thus RVS was born.

RVS is an optical module located between the last mirror (M6) and the 12 RVS CCDs at the right edge of the Gaia focal plane (see Figure 1). RVS consists of six optical elements: a transmission grating (see bottom left in Figure 1), which disperses all the light entering RVS into medium-resolution (λ/Δλ ~ 11,500) spectra; a band-pass filter, which limits the spectra to 845–872 nm (wavelengths visible to the human eye are 400-700 nm); and four lenses/prisms (see top right in Figure 1) to correct the main aberrations of the telescope.

The RVS wavelength range is chosen to include a set of three absorption lines called the calcium triplet (see top left in Figure 1; click here for enlarged version of the spectra). Calcium is made by nuclear fusion in the centre of massive stars. When they explode as supernovae, the calcium can end up in the atmospheres of the next generation of stars (or in our teeth and bones!). If the star is moving away or towards Gaia, the calcium triplet (and all spectral) lines will be shifted by the Doppler effect, compared to their rest wavelengths. The RV of each star can be determined from RVS spectra by measuring these shifts.

The RVS Payload Experts (PEs) are the team, in DPAC, supporting the commissioning of the RVS instrument. The team, geographical locations and responsibilities are mainly split between Observatoire Paris Meudon (OPM) and Mullard Space Science Laboratory (MSSL, part of University College London).

Unfortunately, at least from my personal perspective, we are PEs, rather than Payload Specialists. Payload Specialists were astronauts who flew on the Shuttle with specific payloads. Even if we wanted to visit Gaia, we would not be able to because at the second Sun–Earth Lagrangian point (L2), Gaia is much further than any astronaut has ever been, over 1 million km beyond the Moon’s orbit! Having not been selected to become an ESA astronaut in 2009 (one of my competitors, Alexander Gerst, is on the International Space Station right now!), working on commissioning the RVS is probably the closest I have come to being in space!

One recent commissioning activity that I led was when the RVS CCDs had charge injected into them to check for and calibrate any radiation damage.

The most critical issue for the RVS PEs was to check that the six optical elements of RVS are producing spectra that include the calcium triplet absorption lines, allowing us to derive RVs. If we had not seen the calcium lines, we would have had a bone to pick with RVS (English idiom!). Figure 2 shows what a 2D RVS spectrum looks like on a RVS CCD. RVS spectra are summed in the ACross Scan (AC) direction (y-axis in Figure 2) to produce 1D spectra, like the one in the top left inset of Figure 1 (see blog article “Gaia takes science measurements” for more details on this RVS spectrum – the first to go public). The calcium lines are clearly visible (so ironically no bone to pick!).

Figure 2: Example of a 2D RVS spectrum, extracted by Pasquale Panuzzo (OPM). The three dark vertical lines are where calcium atoms in the atmosphere of this particular star have blocked starlight to form absorption lines in this star’s spectrum.

The biggest surprise for the RVS PEs was finding there was a scattered light problem. The Basic Angle Monitor (BAM) has a laser at the same wavelength as RVS so there was a commissioning activity in January to check the levels of the BAM laser light leaking into RVS. Because of this, we were the first to measure the Gaia instrument background and found it to be much higher than expected from the BAM laser alone. Since then, Gaia’s instrument background has become a hot topic in commissioning and astrometric and photometric PEs have found that their instrument backgrounds are also affected. Various experiments have been conducted to understand the source of the scattered light and the Gaia PEs have been analysing the results of these experiments and feeding the results back to ESA.

OPM have developed most of the RVS offline tools. This involves downloading about 31 Gb of data per day (the total is now 7 Tb), ingesting it into local databases and generating daily “First Look” reports to allow the RVS PEs to have a first look at the data. The offline tools have been used in three other very important ways. Firstly they were used to analyse quickly the RVS spectra during the best focus commissioning activity, which were independently verified by MSSL. RVS spectra do have the required sharp absorption lines, including the calcium lines, which reduce the uncertainty on RV measurements. These spectra were also used by OPM to derive the RVS resolution. In addition, OPM are verifying and optimizing the onboard Video Processing Unit parameters, which control how well the readout windows are centred around RVS spectra to make sure we capture as much dispersed starlight as possible.

