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A drop in the ocean Earth's bulk composition is similar to that of a group of oxygen-poor meteorites called enstatite chondrites, thought to have formed in the early solar nebula. This leads to the suggestion that proto-Earth was dry, and that volatiles including water were delivered by asteroid and comet impacts. The deuterium-to-hydrogen (D/H) ratios measured in six Oort cloud comets are much higher than on Earth, however, apparently ruling out a dominant role for such bodies. Now the Herschel Space Telescope has been used to determine the D/H ratio in the Kuiper belt comet 103P/Hartley 2. The ratio is Earth-like, suggesting that this population of comets may have contributed to Earth's ocean waters. For decades, the source of Earth's volatiles, especially water with a deuterium-to-hydrogen ratio (D/H) of (1.558 ± 0.001) × 10 −4 , has been a subject of debate. The similarity of Earth’s bulk composition to that of meteorites known as enstatite chondrites 1 suggests a dry proto-Earth 2 with subsequent delivery of volatiles 3 by local accretion 4 or impacts of asteroids or comets 5 , 6 . Previous measurements in six comets from the Oort cloud yielded a mean D/H ratio of (2.96 ± 0.25) × 10 −4 . The D/H value in carbonaceous chondrites, (1.4 ± 0.1) × 10 −4 , together with dynamical simulations, led to models in which asteroids were the main source of Earth's water 7 , with ≤10 per cent being delivered by comets. Here we report that the D/H ratio in the Jupiter-family comet 103P/Hartley 2, which originated in the Kuiper belt, is (1.61 ± 0.24) × 10 −4 . This result substantially expands the reservoir of Earth ocean-like water to include some comets, and is consistent with the emerging picture of a complex dynamical evolution of the early Solar System 8 , 9 .

For decades, the source of Earth's volatiles, especially water with a deuterium-to-hydrogen ratio (D/H) of (1.558 ± 0.001) x [10.sup.-4], has been a subject of debate. The similarity of Earth's bulk composition to that of meteorites known as enstatite chondrites (1) suggests a dry proto-Earth (2) with subsequent delivery of volatiles (3) by local accretion (4) or impacts of asteroids or comets (5,6). Previous measurements in six comets from the Oort cloud yielded a mean D/H ratio of (2.96 ± 0.25) x [10.sup.-4]. The D/H value in carbonaceous chondrites, (1.4 ± 0.1) x [10.sup.-4], together with dynamical simulations, led to models in which asteroids were the main source of Earth's water (7), with ≤ 10 per cent being delivered by comets. Here we report that the D/H ratio in the Jupiter-family comet 103P/Hartley 2, which originated in the Kuiper belt, is (1.61 ± 0.24) x [10.sup.-4]. This result substantially expands the reservoir of Earth ocean-like water to include some comets, and is consistent with the emerging picture of a complex dynamical evolution of the early Solar System (8,9).

An improved forced-precession model of Comet 46P/Wirtanen is presented. The nongravitational motion of the comet has been investigated based on the forced precession model of the rotating cometary nucleus. The least squares method applied to observational equations allows us to determine six basic nongravitational parameters: A, eta, I, phi, f_p, and s together with six orbital elements. The solutions were obtained with additional assumptions: 1. The cometary activity with respect to the perihelion is asymmetric and, 2a. The time shift parameter which describes the displacement of maximum activity with respect to the perihelion could change its value during the investigated apparitions or 2b. The real activity of the comet changed during the time interval considered. All eight recorded apparitions of the comet were linked using all astrometric observations covering the period 1948-1997 with a mean RMS residual of 1.6\". According to the best solution, the nucleus of Comet Wirtanen is oblate along the spin-axis with a ratio of equatorial to polar radius of about R_a/R_b=1.1. This precession model yields a value of 4.9 hrs/km for the ratio of P_rot/R_a. Assuming that the nucleus radius of 46P/Wirtanen most likely is in the range of 0.5 - 2.0 km we obtain a range 2.5 - 10.0 hours for the rotational period.

The role of non-gravitational forces in the evolution of orbitalmotion of C/1995 O1 (Hale–Bopp) has been investigated. Inorbital calculations the observational material covering theperiod from April 1993 up to August 2001 was used. To model thenon-gravitational acceleration, observed and theoretical profilesof the H2O production rates were employed. A set of forcedprecession models of a rotating cometary nucleus consistent withthe observed spin axis orientation was fitted to positionalobservations. The non-gravitational models allowed us to constrainthe mass and radius of the comet. The orbitalevolution of Comet Hale–Bopp was investigated over ±400 k yusing two sets of randomly varied orbital elements wellrepresenting all positional observations in the pure gravitationalcase, as well as in the non-gravitational case. The calculationsshowed that the comet's motion is predictable only over an interval ofa few orbital periods. The statistical conclusions changesignificantly when non-gravitational effects are included in the analysis.