SUPPORTED Comet NEAT (C/2001 Q4) is predicted to reach naked-eye visibility in the spring of 2004, under excellent conditions for observations with HST. Although predicting cometary magnitudes more than 1 year in advance is notoriously risky, C/NEAT seems likely to be exceptionally active, allowing us for the first time to perform sensitive measurements of the D/H and OD/OH ratios. Accurate measurements of the deuterium abundance in comets, which can vary in different species, are crucial for determining if comets retain a signature of their possible interstellar origin and if they supplied a significant fraction of the water on Earth. HST observations of C/NEAT will additionally provide accurate abundances for highly volatile ices in the nucleus, such as CO, CO2 (via Cameron band emission), and S2, which also provide important insights on the comet's origin and evolution. This opportunity in cycle 12 is unique in the history of HST and is unlikely to be repeated during its remaining lifetime.

This program was allocated 17orbits. As the target, C/2001 Q4 (NEAT), will be moving relatively fast (~0.1-0.2"/sec), each orbit will be treated as a separate visit. However, the visits will be grouped to provide optimal multispectral coverage and information on the temporal variations. The table below illustrates the strategy. Group1 Group2 Group3 Group4 Group5---------------------------------------------------------------------------------------------------------------- G140L G140L G140L G230MB G230L G230M G230M G230M G230MB E140H G230LB G230LB G230LB G230MB E140H E140H E140H (off-nuc) In addition, each entry in the above table has an associated TA with the STIS/CCD, except for the off-nuc pointing, which doesn't require a TA. For each group, I also take 1 full-frame CCD image. We plan to use the 0.2" wide slit for all observations, and, in particular, the 0.2" x 6" slit for the E140H observations. We need 1 orbit with E140H off the nucleus by ~25 arcmin to measure the H emission where it is optically thin. This latter observation should be made directly after the on-nucleus observations and could be executed under GYRO control, if that were allowed. Comet-NEAT-2001Q4 COMET TYPE=COMET,Q=0.9619255777946496,E=1.0007347157045310,I=99.64195460578811,O=210.27766961861720,W=1.20585663081089,T = 15-May-2004:23:03:17.0,EQUINOX=J2000,EPOCH=26-Apr-2004 Comet-NEAT-2001Q4-OFF COMET TYPE=COMET,Q=0.9619255777946496,E=1.0007347157045310,I=99.64195460578811,O=210.27766961861720,W=1.20585663081089,T = 15-May-2004:23:03:17.0,EQUINOX=J2000,EPOCH=26-Apr-2004 TYPE=POS_ANGLE,RAD=1500.0,ANG=0.0,REF=SUN fill orbit fill orbit fill orbit fill orbit fill orbit fill orbit fill orbit fill orbit fill orbit