# What speed did the (interstellar) comet C/2017 U1 PANSTARRS have at perihelion?

C/2017 U1 PANSTARRS is a comet with an eccentricity of close to 1.2, so it is a strong candidate for having interstellar origin. It's perihelion was at 0.25 AU.

What speed did it have relative to the Sun when it swung around it at the closest?

• twitter.com/AscendingNode/status/923223634386206720 – Erik Oct 26 '17 at 16:24
• @Erik Gaah! Aren't astronomers getting very media trigger happy? Don't they talk about these things with each other internally before they publish? Or do they, but lie in order to set up each other? Eccentricity of 0.18-0.19 looked super high. But the error bar was apparently so large that it couldn't fit inside of the telescope, it seems. – LocalFluff Oct 26 '17 at 18:37
• FWIW there has been significant pushback on that AscendingNode tweet. – Chris Oct 26 '17 at 19:52
• youtu.be/0GPqb-bXhjg – Erik Oct 27 '17 at 0:35

Taking q = 0.2531011 AU (perihelion radius) and e = 1.1937160 from the latest ephemeris for A/2017 U1, we get a semimajor axis of $$a = \frac{r_p}{1-e} = -1.3065575~\textrm{AU}.$$ Using the definitions of 1 AU and $\mu_{\textrm{Sun}}$ from here, we get the heliocentric velocity at perihelion of $$v_p = \sqrt{\mu_{\textrm{Sun}}\left(\frac{2}{r_p} - \frac{1}{a}\right)} = \boxed{87.6872~\frac{\textrm{km}}{\textrm{s}}}$$ (assuming I did my math right). This is subject to change as the orbit gets refined with further observations.
• +1 Very nice, concise, complete, and quantitative answer! Your math looks good to me. – uhoh Oct 27 '17 at 10:46