astorb is a database of osculating orbital elements and ephemeris uncertainties near the current epoch for all known asteroids in the Solar System. It has been hosted at Lowell Observatory since the 1990's and is actively curated to be automatically updated as new objects are discovered. Access to the database, additional documentation, additional data, and associated tools are available at asteroid.lowell.edu. Introduction: The file provided here is an ASCII summary of the database, with each object occupying a 266 column record. Orbits are computed based on fits to astrometric data downloaded from the Minor Planet Center. There are several key features of the astorb database. (1) The database is updated daily. Observations in each new batch of Minor Planet Circulars are used to compute new orbits on a monthly basis, and those in the Minor Planet Electronic Circulars shortly after they are published. Other updates, such as the computation of current ephemeris uncertainties, are being made on a quasi-daily basis. (2) All of the orbits in a given version of the file have an epoch of osculation within 50 days of the present. Consequently, the ephemerides of most non-planet-approaching asteroids can be computed to arcsec accuracy or better within +- 50 days of the epoch using a 2-body ephemeris program. (3) Current and future ephemeris uncertainties are given. Observers will readily be able to estimate whether asteroids are likely to be within their telescope's fields of view, and they will better be able to prioritize astrometric targets. To produce the database, our variable-timestep differential orbit correction program was run in an automatic mode. Perturbation due to all major planets (Mercury through Pluto, Earth and Moon separately), 1 Ceres, 2 Pallas, 3 Juno, 4 Vesta, 10 Hygiea, 15 Eunomia, 31 Euphrosyne, 52 Europa, 511 Davida, and 704 Interamnia were included. Planetary positions were derived from JPL's DE430 planetary ephemeris. Positions of the perturbing asteroids were derived, by iteration, from our own orbits. Relativistic effects have been included. For Near Earth Objects, perturbations due to the Earth's J2 have been included. For the vast majority of asteroids the threshold for inclusion or exclusion of observations in orbit determination is where the great-circle sky-plane residuals exceed 2.3 arc seconds. In rare cases observations with higher residuals are manually accepted. In no cases are residuals greater than 10 arc seconds accepted. Generally higher residuals are accepted from older, pre-CCD data that offer significant arc extension.
