10 Years of SOHO - Nasa

SOHOPerihelion passage of comet NEAT (C/2002 V1) in February2003, as imaged by LASCO C32–100 (depending on wavelength) overthe solar cycle. Detailed knowledge ofthe solar spectral irradiance is crucial forunderstanding climate variability, and todisentangle natural variations fromanthropogenic climate changes.SOHO, the Comet FinderSOHO is providing new measurements notonly about the Sun. On 5 August 2005,Toni Scarmato, a high school teacher fromCalabria, Italy, discovered SOHO’s 999thand 1000th comets. As of February 2006,LASCO had detected over 1100 comets,most of them ‘Sun-grazers’. These cometspass very close to the Sun and growprominent tails as their icy cores areheated. Nearly half of all comets forwhich orbital elements have beendetermined (since 1761) were discoveredby SOHO, and over two-thirds of thoseby amateurs accessing LASCO data viathe Web. This is a field where amateurscan actively contribute to scientificresearch; each day, numerous peoplefrom all over the world download thenear-realtime data to search for newcomets.As the brightest, most spectacularcomet ever observed by SOHO, cometNEAT (C/2002 V1) provided someenticing data for further study, thanks toa grazing encounter with a coronal massejection. The LASCO C3 observationsduring 16–20 February 2003 suggestinteraction between the comet’s ion tailand other magnetic fields in the outercorona at the time of the oblique impactwith the CME. This is the first time thatsuch an event has been imaged, and nocomet has ever been observed closer tothe Sun.The analysis of high-resolution spectroscopicobservations of comet C/2002 X5(Kudo-Fujikawa) from UVCS has revealeda near-spherical cloud of neutral hydrogenand a variable tail of ionised carbon (C +and C + 2) that disconnected from the cometand was subsequently regenerated.SOHO’s SWAN instrument monitoredthe break-up of comet LINEAR C/1999S4. The total amount of water vapourobserved by SWAN from 25 May to12 August 2000 was estimated at3.3 million tonnes. Only about 1% of thiswas left on 6 August, when observationsby the Hubble Space Telescope of thedying comet’s fragments gave an estimateof the total volume. Combining thenumbers gives a remarkably low value forthe density: about 15 kg/m 3 , comparedwith 917 kg/m 3 for familiar non-porousice. Even allowing for an equal amount ofdust grains, 30 kg/m 3 is far less than the500 kg/m 3 often assumed by cometscientists.Conclusions and SOHO’s FutureThe journey has not always been easy. Forexample, an unexpected loss of contactoccurred on 25 June 1998. Fortunately, themission was completely recovered in oneof the most dramatic rescue efforts inspace, and normal operations could beresumed in mid-November 1998 after thesuccessful recommissioning of thespacecraft and all 12 instruments. Despitethe subsequent failures of all threegyroscopes (the last in December 1998),new gyroless control software installed byFebruary 1999 allowed SOHO to return tonormal scientific operations, providing anwww.esa.int esa bulletin 126 - may 2006 31