Coordinated Planetary Defence in action: Colors and lightcurve of asteroid 2024 YR4

The Near-Earth Object 2024 YR4 (hereafter YR4) was discovered by the ATLAS survey [1] on December 27 2024. With an H magnitude estimated around H = 24 [2], YR4 size is estimated to be between 40 to 90 meters [2].  At time of discovery its brightness was around 16.5 magnitude and moving across the sky at high speed. Shortly after discovery, YR4 was found to display a high probability of impacting the Earth in 2032 and was classified as a Torino scale 1 virtual impactor [3].  For a few hours to days, its visual magnitude was ideal for detailed physical characterization observations such as polarimetry or spectroscopy. However, due to the timing of its discovery during the end of the year holidays, very few observations were performed at that time. A few days later, YR4 was already a few magnitudes fainter, limiting the number of telescopes able to gather physical characterization observations.   On January 7, we obtained color observations with the Lowell Discovery Telescope when YR4 was already much fainter at a V~20 magnitude. The observations showed that YR4 could potentially be an S-type or L, K-type. S-types tend to have larger albedo than L or K-type (however, very few albedo of L and K-types for NEOs have been determined). It is thus highly important to distinguish between these two taxonomic classifications.  To gather more information on the composition of YR4, a Director Discretionary Proposal (DDT) was submitted to the ESO Very Large Telescope to obtain Near Infra-Red colors of YR4 using the HAWK-I instrument [4]. The proposal was submitted on Sunday January 19 at night, accepted during the day of January 20 and observed during the night of the January 20 to 21.  Analysis of the NIR color dataset shows good agreement with the S or L, K-type composition, but is unfortunately not sufficient to be able to distinguish between the two. To be able to assess if YR4 is a S or L, K-type, spectroscopy in both the visible and near-infrared would be needed. Visible spectroscopy has been obtained for YR4, but no NIR spectroscopy is available.  Simultaneously to the HAWK-I observations, we also obtained observations in the R band with the FORS2 instrument on the VLT. These observations were used to connect the visible colors/spectroscopy of YR4 with our new NIR colors observations. We also used these observations to obtain a high signal to noise ratio lightcurve. Our lightcurve shows a rotation period around P=19.5 minutes with an amplitude of 0.4 magnitude.  [1] Tonry, J. L. et al. ATLAS: A High-cadence All-sky Survey System. PASP 130, 064505 (2018)[2] https://neo.ssa.esa.int/search-for-asteroids?sum=1&des=2024YR4[3] https://neo.ssa.esa.int/risk-list[4] Pirard, J.-F. et al. HAWK-I: A new wide-field 1- to 2.5-μm imager for the VLT. in vol. 5492 1763–1772 (2004)