Caption: This is a light curve* of comet Hartley 2 derived from the Deep Impact (DI) spacecraft Medium Resolution Instrument (MRI) data from the beginning of the approach phase, September 5, through October 14. There are two curves plotted, one that depicts dust (black) released from the nucleus and one taken with the CN filter, sensitive to both the dust continuum and CN gas (blue), which is not directly produced by the nucleus. The data are plotted in arbitrary flux units and scaled to align the curves. Between 9 Sep and 17 Sep 2010, CN gradually increased by a factor of 5 and then slowly decreased, returning to its prior trend line by 24 Sep, with no apparent change in the dust continuum. This activity is different from the quick, discrete dusty outbursts seen at comet Tempel 1 by DI and is not apparently associated with the dust-free CN jets observed in this and other comets. Only time will tell if this activity will repeat itself.
The gradual underlying increase in the dust and gas in the light curve is due to the decreasing range from the spacecraft to the comet and the increasing activity of the nucleus as it approaches the Sun. There are also periodic fluctuations associated with the rotation of the nucleus. The gap in data from DOY 268 through 274 (25 Sept through 1 Oct, E-40d through E-34d) was due to a scheduled break in the observations for calibration observations, a trajectory correction maneuver, and preparations for beginning infrared spectroscopy observations in addition to the continuation of visual observations of the comet.
* In astronomy, a light curve shows any changes in the brightness of a target with respect to time.
(Reported in CBET 2512, which requires a subscription to view.)