Occultation astrometry - major goals

The power of Gaia-based predictions was demonstrated by the successful observation of a MU69/Arrokoth occultation (New Horizons/NASA target), with an array of low-cost, 11”-16” telescopes, operated by enthusiastic students, amateurs and locals; the same strategy is now followed for Lucy targets.

Near Earth Asteroids

In October 2019, an occultation by the ~6 km-sized NEA Phaethon with duration only ~0.2 sec was successfully recorded, resulting in formal errors <50m in diameter and <70 μas/s in apparent motion; a ~1,000 euros, high-fps, GPS-enabled camera was used. Some NEAs are of special interest to our proposal: Apophis, expected to fly-by the Earth in 2027 and Didymos, the binary system-target of the DART and Hera missions. Both objects are expected to cast their shadows over SE Europe in spring 2021, while several events are predicted for Didymos in mid-late 2022. At present, Didymos has a relatively ‘poor’ orbit, the projected cross-track uncertainty being several times larger than its dimensions. Data obtained by successful occultations can potentially pin down Didymos’ pre-impact, heliocentric orbit to an accuracy ~100 meters or better. This would produce very precise predictions for future (post-impact) events. Then, systematic differences between predicted and actual event times, could lead to direct estimation of the heliocentric deflection of Didymos, a result to be fully quantified and validated by Hera.

Asteroid families in the Main Belt

The current attempts to model the chronology of the catastrophic collisions that created the Main Belt families, are affected by a lack of direct measurements of the radial drift of small family members, induced by the Yarkovsky effect (YE). However, an important limitation is that the chronology itself is based on a rescaling of the drift rate da/dt on the base of poorly known physical properties (such as density), using a few YE measurements on well-characterized NEAs as a reference (such as the near-Earth asteroid 101955 Bennu). The current Holy Grail in this domain, is to avoid this procedure, and measure the YE drift da/dt directly on some MBAs and family members. Important differences can be present between MBAs and NEAs, for instance in the thermal inertia (Delbo and Tanga 2009). We will have to consider family members in the 4-6 km range as smaller asteroids diffuse faster in the background.

Some 100s MBAs in the above size range, have a semi-major axis standard deviation sa < 5 x 10-9 au. For them, new accurate observations will make YE detectable: 20 of these MBAs are already present in the data published in Gaia DR2 (purple points in the rightmost portion of the yellow region in Fig. 3), but this number will grow to >100 in following Gaia releases, and by at least another order of magnitude with the occultation astrometry.

Remote small bodies and support to planetary missions

We participate to campaigns devoted to characterize Centaurs and TNOs (such as the Lucky Star project). Some of them have supported the NASA mission New Horizons during its navigation to the target Arrokoth. Occultations by this object have been made possible by the anticipated pre-release of Gaia stellar astrometry, and have permetted the correct planning of the spacecraft approach and imaging sequence during the very critical flyby operations. As mentioned above, we have obtained in 2019 one of the most accurate occultation chords by the NEA Phaethon, target of the DESTINY+ mission (JAXA). We also support occultations by targets of the Lucy mission (trojan asteroids of Jupiter), and of DART/Hera (Didymos).

-- PaoloTanga - 27 Sep 2020