How big will Apophis appear in the sky at its closest point of approach in 2029? Will it compare to the moon? How long will it linger as it passes?
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2$\begingroup$ According to en.wikipedia.org/wiki/99942_Apophis#Physical_characteristics it will peak at magnitude 3.4, so only as bright as an average star. More details: newton.dm.unipi.it/neodys/… $\endgroup$– user21Apr 26, 2016 at 4:27
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Current estimates put Apophis's diameter around 325 m and its 2029-04-13 approach about 38000 km from the center of the Earth. I figure an angular size <= 2 arcsec, almost starlike even if you manage to track it in a telescope. Ephemerides show it at apparent magnitude 3.5 or so just before closest approach - visible but not outstanding to the unaided eye at night - and spending about 36 hours within 1 lunar distance of the Earth.
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$\begingroup$ I'm not sure if the Hubble telescope will be operational then, but it would be a few dozen pixels wide at that size and so nicely imageable in visible light. fyi I've just asked Has Hubble ever been used to try to image a near Earth asteroid? $\endgroup$– uhohJun 23, 2019 at 2:17