Image of the Week


Apophis' Yarkovsky acceleration improved through stellar occultation

Figure 1: Evolution in time of our knowledge of the average Yarkovsky acceleration for 99942 Apophis. The light blue data represent the early theoretical estimates from approximate models of the physical properties of Apophis1. Note that the sign of the acceleration is not known for the early estimates. The other data are measurements enabled by the collection of more optical and radar astrometry. On the horizontal axis, close encounters with the Earth (enabling collection of accurate astrometry) are marked. The inset shows the last estimates compared to our value, in red, obtained from all the observations available on March 15, including the occultation observed on March 7, 2021. The other values shown here are the best measurements available in January (A), the improvement with subsequent optical data during the close encounter (B), and with the addition of radar (C). Considering the occultation results, the error is reduced by nearly a factor 2 (red point) with respect to (C), and a factor 20 compared to (A). The (D) solution, with the same data used for our red point, but computed independently by JPL, is compatible with our results, within the error bars. Image credit: P. Tanga, based on existing data and additional measurements.


Well before Gaia published its first astrometric releases, its promising impact for observing stellar occultations by a variety of bodies of the Solar System was thrilling. Those expectations were fully met, and a recent breakthrough shows that reality even surpassed our prospects. On 7 March 2021, and for the first time ever, the occultation of a star by the “hazardous” asteroid 99942 Apophis has been recorded.

Why should we consider this, in many respect, as an absolute first? Occultation predictions are generally challenging, as they result from a very fine-tuned alignment of a star and a body - for instance an asteroid - as seen from specific locations on Earth. The uncertainty on where this alignment will be visible (the “path” of the occultation shadow on the ground, akin to the path of the Moon shadow during solar eclipses) should not exceed by much the size of the occulting object itself. In that way, telescopes can be activated along the path at surgical precision, or even transported in that path, allowing to acquire data at the right place and at the right moment.

Figure 2: The map of the predicted shadow patch of the stellar occultation by Apophis on 7 March 2021, as published by the Lucky Star project. The straight and continuous lines show the limits of the shadow path as projected on Earth. The big blue dot indicates the nominal occultation time, and the other blue dots are spaced by one minute each. More information on this plot can be found from the Lucky Star Project. Image source: LESIA, Observatoire de Paris, Lucky Star Project


When it works, a success in the observation is extremely rewarding. By timing the disappearance and reappearance of the star, a precise shape and size of the occulting asteroid can be derived. In some cases, rings or a tenuous atmosphere can be revealed and studied. For distant minor planets, such as the Trans-Neptunian Objects (TNOs), occultations represent the only accurate tool at hand to characterize their shape, satellites, atmospheres and rings. No other technique (with the exception of in situ space missions) can actually provide the same level of details.

With the use of Gaia DR1 and Gaia DR2, many challenging occultation events have been predicted and observed, and some results have been illustrated on these pages in the recent years. In that vein, the successful occultation by the 30 kilometre trans-neptunian object (TNO) Arrokoth, that have granted a successful flyby of the New Horizons mission (NASA), stands out. It probably represents the ultimate occultation challenge that Gaia astrometry can face for the outer Solar System. But, another frontier remained to be explored.

At the opposite of the distance scale: the population of Near Earth Asteroids (NEAs). They are in general so small and so fast that their occultations are particularly difficult to predict, and have durations of the blinking of an eye: a few tenths of a second at most, instead of several seconds for Main Belt Asteroids or TNOs. Meanwhile, NEAs are interesting in many respects: some of them are the targets of space missions, and some come close to Earth as potential impactors.

The NEA 99942 Apophis, named after the evil serpent dubbed “Uncreator” in ancient Egypt, is the only asteroid to have briefly topped at level 4 in the Torino impact hazard scale shortly after its discovery. Follow up observations currently rule out the possibility of an impact on our planet on the short term, but this asteroid is still of exceptional interest for its repeated close encounters with Earth – in particular the one on April 13th, 2029, when it will pass 31,600 km above the Earth surface and will become easily visible to the naked eye. Although no observations of Apophis by Gaia are available, the Gaia EDR3 and Gaia DR2 catalogues provided the astrometry of stars surrounding Apophis’ trajectory as seen on the sky. Those stars have been used as references to pin down Apophis’ orbit with exquisite accuracy.


Figure 3: The trajectory of Apophis during the close encounter in 2029, with respect to the Earth and the Moon orbit. It will be so close that Earth gravity could change its shape or its rotation. It will pass closer than the distance to the geostationary satellites. Image credit: Marco Polo, Public domain, via Wikimedia Commons.