MSSL has developed a local infrastructure and integrated the official on-ground RVS data processing pipeline, which includes modules by all of Co-ordination Unit 6 (“Spectroscopic Processing”). MSSL are now running it to investigate the RVS data. The pipeline consists of more than 60,000 lines of code, described in over 800 pages of documentation (Software Design Descriptions). It has involved a lot of debugging of software to ensure the pipeline can process real RVS spectra. The commissioning period included 14 days of undisturbed data obtained between 9 and 23 May. The challenge now for MSSL, having ingested all these spectra (more than 111 million!) into our databases, is to process them through the RVS pipeline to estimate how accurate the RVS RVs will be (as a function of stellar brightness). RVS was never designed to measure the RVs of all Gaia’s 1 billion stars. We will soon know the number of stars for which RVS will be able to measure an RV and what the RV accuracy will be. Whatever the answer, it should be tens of millions, making RVS the largest RV survey in history!

I was fortunate enough to be in French Guiana to see the Gaia launch but like all the RVS PEs, we have never actually seen the completed RVS instrument with our own eyes! Now we will never see it, as Gaia will not return to Earth. Nevertheless, we have been getting to know RVS in the last six months of commissioning through the data it is returning. Over the next five years (or more) of the mission, we will get to know the instrument in ever-greater detail. This will allow us continually to optimize its operation onboard Gaia and also the on-ground algorithms that process the data, ensuring RVS reaches its enormous scientific potential.

Written by George Seabroke (MSSL) on behalf of the RVS PEs: Kevin Benson (MSSL), Mark Cropper (MSSL), Chris Dolding (MSSL), Joris Gerssen (Potsdam), Alain Guéguen (OPM), Leanne Guy (Geneva), Howard Huckle (MSSL), Katja Janssen (Potsdam), David Katz (OPM), Olivier Marchal (OPM), Pasquale Panuzzo (OPM, RVS PE Co-ordinator), Paola Sartoretti (OPM), Mike Smith (MSSL).


Preliminary analysis of stray light impact and strategies

Mon, 2014-06-16 13:34

Update from the Gaia Project Team

A series of exhaustive tests have been conducted over the past few months to characterise some anomalies that have been revealed during the commissioning of Gaia following its successful launch in December 2013, as have been discussed in previous blog posts.

Annotated diagram of the Gaia payload module. Click for more information.

Key among these are an increased background seen in Gaia’s focal plane assembly due to stray light entering the satellite and reduced transmission of the telescope optics. In an effort to understand both problems, much of the diagnostic work has been focussed on contamination due to small amounts of water trapped in the spacecraft before launch that has been “outgassing” now that Gaia is in a vacuum.

The water vapour freezes out as ice on cold surfaces and since Gaia’s payload sits at temperatures between –100 and –150°C in the dark behind the big sunshield, that is where it ends up, including on the telescope mirrors. The ice initially led to a significant decrease in the overall transmission of the optics, but this problem was successfully dealt with by using heaters on Gaia’s mirrors and focal plane to remove the ice, before letting them cool down to operational temperatures again.

Some ice on the mirrors was expected – that is why the mirrors are equipped with heaters – but the amount detected was higher than expected. As the spacecraft continues to outgas for a while, future ‘decontamination’ campaigns are foreseen to keep the transmission issue in check using a much lighter heating procedure to minimise any disturbing effect on the thermal stability of the spacecraft.

With regards to the stray light, our analysis of the test data indicates that it is a mixture of sunlight diffracting over the edge of the sunshield and brighter sources in the ‘night sky’ on the payload side, both being scattered into the focal plane. A model has been developed which goes some way to explaining the stray light seen in the focal plane, but not all aspects are yet understood.

One key working hypothesis was that ice deposits have built up on the ceiling of the thermal tent structure surrounding the payload, and that scattering off this ice might enhance the stray light. Although there is no way to directly confirm that this is indeed the situation, the Gaia project team nevertheless considered ways of removing any such ice.

Unlike the mirrors and focal plane, the thermal tent does not have any heaters, so alternative solutions had to be explored. One option analysed in detail would involve altering the attitude of the spacecraft to allow sunlight to directly enter the thermal tent in order to remove any ice that might be there. The risks associated with this concept were assessed, and software and procedures developed to carry it out safely, but there is currently no plan to do so.

