---
author:
- Eric Mamajek
title: 'Kinematics of the Interstellar Vagabond 1I/$\!$‘Oumuamua (A/2017 U1)'
---

The discovery of an asteroid of likely interstellar origin was recently made by the Pan-STARRS survey – A/2017 U1 = 1I/[$\!$‘Oumuamua]{}[^1]. [*Can [$\!$‘Oumuamua]{}’s velocity before it entered the solar system provide any clues to its origin?*]{} The best available orbit from the JPL Small-Body Database Browser[^2] (solution JPL-13 produced by Davide Farnocchia) lists perihelion distance $q$ = 0.255287 $\pm$ 0.000079 au, eccentricity $e$ = 1.19936 $\pm$ 0.00021 and semi-major axis $a$ = -1.28052 $\pm$ 0.00096 au. This value of $a$ is consistent with an initial velocity before encountering the solar system of $v_{\circ}$ = 26.3209$\pm$0.0099 kms$^{-1}$, assuming no non-gravitational forces. The ephemeris shows that the object entered the solar system from the direction $\alpha_{ICRS}$, $\delta_{ICRS}$ = 279$^{\circ}$.804, +33$^{\circ}$.997 ($\pm$0$^{\circ}$.032, $\pm$0$^{\circ}$.015; 1$\sigma$). This divergent point and $v_{\circ}$ value translates to a heliocentric Galactic velocity [@Perryman98 $U$ towards Galactic center] of $U, V, W$ = -11.457, -22.395, -7.746 kms$^{-1}$ ($\pm$0.009, $\pm$0.009, $\pm$0.011 kms$^{-1}$).\
[*Could [$\!$‘Oumuamua]{} be a member of the Oort Cloud of the $\alpha$ Centauri system?*]{} Such a scenario might not be unexpected as the tidal radius for the 2.17 $M^{N}_{\odot}$ triple system [@Kervella17] is of order $r_t$ $\simeq$ 1.7 pc [@Mamajek13]. As the system lies only 1.34 pc away, the solar system may be on the outskirts of $\alpha$ Cen’s cometary cloud [see @Hills81; @Beech11]. @Kervella17 calculated updated heliocentric Galactic velocities for $\alpha$ Cen AB of $U, V, W$ = -29.291, 1.710, 13.589 ($\pm$0.026, $\pm$0.020, $\pm$0.013) kms$^{-1}$ and for Proxima Centauri ($\alpha$ Cen C) of $U, V, W$ = -29.390, 1.883, 13.777 ($\pm$0.027, $\pm$0.018, $\pm$0.009) kms$^{-1}$. The velocity difference of 36.80$\pm$0.04 kms$^{-1}$ between [$\!$‘Oumuamua]{} and the $\alpha$ Cen system, and the fact they were further apart in the past ($\Delta$ $\simeq$ 5 pc 100 kyr ago), [*argues that it has no relation to $\alpha$ Cen*]{}. Members of $\alpha$ Cen’s cometary cloud would appear to have motions diverging from the vicinity of $\alpha$, $\delta$ = 293$^{\circ}$, -42$^{\circ}$ with $v_{\circ}$ $\simeq$ 32 kms$^{-1}$.\
The Galactic velocity of $\!$‘Oumuamua is plotted against those of the nearest stars (parallax $>$ 300 mas) in Fig. \[fig1\]. Besides the velocity of $\alpha$ Cen AB and C from @Kervella17, velocities for the nearest stars are drawn from @Anderson12 and @Hawley97. The velocity for the substellar binary Luhman 16 is calculated using data from @Garcia17 and @Kniazev13: $U, V, W$ = -18.3, -27.5, -6.9 kms$^{-1}$. [$\!$‘Oumuamua]{}’s velocity is more than 20 kms$^{-1}$ from any of the stars, and 9 kms$^{-1}$ off from Luhman 16, so [*[$\!$‘Oumuamua]{} does not appear to be comoving with any of these nearest systems*]{}.\
[*What velocities might be expected of interstellar field objects?*]{} We might first suspect that interstellar planetesimals share the velocity distribution of nearby stars. The XHIP catalog [@Anderson12] contains velocities for 1481 stars within 25 pc with distances of $<$10% accuracy. The XHIP sample has median velocity $U, V, W$ = -10.5, -18.0, -8.4 kms$^{-1}$ ($\pm$33, $\pm$24, $\pm$17 kms$^{-1}$; 1$\sigma$ range), similar to that for volume-limited samples of nearby M dwarfs [$U, V, W$ = -9.7, -22.4, -8.9 kms$^{-1}$ ; $\pm$37.9, $\pm$26.1, $\pm$20.5; 1$\sigma$; @Reid02]. @BlandHawthorn16 provides a recent consensus estimate for the Local Standard of Rest (LSR) of $U, V, W$ = -10.0, -11.0, -7.0 kms$^{-1}$ ; $\pm$1, $\pm$2, $\pm$0.5; 1$\sigma$). An object with the median velocity of the local XHIP sample would have speed 22.5 kms$^{-1}$ coming from $\alpha$, $\delta$ = 273$^{\circ}$, +33$^{\circ}$, within only $\sim$6$^{\circ}$ of [$\!$‘Oumuamua]{}’s divergent point. The velocity is very close to the median for the XHIP sample ($\Delta
v$ $\simeq$ 4.5 kms$^{-1}$; $\chi^2$/$\nu$ = 0.036/3; P = 0.0018), the mean for the local M dwarfs ($\Delta v$ $\simeq$ 2.1 kms$^{-1}$; $\chi^2$/$\nu$ = 0.0053/3; P = 0.0001) and the LSR ($\Delta v$ $\simeq$ 11.5$\pm$2.3 kms$^{-1}$), compared to the typical 3D velocity of nearby stars. Compared to the LSR, [$\!$‘Oumuamua]{} has negligible radial and vertical Galactic motion[^3], and its sub-Keplerian circular velocity trails by 11 kms$^{-1}$.\
Robotic reconnaissance of [$\!$‘Oumuamua]{}, or future interstellar planetesimals passing through the solar system, might constitute logical precursors to interstellar missions, and provide the opportunity to conduct chemical studies and radiometric dating of extrasolar material which formed around other stars.\

![Galactic velocities for 1I/[$\!$‘Oumuamua]{} (filled triangle), nearby stars (open circles), and LSR (cross)\[fig1\].](f1.pdf)

This research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration. EEM acknowledges support from the NASA NExSS program, and thanks Davide Farnocchia (JPL) for discussions. This work used the JPL Small-Body Database Browser, HORIZONS system, and Vizier.\

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[^1]: See: http://www.minorplanetcenter.net/mpec/K17/K17UI1.html, https://www.minorplanetcenter.net/mpec/K17/K17V17.html.

[^2]: https://ssd.jpl.nasa.gov/sbdb.cgi?sstr=A%2F2017%20U1

[^3]: @Gaidos17 have proposed that the object’s velocity is due to birth in a nearby $\sim$40 Myr stellar association.
