Spectral Portrait of Apophis: Groundwork for the 2029 Earth Flyby

  David Polishook  ,  the MITHNEOS team  
Weizmann Institute of Science


The April 2029 near-Earth encounter by the asteroid Apophis is a once-in-a-lifetime event where a relatively large asteroid will approach Earth to within a distance of ~31K km. Estimates are that a 300m asteroid approaches this close about once-per-thousand years. The scientific and physical significance of Apophis’ 2029 approach is that Earth’s tidal forces will modify Apophis’ spin state, and possibly its shape, and creating the potential for generating seismic waves in its interior. This makes the Apophis’ 2029 flyby a unique opportunity to study surface altering effects on asteroids, and if instrumented, the first direct measures of an asteroid’s interior. Apophis has a ’weathered’ surface (Sq-type spectrum), that may reveal its true ’fresh’ character (Q-type spectrum) due to quakes imposed by Earth’s tidal forces. Therefore, since surface ’refreshing’ due to encounter could cause global spectral changes or local spectral changes, any current, pre-flyby variability is extremely important to establish, with precise Apophis pre-encounter spectral characteristics.
Early 2021 presents the only opportunity to closely scrutinize Apophis ahead of the 2029 flyby, when Apophis will be at a distance of 0.11 AU and Vmag of 15.4. We proposed to conduct an extensive observational campaign on NASA's 3m InfraRed Telescope Facility (IRTF) on Mauna Kea and obtained 38 hours spreading during 12 nights on March-April. The campaign will allow us to conduct a full spectrally rotational coverage of Apophis surface along its rotation period of ~30.5 h; to correct systematic errors of the measured spectral slope by observing during a long range of phase angles; and to determine if spectral homogeneity exist on the entire surface ahead of the 2029 flyby.