The reason is that we have also been conducting tests in our laboratories at ESTEC to try to replicate and better understand the situation. We have added layers of ice of varying thickness to representative samples of the same black paint that covers the inside of the Gaia thermal tent, to assess the ways it might be affecting the stray light. There is no evidence to suggest that thin layers of ice would in fact enhance the stray light and thus no evidence that an attempt to remove the hypothesised ice contamination in the tent would yield any benefit, hence the decision not to carry out this procedure.

Under the assumption that the stray light cannot be completely eliminated, we are investigating a variety of modified observing strategies to help reduce its impact over the course of the mission, along with modified on-board and ground software to best optimise the data that we will collect. As stated in earlier posts, even if we do have to work with the stray light, we already know that it will only affect the quality of the data collected for the faintest of Gaia’s one billion stars.

Stray light increases the background detected by Gaia and thus the associated noise. The impact is largest for the faintest stars, where the noise associated with the stellar light itself is comparable to that from the background, but there is minimal impact on brighter ones, for which the background is an insignificant fraction of the total flux.

The stray light is variable across Gaia’s focal plane and variable with time, and has a different effect on each of Gaia’s science instruments and the corresponding science goals. Thus, it is not easy to characterise its impact in a simple way.

Broadly speaking, however, our current analysis is that if the stray light remains as it is today, its impact will be to degrade the astrometric accuracy of a solar-type star at magnitude 20, the faint limit of Gaia, by roughly 50%, from 290 microarcsec to 430 microarcsec by the end of the mission. Things improve as you move to progressively brighter stars, and by magnitude 15, the accuracy will remain unaltered at approximately 25 microarcsec.

Credit: ESA/ATG medialab; background: ESO/S. Brunier

It is important to realise that for many of Gaia’s science goals, it is these relatively brighter stars and their much higher accuracy positions that are critical, and so it is good to see that they are essentially unaffected. Also, the total number of stars detected and measured will remain unchanged.

For brightness and low-resolution spectroscopic measurements made by Gaia’s photometric instruments, current indications are that the faintest stars at magnitude 20 will have been measured to roughly the 6–8% level by the end of the mission, rather than a nominal 4%, while brighter stars will remain more accurate at about 0.4%.

The radial velocity spectrometer is most affected by the stray light and about 1.5 magnitudes of sensitivity could be lost, although the number of stars that that translates into will not be known until on-going data analysis is complete.

Finally, Gaia also contains a laser interferometer called the ‘basic angle monitor’, designed to measure the angle of separation between Gaia’s two telescopes to an accuracy of 5 microarcseconds every few minutes. This is necessary in order to correct for variations in the separation angle caused by ‘normal’ thermal changes in the payload as Gaia spins. The system is working as planned, but is seeing larger-than-expected variations in the basic angle. We are currently examining these data to discover if this issue will have any impact.

A comprehensive understanding of these issues will be given when a thorough analysis of all engineering tests is complete. Gaia has nearly completed its performance verification data taking, and is about to start a month-long dedicated science observation run. Once the data have been fully analysed, we will be able to provide a detailed quantitative assessment of the scientific performance of Gaia.

While there will likely be some loss relative to Gaia’s pre-launch performance predictions, we already know that the scientific return from the mission will still be immense, revolutionising our understanding of the formation and evolution of our Milky Way galaxy and much else.


Gaia takes science measurements

Thu, 2014-06-05 12:04

As part of the on-going commissioning tests, we are happy to be able to report on the first spectroscopy observations made by Gaia.

You will have seen the ‘first light’ images from the early phases of commissioning already, but as part of these activities we have also started taking test spectroscopic measurements of known stars.

While astrometric measurements will determine the positions and motions of stars, Gaia will use spectroscopy to measure key physical properties, such as brightness, temperature, mass, age, and chemical composition.

This is achieved by studying stellar spectra – the fingerprints of stars. Typically, a star’s spectrum includes a broad continuum spanning a wide range of wavelengths coming from the hot gas at the surface of the star. This is then interspersed with dips at specific wavelengths, where cooler atoms and molecules in the ‘atmosphere’ of the star absorb some of the continuum light. Occasionally, brighter emission lines can also be seen. The absorption and emission lines provide an indication of the elements present in the object and under what temperature and pressure conditions they exist.