During its approach over the last several weeks, the object has been observed several times by telescopes on the ground. Around the time of its closest distance (early March) its position was accurately measured by radar ranging too. In this way, it was possible to predict the occultation of the star NY Hydrae (magnitude 8.4) by Apophis on 7 March of this year, and to draw the occultation shadow path on Earth with an uncertainty of about 1 km. Most important, that star was also observed by Gaia (Gaia EDR3 5746104876938414592) and this provided the accuracy essential to the delicate task. A few bi-static radar measurements of Apophis secured by the Green Bank and Goldstone radio telescopes, and their prompt processing, were fundamental to ensure a last-minute, accurate astrometry.


Figure 4: Rendering of the Apophis shape model as oriented at the epoch of the European occultation event (March 11, 2021). The model is derived from radar observations (M. Brozovic et al., Icarus 300 (2018) 115–128) and shows a very elongate shape along the red axis. The purple arrow shows the orientation of the pole of rotation. Image credit: M. Brozovic (JPL, USA)

Three different orbit computations were compared for this critical prediction, with compatible results, in a collaboration involving Observatoire de la Côte d’Azur, France (Paolo Tanga), the Lucky Star project (PI Bruno Sicardy, LESIA, Observatoire de Paris, France, with Josselin Desmars and Damya Souami), the Aristotle University in Thessaloniki, Greece (Kleomenis Tsiganis), the Southwest Research Institute of Boulder, USA (Marc Buie, Brian Keeney), Jet Propulsion Laboratory (Marina Brozovic, Davide Farnocchia, Jon Giorgini), Lowell observatory (Larry Wasserman) several members of the International Occultation Timing Association (David Dunham, David Herald, John Irwin, Steve Preston) and of the SETi Institute (Franck Marchis).

As the path was predicted to cross through continental US, more than 40 observers, most of them citizen astronomers, deployed telescopes at a regular spacing across the occultation track, some of them with telescopes as small as 8 cm in diameter, which was made possible thanks to the unusual brightness of the occulted star. The invested efforts paid off: three locations in Louisiana, where part of the observers converged, eventually recorded the much awaited star blinking for about 0.2 seconds!

So now, what does this single observation contribute to?

First, useful science. Our occultation measurement provides an extremely accurate position of the object in space. In the case of Apophis – and everything considered – the uncertainty is ~0.3 milliarcsec (mas) only, corresponding to 25 m at its distance, a measurement that compares with the Gaia quality level.

Secondly, we have computed an improved orbit for the object, and then obtained a much better value of the Yarkovsky acceleration, a minute effect caused the emission of thermal radiation from the body. Combining the results obtained from the occultation and radar ranging, the error on the measurement of that acceleration was reduced to 0.5%, a factor 20 times smaller when compared to what was available just a couple of months ago!

What are the consequences of those findings? The exact geometry (timing and distance) of the forthcoming close encounters with Earth (in 2029, and then in 2036…) is extremely sensitive to the amplitude of the Yarkovsky effect, that was particularly difficult to measure in the case of Apophis. A precise knowledge of this effect will be thus very useful to predict the geometry of the next near-miss of Apophis with our planet.

Eventually, this occultation opens bright perspectives: it was the first time that such a small NEA (approximately 340 x 170 m) was successfully observed by this technique, pushing the limits of the approach and paving the way to other detections. The previous record, set in 2019 with a sequence of occultations by the 5-km NEA 3200 Phaethon, is pulverized. The ten-fold increase of accuracy means access to many more occultations by smaller NEAs.

For the sake of completeness, we add that a new positive observation of an occultation by Apophis has been secured in the US on March 22, while an attempt in Europe failed mainly due to heavy clouds, preventing to adequately cover the predicted path (negative recordings were obtained at 2 stations in Greece). Our teams are now preparing future occultations by NEAs in the ~km size range, including objects such as the binary asteroid 65803 Didymos, the target of the NASA/DART and the ESA/Hera missions. Last but not least, as illustrated once more by Apophis, amateur astronomers with modest equipment can contribute to outstanding results in a citizen-science approach. This collaboration is so powerful that it can obtain astrometric measurements of asteroids at the accuracy of Gaia, allowing an improved assessment of the hazards posed by these little interlopers!


Figure 5: Animation of the orbit of Apophis around the Sun. ​​​​​​Animation license: CC BY-SA 4.0. Animation attribution: Phoenix7777, via Wikimedia Commons.


Further reading:


Credits: ESA/Gaia/DPAC; P. Tanga from Observatoire de la Côte d'Azur, France; B.Sicardy, J. Desmars and D. Souami from the Lucky Star Project; K. Tsiganis from Aristotle University in Thessaloniki, Greece; M. Buie and B. Keeney from the Southwest Research Institute of Boulder, USA; M. Brozovic, D. Farnocchia, J. Giorgini from the Jet Propulsion Laboratory; L. Wasserman from the Lowell Observatory; D. Dunham, D. Herald, J. Irwin and S. Preston from the International Occultation Timing Association; F. Marchis from the SETi Institute; and Federica Spoto, Minor Planet Center, Harvard & Smithsonian; and the OrbFit Consortium.

