After months of endeavour, astronomers have succeeded in capturing the momentary shadow cast by the Didymos asteroid, from tens of meg km away as it passed in front end of far-distant stars – a feat of ascertainment simply made possible when both the trajectory of the asteroid and the precise location of the stars are known. Even in that case, to have a chance of success, several observers had to exist placed in meticulously predicted locations across the path of the shadow, to glimpse the fleeting fading of the star within merely a fraction of a second.
Why fifty-fifty attempt such a ambitious challenge? Because ‘stellar occultations’, as they are called, are an boggling tool to obtain data on shapes and positions of Solar System objects, with an accuracy otherwise impossible for distant observers. And, crucially, the ultra-precise three-dimensional star maps generated by ESA’s Gaia satellite have made this technique much more than viable in contempo years.
To target the occultations of About-World asteroids in detail, an ESA-backed project called ACROSS, Asteroid Collaborative Research via Occultation Systematic Survey, was supported by ESA’s Discovery programme through a telephone call on the Agency’due south Open Space Innovation Platform, seeking out promising infinite enquiry ideas for support.
Paolo Tanga of Observatoire de la Côte d’Azur, project leader of Across and responsible for Solar System data processing in Gaia, comments: “Astrometry based on observing ‘stellar occultations’ was initially exploited for asteroids in the main belt between Mars and Jupiter, then for far-away Trans-Neptunian objects, but Beyond is extending its systematic exploitation to Near-Globe asteroids every bit well. This is the challenge: because NEAs motion fast and are small, thus producing shorter events and much narrower shadows projected on the basis.”
How to catch NEAs then, using occultations? By improving the accuracy of their trajectories, past diffusing accurate predictions well-nigh where and when the events are visible and by mobilising networks composed both by citizen scientists (motivated apprentice astronomers) every bit well as professionals, on a global scale.
ACROSS has a special focus on Didymos – really a binary asteroid system, with its 780-m bore main asteroid orbited past a smaller 160-k diameter moonlet called Dimorphos – the target of NASA’southward Double Asteroid Redirect Examination, DART. On the night of 26 September the DART spacecraft slammed into the smaller of the ii asteroids shifting its orbit. ESA volition so fly the Hera spacecraft to the Didymos arrangement to perform a shut-up survey of the post-affect asteroid. Repeating occultations over several years will fifty-fifty allow measurements of the deviation of the arrangement’s trajectory around the Sun induced by DART’southward impact – something that cannot be done past DART or Hera alone.
ACROSS co-investigator Prof. Kleomenis Tsiganis of Aristotle University of Thessaloniki, AUTh, adds: “To make such campaigns possible, nosotros needed to perform independent orbit improvement calculations from the currently bachelor data, shrink the uncertainty of the occultation path to a few kilometres, and deploy observers accordingly to take hold of a momentary blinking out of the star that has to be measured in milliseconds.”
Logistic efforts accept been enormous and unsuccessful for several weeks, such as in the example of the 45 observers deployed in Portugal, Kingdom of spain and Algeria on 25 August, simply prevented from observing past adverse meteorological conditions.
In the meantime, efforts on all fronts including a shut collaboration with the NASA Jet Propulsion Laboratory’s Steve Chesley of the Dart team, improved the predictions further.
And then, success came: The first confirmation came on 15 October from the deployment of six telescopes, five of which were overseen by a unmarried observer, Roger Venable, in northern Oklahoma, Us, the other by Robert Dunford. One of them – at nine:02 CEST – clearly caught the fading of the star over about 0.thirteen seconds, only slightly shorter than the maximum possible duration.
Then, on 18 October came the turn of a squad led past Jose-Luis Ortiz observing an event very close to his home plant, the Instituto de Astrofisica de Andalucia-CSIC, near Granada in Spain. Miguel Sanchez, equipped with a portable telescope of 28 cm aperture, clearly recorded the occultation with an excellent agreement to predictions.
Later, on the same twenty-four hours, in Japan, some seven telescopes were deployed; those of Hayato Watanabe and Miyoshi Ida through the Japan Occultation Information Network (Join) were the lucky ones – this being the showtime event to be observed past two sites.
With Didymos’ eclipsing of the stars behind it caught by the first time by astronomers, future events will be easier to catch. Observing more of them should ensure a much more accurate tracking of Didymos orbit, possibly allowing the detection of a tiny change in the heliocentric orbit of Didymos due to the Dart touch, and even hope to learn more about the scars left by Dart’due south affect.
The Across team: P. Tanga (project coordinator, Université Côte d’Azur, OCA, CNRS/Lab. Lagrange), 1000. Tsiganis (partner coordinator, Aristotle University of Thessaloniki), D. Souami (ACROSS postal service-doc, Université Côte d’Azur, OCA, CNRS/Lab. Lagrange), J. Ferreira (mail service-doc, Aristotle Academy of Thessaloniki), A. Siakas (educatee, Aristotle University of Thessaloniki.
P. Tanga, K. Tsiganis, and D. Souami are as well members of the Dart It (Investigation Team). Data collected by Dart IT were used to excerpt astrometry of Didymos using the high accuracy pipeline adult by S. Bouquillon (Observatoire de Paris) for the ground-based follow-upwards of the Gaia satellite http://gbot.obspm.fr/
Steve Chesley is at JPL / Solar System Dynamic group https://ssd.jpl.nasa.gov
Roger Venable and Robert Dunford are occultation observers affiliated to International Occultation Timing Association (IOTA). The campaign in Japan was led by Hiroyuki Watanabe from the Nihon Occultation Information Network (JOIN).
Across heavily relies on the orbFit software for orbit comeback (F. Spoto, Pocket-size Planet Center), on the Horizons web service by JPL (
) and on astrometry provided past ESA/Gaia.
ACROSS acknowledges back up by ESA contract ii-1775/21/NL/GL/ov