In addition, the lines can all be shifted from their normal wavelengths – that is, the corresponding wavelength at which the same line is observed in the laboratory – if the star is moving towards us or away from us. These stellar radial velocities can be used to determine the velocity of stars with respect to the Sun, and are therefore essential to understand stellar motions in our Galaxy.

The two plots shown here give an idea of the kind of spectroscopic information that Gaia will return over its 5-year mission.

Gaia RVS data (top) compared with high-res ground-based observations (bottom) for the star HIP 86564.
Credits: ESA/Gaia/DPAC/Airbus DS

The first (above) is a radial velocity spectrum for a bright star (HIP 86564), with key elements identified. The RVS only covers a very narrow spectral range at wavelengths centred near 860nm, just beyond the visible red, but provides high enough spectral resolution to make it possible to measure stellar velocities to within a few kilometres per second. The most prominent spectral lines correspond to iron (labelled Fe), titanium (Ti), and calcium (Ca). The ‘triplet’ of calcium lines is particularly important, as they appear in almost all stars. The Gaia plot (top) is compared with high-resolution ground-based observations of the same star, by the NARVAL instrument at the Pic-du-Midi Observatory (bottom), showing that the Gaia RVS is working as expected.

The second plot (below) shows temperature information for seven different bright stars (labelled, along with their spectral types – click here for background on stellar types). This information is extracted from Gaia’s photometric instrument, which generates two low-resolution spectra, one covering blue wavelengths and the other red wavelengths. The blue photometer (BP) receives light with shorter wavelengths (from 330 nm to 680 nm), and the red photometer (RP) receives light with longer wavelengths (from 640 to 1050 nm). The photometers record the total intensity of each star across these wavelengths, and also make it possible to determine the stellar temperatures.

Gaia BP/RP data for seven bright stars.
Credits: ESA/Gaia/DPAC/Airbus DS

A pair of red and blue spectra is shown here for each of the seven stars. The plot is arranged with cool stars (approximately 3000ºC) at the top, to hotter stars (around 8000ºC) at the bottom. As expected, the hottest stars are relatively stronger in Gaia’s blue photometer, and weaker in the red photometer. Conversely, the cooler stars are brighter in the red photometer. Data like these will be used to determine the temperatures for millions of stars in the Milky Way that have not yet been studied in detail.

More details and original spectra are available here.

Within the next few days we will provide an update on the stray light issues discussed in previous blog posts. 

Posted on behalf of the Gaia Project Team 


Commissioning update

Thu, 2014-04-24 12:58

The Gaia project team provides an update on the ongoing commissioning activities of ESA’s billion star surveyor…

The work done to bring online all components of the Gaia service module, which houses equipment needed for the basic control and operation of the satellite, has gone very smoothly. The chemical and micro propulsion systems function well, with the latter providing tiny (micro-Newton) thrusts to maintain Gaia’s spin rate, compensating for torques due to solar radiation pressure. The phased array antenna is operating very well, ensuring that we can maintain the high data rates that are needed to downlink all the science data. And the essential rubidium atomic clock is also working to specification.

Exploded view of Gaia spacecraft. Click for more information.

The Gaia scientific payload is also functioning very well. This includes all 106 CCD detectors and the associated electronics units, as well as the seven on-board computers that manage the CCD’s. Alignment and co-focusing of the two telescopes through their movable secondary mirrors is working as expected. Following the last displacement of one of the secondary mirrors by just 3 micrometres, we are currently at the optimal image quality that Gaia can deliver, well balanced across the large focal plane and the three instruments. This is no small achievement considering the complexity of the optics!

However, a few other aspects of the commissioning have been progressing somewhat less smoothly.

In order to deliver exquisitely precise measurements of the positions of stars on the sky, we need in turn to know where Gaia itself is in space very accurately at any given moment. The distance part of Gaia’s orbit is readily determined from radio signals sent back and forth, but the position on the plane of the sky needs ground-based telescope observations of the satellite.