[Published: 26/03/2021]

Image of the Week Archive


30/03: First transiting exoplanet by Gaia

26/03: Apophis' Yarkovsky acceleration improved through stellar occultation

26/02: Matching observations to sources for Gaia DR4


22/12: QSO emission lines in low-resolution BP/RP spectra

03/12: Gaia Early Data Release 3

29/10: Gaia EDR3 passbands

15/10: Star clusters are only the tip of the iceberg

04/09: Discovery of a year long superoutburst in a white dwarf binary ​​​​​​​

12/08: First calibrated XP spectra

22/07: Gaia and the size of the Solar System

16/07: Testing CDM and geometry-driven Milky Way rotation Curve Models

30/06: Gaia's impact on Solar system science

14/05: Machine-learning techniques reveal hundreds of open clusters in Gaia data

20/03: The chemical trace of Galactic stellar populations as seen by Gaia

09/01: Discovery of a new star cluster: Price-Whelan1

08/01: Largest ever seen gaseous structure in our Galaxy

20/12: The lost stars of the Hyades
06/12: Do we see a dark-matter like effect in globular clusters?
12/11: Hypervelocity star ejected from a supermassive black hole
17/09: Instrument Development Award
08/08: 30th anniversary of Hipparcos
17/07: Whitehead Eclipse Avoidance Manoeuvre
28/06: Following up on Gaia Solar System Objects
19/06: News from the Gaia Archive
29/05: Spectroscopic variability of emission lines stars with Gaia
24/05: Evidence of new magnetic transitions in late-type stars
03/05: Atmospheric dynamics of AGB stars revealed by Gaia
25/04: Geographic contributions to DPAC
22/04: omega Centauri's lost stars
18/04: 53rd ESLAB symposium "the Gaia universe"
18/02: A river of stars
21/12: Sonification of Gaia data
18/12: Gaia captures a rare FU Ori outburst
12/12: Changes in the DPAC Executive
26/11:New Very Low Mass dwarfs in Gaia data
19/11: Hypervelocity White Dwarfs in Gaia data
15/11: Hunting evolved carbon stars with Gaia RP spectra
13/11: Gaia catches the movement of the tiny galaxies surrounding the Milky Way
06/11: Secrets of the "wild duck" cluster revealed
12/10: 25 years since the initial GAIA proposal
09/10: 3rd Gaia DPAC Consortium Meeting
30/09: A new panoramic sky map of the Milky Way's Stellar Streams
25/09: Plausible home stars for interstellar object 'Oumuamua
11/09: Impressions from the IAU General Assembly
30/06: Asteroids in Gaia Data
14/06: Mapping and visualising Gaia DR2

