We describe here the most ambitious survey currently planned in the optical, the Large Synoptic Survey Telescope (LSST). The LSST design is driven by four main science themes: probing dark energy and dark matter, taking an inventory of the solar system, exploring the transient optical sky, and mapping the Milky Way. LSST will be a large, wide-field ground-based system designed to obtain repeated images covering the sky visible from Cerro Pachón in northern Chile. The telescope will have an 8.4 m (6.5 m effective) primary mirror, a 9.6 deg 2 field of view, a 3.2-gigapixel camera, and six filters (ugrizy) covering the wavelength range 320-1050 nm. The project is in the construction phase and will begin regular survey operations by 2022. About 90% of the observing time will be devoted to a deep-wide-fast survey mode that will uniformly observe a 18,000 deg 2 region about 800 times (summed over all six bands) during the anticipated 10 yr of operations and will yield a co-added map to r∼27.5. These data will result in databases including about 32 trillion observations of 20 billion galaxies and a similar number of stars, and they will serve the majority of the primary science programs. The remaining 10% of the observing time will be allocated to special projects such as Very Deep and Very Fast time domain surveys, whose details are currently under discussion. We illustrate how the LSST science drivers led to these choices of system parameters, and we describe the expected data products and their characteristics.

We present observations of the interstellar interloper 1I/2017 U1 ('Oumuamua) taken during its 2017 October flyby of Earth. The optical colors B -V=0.70±0.06, V -R=0.45±0.05, overlap those of the D-type Jovian Trojan asteroids and are incompatible with the ultrared objects that are abundant in the Kuiper Belt. With a mean absolute magnitude H V =22.95 and assuming a geometric albedo p V =0.1, we find an average radius of 55 m. No coma is apparent; we deduce a limit to the dust mass production rate of only ∼2×10 −4 kg s, ruling out the existence of exposed ice covering more than a few m 2 of the surface. Volatiles in this body, if they exist, must lie beneath an involatile surface mantle 0.5 m thick, perhaps a product of prolonged cosmic-ray processing in the interstellar medium. The light curve range is unusually large at ∼2.0±0.2 mag. Interpreted as a rotational light curve the body has axis ratio ³ -+ 6.3 1.1 1.3 :1 and semi-axes ∼230 m×35 m. A 6:1 axis ratio is extreme relative to most small solar system asteroids and suggests that albedo variations may additionally contribute to the variability. The light curve is consistent with a two-peaked period ∼8.26 hr, but the period is non-unique as a result of aliasing in the data. Except for its unusually elongated shape, 1I/2017 U1 is a physically unremarkable, sub-kilometer, slightly red, rotating object from another planetary system. The steady-state population of similar, ∼100 m scale interstellar objects inside the orbit of Neptune is ∼10 4 , each with a residence time of ∼10 years.

We report the results of a sensitive K-band survey of Herbig Ae/ Be disk sizes using the 85 m baseline Keck Interferometer. Targets were chosen to span the maximum range of stellar properties to probe the disk size dependence on luminosity and effective temperature. For most targets, the measured near-infrared sizes (ranging from 0.2 to 4 AU ) support a simple disk model possessing a central optically thin (dust-free) cavity, ringed by hot dust emitting at the expected sublimation temperatures (T s $ 1000-1500 K). Furthermore, we find a tight correlation of disk size with source luminosity R / L 1 = 2 for Ae and late Be systems (valid over more than two decades in luminosity), confirming earlier suggestions based on lower quality data. Interestingly, the inferred dust-free inner cavities of the highest luminosity sources (Herbig B0-B3 stars) are undersized compared to predictions of the ''optically thin cavity'' model, likely because of optically thick gas within the inner AU.

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We present imaging and spectroscopic observations of 6478 Gault, a ∼6 km diameter inner main-belt asteroid currently exhibiting strong, comet-like characteristics. Three distinct tails indicate that ultra-slow dust (ejection speed 0.15±0.05 m s −1 ) was emitted from Gault in separate episodes beginning UT 2018 October 28±5 (Tail A), UT 2018 December 31±5 (Tail B), and UT 2019 February 10±7 (Tail C), with durations of ∆T ∼ 10 to 20 days. With a mean particle radius a ∼ 200 µm, the estimated masses of the tails are M A ∼ 4 × 10 7 kg, M B ∼ 6 × 10 6 kg and M C ∼ 6 × 10 5 kg, respectively, and the mass loss rates from the nucleus are 20 to 40 kg s −1 for Tail A, 4 to 6 kg s −1 for Tail B and ∼0.4 kg s −1 for Tail C. In its optical colors Gault is more similar to C-type asteroids than to S-types, even though the latter are numerically dominant in the inner asteroid belt. A spectroscopic upper limit to the production of gas is set at 1 kg s −1 . Discrete emission in three protracted episodes effectively rules out an impact origin for the observed activity. Sublimation driven activity is unlikely given the inner belt orbit and the absence of detectable gas. In any case, sublimation would not easily account for the observed multiple ejections. The closest similarity is between Gault and active asteroid 311P/(2013 P5), an object showing repeated but aperiodic ejections of dust over a 9 month period. While Gault is 10 times larger than 311P/(2013 P5), and the relevant timescale for spin-up by radiation torques is ∼100 times longer, its properties are likewise most consistent with episodic emission from a body rotating near breakup.