It turns out that Gaia is much fainter in the sky than hoped for, at magnitude 21 rather than 18, and thus the smaller 1 metre diameter class telescopes planned to be used by Gaia’s GBOT network  are not big enough to detect Gaia in a reasonable amount of time. But by shifting the bulk of the observations to the 2.0-m Liverpool Telescope on La Palma and ESO’s 2.6-m VST on Paranal, as well as introducing Very Long Baseline Interferometry radio measurements, the problem is now under control.

Near the beginning of commissioning, a steady drop in the transmission of Gaia’s telescopes was seen, due to water-ice deposits building up on the mirrors as trapped water vapour was liberated from the satellite after launch. The transmission was fully recovered following a decontamination campaign, during which the payload was heated to remove the ice from the optics.

Annotated diagram of the Gaia payload module. Click for more information.

Ice deposits are thought to play a part in another concern, in which unanticipated ‘stray light’ is seen hitting parts of the Gaia focal plane. Some of the stray light is thought to come from sunlight diffracted around the edges of the sunshield and entering the telescope apertures. There also seems to be a smaller contribution from night sky sources reaching the focal plane via unexpected paths.

Although the diffracted sunlight component was foreseen, we think that it is enhanced by reflections off ice deposits on the ceiling of the ‘thermal tent’ structure surrounding the payload, allowing it to reach the focal plane. It was hoped that the decontamination campaign would also remove this ice layer, but unfortunately the stray light is still there at the moment.

Careful preparations are being made for one more attempt to remove the water ice and, hopefully, the stray light. But in parallel, we are now continuing with the nominal commissioning and a detailed performance verification phase. Even if the stray light remains, the current best assessment is that degradation in science performance will be relatively modest and mostly restricted to the faintest of Gaia’s one billion stars.

We will, of course, provide an update on the blog when we have new information to share.

Posted on behalf of the Gaia project team.

Gaia Live in School: Inspiring the next generation of European Space Scientists

Fri, 2014-04-04 11:03

Students following 'Gaia Live in School' event

Will Gaia discover planets that humans would be able to live on? What is a quasar? How many people are actually working on the mission at the moment? These are just some of the varied questions that school students put to some of ESA’s Gaia experts during the Gaia Live in School Event on 25 March 2014.

More than 2000 students, mainly aged 10-12 years old, from 34 schools in 10 European countries followed a live webcast from the Gaia mission planning room at ESOC, ESA’s spacecraft operations centre in Germany. This special webcast gave students a unique opportunity to see behind the scenes of the Gaia mission, with Timo Prusti, the Gaia Project Scientist, and David Milligan, the Gaia Spacecraft Operations Manager answering many of the students’ questions.

Each school participating in the Gaia Live event was linked to a leading research institute in its area. On the day of the event, two postgraduate students, the ‘Gaia Explainers’ from each institute, went into the schools to deliver lively and interactive presentations about Gaia. Hands-on demonstrations and videos introduced the school students to the mission, and to key concepts such as the Solar System, the Milky Way and parallax, to aid their understanding of the science of Gaia before linking up to the live webcast.

Thumbs up at 'Gaia Live in School' event

In the first part of the live webcast students watched David Milligan describe Gaia’s journey to its orbit about L2, a gravitational equilibrium point that is 1.5 million kilometres from Earth, how the spacecraft is operated, and how data are sent to and from the satellite. Timo Prusti continued by explaining why it is important to make a 3D map of the Milky Way, how Gaia will help to reveal our Galaxy’s history, and the other exciting discoveries Gaia will make.

Timo and David then answered a range of excellent questions from the schools, which had been submitted in advance of the event. The webcast further stimulated the students’ curiosity, and even more questions for the experts came streaming in from all 34 schools to ESOC by web chat – as many as possible were answered live on air.

Following the webcast, the postgraduate students completed their sessions in the schools with another question and answer session, as well as further demonstrations and activities.

Finnish students participating in 'Gaia Live in Schools' event

In preparation for the event the postgraduate students participated in an intensive training course, held at ESTEC, where they explored how to present science concepts to groups of school students. Working together with the teachers involved at each school, the local event programmes were adapted to ensure that they were relevant for each participating school audience. The enthusiasm of the teachers helped ensure the success of the event at each school.

The event was organised as a partnership between the Gaia Research for European Astronomy Training Network (GREAT) and ESA, with many of the ‘Gaia Explainers’ being students in the GREAT Initial Training Network.


