Rosetta begins its Comet Tale

摘要: Comets are the best sample of primitive solar nebula material presently available to us, dating back 4.57 billion years origin our planetary system. Past missions comets have all been “fast flybys”: They provided only a snapshot view dust and ice nucleus, nebulous coma surrounding it, how wind interacts with both these components. Such space-based investigations began in 1980s flotilla spacecraft: European Space Agency's (ESA's) first deep space mission, Giotto, which pursued comet 1P/Halley; Deep 1 at 19P/Borrelly; Stardust 81P/Wild 2; Impact NeXT 9P/Tempel; EPOXI 103P/Hartley 2. ![Figure][1] PHOTO: ESA/ROSETTA/PHILAE/CIVA Rosetta is now taking more prolonged look. The spacecraft an ESA contributions from member states NASA, it currently orbits Jupiter family 67P/Churyumov-Gerasimenko (67P). Rosetta met nucleus on 6 August 2014, 3.7 astronomical units (AU) Sun, delivered Philae lander surface 12 November when was 3.0 AU Sun. Rosetta uniquely positioned further understanding bodies, having revealed unusual fascinating object. After rendezvous, moved 100 km above bound orbit ~10 away. This early period mission has previously unseen details as Rosetta's instruments recorded measurements that were once impossible. issue Science contains published scientific results 67P. The shows evidence many active processes highly complex. solid object for neither horizontal nor vertical variations modest (Thomas et al. , this issue). current shape model suggests mass 1013 kg (about million times international station), bulk density ~470 kg/m3 (similar cork, wood, or aerogel). low values strongly constrain composition internal structure implying relatively fluffy nature, porosity 70 80% (Sierks itself appears rich organic materials, little sign water (Capaccioni issue). The produced by ices sublimating variable, displaying large diurnal possibly seasonal changes. For example, atomic H O detected close vary time, probably stemming electron impact dissociation venting H2O vapor. total gas production rates varied × 1025 molecules per second June 2014 4 August, broadly consistent predictions. In outflow factor least 5, owing effects terrain, shape, daily illumination changes other factors (Gulkis science team reports detection several molecules, including H2 17O, 18O, CO, CO2, assessed their time variability heterogeneous distribution (Hassig A high D/H ratio water, 5.3 10−4, measured, precludes idea contain solely Earth ocean–like (Altwegg As observed 3.6 cloud about 105 grains (larger than 5 cm) surrounds orbits, likely previous perihelion passage. emits up 2 cm size, giving dust/gas ± averaged over sunlit (Rotundi higher generally accepted comets. progressive series observations, emergence energetic ion environment low-activity under influence (Nilsson data presented here allow us build detailed portrait 67P. These initial observations provide reference description global morphology composition, physical properties nucleus. Subsequent orbiter will describe time. follow range through its closest approach perihelion, 2015, then moves away Sun. perform flybys onboard measure evolution respect comet's state, defined here. The begun explore origins, thanks efforts thousands people ESA, industrial partners, agencies engineers scientists around world. 25 years, they dreamed moments designed, developed, launched followed interplanetary journey, watched long sleep, woke hibernation. papers dedicated them. [1]: pending:yes