Object 1I/2017 U1 ( ‘ Oumuamua ) is the first interstellar small body ever discovered in the solar system .
By the time of discovery , it had already passed perihelion .
To investigate the behavior of ‘ Oumuamua around perihelion , we searched for it in Solar and Heliospheric Observatory ( SOHO ) and Solar TErrestrial RElations Observatory ( STEREO ) images from early 2017 September ( preperihelion ) , but did not detect it .
The nondetection of ‘ Oumuamua by STEREO renders more stringent constraints on its physical properties thanks to the extreme forward-scattering observing geometry .
Assuming geometric albedo p _ { V } = 0.1 , the effective scattering cross-section of any dust coma was \la \left ( 2.1 \pm 0.2 \right ) \times 10 ^ { 4 } m ^ { 2 } .
Assuming it behaved like a typical solar-system comet this would correspond to a total mass of \la 20 \pm 2 kg , and a water production rate of \la \left ( 6.1 \pm 0.5 \right ) \times 10 ^ { 25 } s ^ { -1 } at heliocentric distance r _ { H } = 0.375 au .
If scaled to post-discovery r _ { H } , the water production rate would be smaller than any of the previously reported upper limits by at least an order of magnitude .
To exhibit the reported nongravitational motion with our default assumptions requires a nucleus bulk density \la 40 kg m ^ { -3 } ; higher bulk densities are possible for other assumptions .
Alternatively , we show that thermal fracturing could have plausibly removed an inert surface layer between these observations and discovery , thus initiating activity after ‘ Oumuamua left the field of view of STEREO .