Watch the replay of the ESOC part of the Gaia Live event here.

For more information about the event and the schools taking part, visit the GREAT event web page.

Authors: Rebecca Barnes (HE Space Operations for ESA), Nicholas Walton (Institute of Astronomy, University of Cambridge)

Detecting sources

Thu, 2014-04-03 09:08

As part of the ongoing commissioning activities of Gaia, the satellite’s Sky Mapper has briefly been used in a special mode to image a patch of the sky close to the Galactic plane (below).

Detecting sources with Gaia's Sky Mapper.
Credit: ESA

The job of the Sky Mapper is to detect stars entering Gaia’s field-of-view as the satellite slowly rotates. There’s one column of Sky Mapper CCD detectors for each of the two telescopes, in order to keep track of which star is coming from which telescope when the two beams are combined onto Gaia’s big camera.

In normal operations, the Sky Mapper is continuously operating, but the image data are not transmitted back to Earth like this. Instead, Gaia’s on-board computers analyse the data stream “on the fly” to detect the point sources, and to prepare small tracking “windows” which follow the stars as they move across the main astrometric CCDs.

Gaia focal plane
Credit: ESA – A.Short

See the image above – or watch this movie – for a reminder of the layout of the Gaia focal plane and how the various parts of it work together.

The figure below shows the same Sky Mapper image, but this time also includes an overlay of the sources which were confirmed as they tracked across the first column of astrometric CCDs.

Red markers indicate stars brighter than magnitude 13: these get downlinked from Gaia as small images centred on the star. Yellow markers then indicate intermediate-brightness stars with magnitudes 13–16, while cyan markers circle the faintest stars, with magnitudes 16–20. For the fainter stars, only the brightness profile in the scanning direction is downlinked.

Finally, a small number of the detected stars – those marked here with squares – are used to measure Gaia’s spin rate and to help keep that very precisely constant via the attitude control system.

Confirmed sources in the Sky Mapper image.
Credit: ESA

These images provide visual confirmation of the on-going tests that show that the critical star detection and tracking hardware and software built into Gaia is working very well indeed.

Meanwhile and also part of the commissioning activities, work is continuing to provide a better understanding of the source of the stray light ‘contamination’ noticed earlier. Further tests have involved tilting the spacecraft to different angles with respect to the Sun to see how that might affect the amount of unwanted light reaching Gaia’s detectors, and the heating of various payload elements to help drive off any residual water ice in the system.

Both actions require considerable time to plan and execute, and also to interpret the results and respond accordingly. We will provide a further update once we have analysed all of the information.

Posted on behalf of Gaia project team. The new images were also featured today as "Image of the week" on the Gaia community pages.

Gaia tilt watched from Earth

Tue, 2014-03-04 12:28

As part of the tests to try and diagnose the stray light issue noticed in the first month of instrument commissioning, the Gaia operations team is making a series of spacecraft orientation changes.

The spacecraft was first tilted from 45 to 42 degrees and then to 0 degrees, that is, facing its sunshield directly at the Sun. Then the spacecraft was returned to 45 degrees.

Gaia tilt seen from Earth

Astronomers Peter Veres and Bryce Bolin, who were following a call for Earth-bound observations to improve the prediction of Gaia’s brightness under different viewing conditions, used the 2.24m  telescope on Mauna Kea in Hawaii to capture Gaia’s tilt from 0 to 45 degrees on 27 February.

The resulting movie nicely illustrates the change in brightness of the spacecraft over a period of around half an hour (12:14:52 UT to 12:42:06 UT), as Gaia’s sunshield tilted away from the Earth. Gaia is the bright object in the centre of the movie and moves downwards.

Dave Tholen, who processed the images, said: “We started with 10 second exposures for the first 30 exposures, then increased the exposure time to 20 seconds to get images 31 to 35, then increased again to 40 seconds for images 36 to 40. The last three exposures were 80 seconds each.”

The observations also captured a main belt asteroid (2002 RS34) moving from top centre to the right of the field of view in the movie.

As for the issue of stray light, the data are still being analysed. The tilting process will be repeated again, at a much slower rate, in order to gather more information from on-board systems during the transition period.