25/04: In-depth stories on Gaia DR2

14/04: Gaia tops one trillion observations
16/03: Gaia DR2 Passbands
27/02: Triton observation campaign
11/02: Gaia Women In Science
29/01: Following-up on Gaia
19/12: 4th launch anniversary
24/11: Gaia-GOSA service
27/10: German Gaia stamp in the making
19/10: Hertzsprung-russell diagram using Gaia DR1
05/10: Updated prediction to the Triton occultation campaign
04/10: 1:1 Gaia model arrives at ESAC
31/08: Close stellar encounters from the first Gaia data release
16/08: Preliminary view of the Gaia sky in colour
07/07: Chariklo stellar occultation follow-up
24/04: Gaia reveals the composition of asteroids
20/04: Extra-galactic observations with Gaia
10/04: How faint are the faintest Gaia stars?
24/03: Pulsating stars to study Galactic structures
09/02: Known exoplanetary transits in Gaia data
31/01: Successful second DPAC Consortium Meeting
23/12: Interactive and statistical visualisation of Gaia DR1 with vaex
16/12: Standard uncertainties for the photometric data (in GDR1)
25/11: Signature of the rotation of the galactic bar uncovered
15/11: Successful first DR1 Workshop
27/10: Microlensing Follow-Up
21/10: Asteroid Occultation
16/09: First DR1 results
14/09: Pluto Stellar Occultation
15/06: Happy Birthday, DPAC!
10/06: 1000th run of the Initial Data Treatment system
04/05: Complementing Gaia observations of the densest sky regions
22/04: A window to Gaia - the focal plane
05/04: Hipparcos interactive data access tool
24/03: Gaia spots a sunspot
29/02: Gaia sees exploding stars next door
11/02: A new heart for the Gaia Object Generator
04/02: Searching for solar siblings with Gaia
28/01: Globular cluster colour-magnitude diagrams
21/01: Gaia resolving power estimated with Pluto and Charon
12/01: 100th First-Look Weekly Report
06/01: Gaia intersects a Perseid meteoroid
18/12: Tales of two clusters retold by Gaia
11/11: Lunar transit temperature plots
06/11: Gaia's sensors scan a lunar transit
03/11: Celebrity comet spotted among Gaia's stars
09/10: The SB2 stars as seen by Gaia's RVS
02/10: The colour of Gaia's eyes
24/09: Estimating distances from parallaxes
18/09: Gaia orbit reconstruction
31/07: Asteroids all around
17/07: Gaia satellite and amateur astronomers spot one in a billion star
03/07: Counting stars with Gaia
01/07: Avionics Model test bench arrives at ESOC
28/05: Short period/faint magnitude Cepheids in the Large Magellanic Cloud
19/05: Visualising Gaia Photometric Science Alerts
09/04: Gaia honours Einstein by observing his cross
02/04: 1 April - First Look Scientists play practical joke
05/03: RR Lyrae stars in the Large Magellanic Cloud as seen by Gaia
26/02: First Gaia BP/RP deblended spectra
19/02: 13 months of GBOT Gaia observations
12/02: Added Value Interface Portal for Gaia
04/02: Gaia's potential for the discovery of circumbinary planets
26/01: DIBs in three hot stars as seen by Gaia's RVS
15/01: The Tycho-Gaia Astrometric Solution
06/01: Close encounters of the stellar kind
12/12: Gaia detects microlensing event
05/12: Cat's Eye Nebula as seen by Gaia
01/12: BFOSC observation of Gaia at L2
24/11: Gaia spectra of six stars
13/11: Omega Centauri as seen by Gaia
02/10: RVS Data Processing
12/09: Gaia discovers first supernova
04/08: Gaia flag arrives at ESAC
29/07: Gaia handover
15/07: Eclipsing binaries
03/07: Asteroids at the "photo finish"
19/06: Calibration image III - Messier 51
05/06: First Gaia BP/RP and RVS spectra
02/06: Sky coverage of Gaia during commissioning
03/04: Gaia source detection
21/02: Sky-background false detections in the sky mapper
14/02: Gaia calibration images II
06/02: Gaia calibration image I
28/01: Gaia telescope light path
17/01: First star shines for Gaia
14/01: Radiation Campaign #4
06/01: Asteroid detection by Gaia
17/12: Gaia in the gantry
12/12: The sky in G magnitude
05/12: Pre-launch release of spectrophotometric standard stars
28/11: From one to one billion pixels
21/11: The Hipparcos all-sky map
15/10: Gaia Sunshield Deployment Test
08/10: Initial Gaia Source List
17/09: CU1 Operations Workshop
11/09: Apsis
26/08: Gaia arrival in French Guiana
20/08: Gaia cartoons
11/07: Model Soyuz Fregat video
01/07: Acoustic Testing
21/06: SOVT
03/06: CU4 meeting #15
04/04: DPCC (CNES) 
26/03: Gaia artist impression 
11/02: Gaia payload testing  
04/01: Space flyby with Gaia-like data
10/12: DPAC OR#2. Testing with Planck
05/11: Galaxy detection with Gaia
09/10: Plot of part of the GUMS-10 catalogue
23/07: "Gaia" meets at Gaia
29/06: The Sky as seen by Gaia
31/05: Panorama of BAM clean room
29/03: GREAT school results
12/03: Scanning-law movie
21/02: Astrometric microlensing and Gaia
03/02: BAM with PMTS
12/01: FPA with all the CCDs and WFSs
14/12: Deployable sunshield
10/11: Earth Trojan search
21/10: First Soyuz liftoff from the French Guiana
20/09: Fast 2D image reconstruction algorithm
05/09: RVS OMA
10/08: 3D distribution of the Gaia catalogue
13/07: Dynamical Attitude Model
22/06: Gaia's view of open clusters
27/05: Accuracy of the stellar transverse velocity
13/05: Vibration test of BAM mirrors
18/04: L. Lindegren, Dr. Honoris Causa of the Observatory of Paris
19/01: Detectability of stars close to Jupiter
05/01: Delivery of the WFS flight models
21/12: The 100th member of CU3
17/11: Nano-JASMINE and AGIS
27/10: Eclipsing binary light curves fitted with DPAC code
13/10: Gaia broad band photometry
28/09: Measuring stellar parameters and interstellar extinction
14/09: M1 mirror
27/08: Quest for the Sun's siblings
Please note: Entries from the period 2003-2010 are available in this PDF document.