Comet 17P/Holmes suffered a massive outburst in October 2007. Its total brightness increased from about 17th to 2nd magnitude (that is a factor of 1 million increase) over a period of only two days. During that interval 17P released about 1-10% of its mass into space in the form of dust. Several theories have been proposed to explain the event but the exact cause for the outburst remains unknown. 17P had suffered a similar outburst in 1892, which led to its discovery by Holmes. These unusual and violent explosions have rendered this otherwise unremarkable Jupiter family comet an interesting target of study, because it may provide clues to the activity in other comets.
On 29 October 2007, the nucleus of comet 17P passed within <1 arc second of a background star. We used observations taken at the Univ. of Hawaii 2.2m telescope located atop Mauna Kea to measure the brightness of the star as it crossed the coma of 17P in an attempt to estimate the optical depth and albedo of the dust.
Here is a plot of the brightness of the star plotted against distance from the nucleus. Negative distance means approaching.
The stellar brightness dips by about 4% at about 1.5 arcseconds from the nucleus (we trust the point with the smaller error bar better). This indicates that the coma optical depth through the line of sight that misses the nucleus by about 1,750 km is 0.04. If we extrapolate this measurement we obtain an optical depth about 10 towards the nucleus itself. Optical depth larger than 1 implies direct sunlight does not get through, so the nucleus was in shadow at this time. The circular region where the optical depth is larger than 1 is about 10 km in radius, larger than the nucleus which is about 1.6 km in radius. These measurements place the most stringent constraints on the extinction optical depth of any cometary coma.
The other thing we were able to measure was the coma dust albedo (or reflectivity). The 4% absorption in starlight measured above is approximately proportional to the total cross-section of the dust blocking the light. On the other hand, the amount of sunlight that the dust reflects back to us is proportional to the cross-section of the dust multiplied by its reflectivity. So, we can measure the amounts of absorbed and reflected light and calculate the albedo from their ratio. We measure an albedo of about 1%. The dust is very, very dark. One possibility is that the dust is made of porous carbon grains. The picture below shows a grain of carbon aerogel (also known as aerographite) which has an albedo of 0.3%.
Comets carry possibly some of the oldest (least processed) material in the solar system. These clues we got about comet dust may be useful in the future to help us understand the material present when the planets were forming.