# 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?

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.