Comet 8P/Tuttle is a returning nearly isotropic comet (NIC) with an outstanding apparition in cycle 16, passing within 0.25 AU of the Earth. We propose a 12-orbit Hubble investigation that will allow us to determine the size, shape, rotational period, and color (UBVRI) of 8P, thereby providing the most detailed view of a NIC nucleus since the spacecraft flyby of 1P/Halley in 1986. The return of 8P is a rare opportunity, and we expect many other observatories, including Spitzer, to be investigating this comet. Combining the Hubble results with those from other observatories should yield a comprehensive picture of this NIC that can be compared to the detailed data collected on ecliptic comets (ECs) during the past 3 decades. The differences and similarities between NICs and ECs should yield valuable insights into the origin and evolution of comets.

Shape and rotational information on the nucleus can only be obtained by measuring the lightcurve. As no rotational information exists for 8P, we must take observations that sample periods ranging from hours up to days. Given the precious nature of HST observing time, we make some compromises in the lightcurve sampling strategy. Our preferred approach is to sample the nucleus over a total of 12 orbits, the first 6 being consecutive followed by 6 observations every other orbit (i.e., observe during orbit numbers 1, 2, 3, 4, 5, 6, 8, 10, 12, 14, 16 and 18). This will cover approximately a full day of clock time. If the nucleus has a rotational period of about 2 days, we will then sample half its rotation, which is a full sinusoid because of the double-peaked nature of the lightcurve. This strategy is inherently flexible, and can therefore be adapted to satisfy external constraints, such as SAA impacts or the availability of guide stars. We have extensive experience, gained over the course of 16 years of successful HST observations of comets, and will work with the STScI?s planning personnel to optimally schedule the observations of 8P. Regarding the geometrical circumstances, 8P will be observed a few days before its closest approach to Earth (0.25 AU) in order to avoid times when the comet?s proper motion approaches HST?s maximum tracking rate, and in order to benefit from a more favorable phase angle ( 50 deg). Regarding the ephemerides, comet 8P has been observed at all returns since its discovery, with only one exception because of very unfavorable conditions. Furthermore its orbit is very stable as no encounter with Jupiter is possible. The accuracy of the presently available ephemerides is therefore already adequate for our HST observations. The total number of orbits requested is 12. 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