Hubble Space Telescope Observations
of Comet Hyakutake (1996 B2)

Observations of comet Hyakutake, the great comet of 1996, were made on April 4, 1996, using three of the instruments on the Hubble Space Telescope to study water photochemistry. The investigation was designed to make measurements simultaneously of hydrogen (H) and hydroxyl (OH) in the coma (or atmosphere) of Comet Hyakutake. Hydroxyl radicals are made up of a one hydrogen and one oxygen atom. These are the most abundant constituents in the coma (or atmosphere) of a comet, being produced when ultraviolet light from the sun breaks apart the water molecules that are evaporated from the comet nucleus. A water molecule is made up of two hydrogen atoms and one oxygen atom.

False Color Composite Image of Comet Hyakutake (C/1996 B2)

Above is a color composite image of the inner coma of Comet Hyakutake made from two images using the Hubble Space Telescope Wide Field Planetary Camera 2 (WFPC2): one with a red-filter showing the dust coma and a second with an ultraviolet "Woods" filter image showing the distribution of scattered ultraviolet radiation by hydrogen atoms in the inner coma.

The Dust Coma of Comet Hyakutake (C/1996 B2)

Below is the image of the comet recorded using the WFPC2 and a red (F675W) filter which shows the distribution of dust in the coma. The square view is about 14,000 km on a side and the sun is toward the upper right corner of the image. Spiral dust jets are seen being ejected from the sunward side of the comet. From the curvature of the spirals and the reported rotation period of about 6 1/4 hours, dust particle velocities of about 400 meters per second were derived. Also seen pointing toward the lower left is the long spike of trailing grains, as well as at least two clumps of material moving slowly away from the nucleus in the direction away from the Sun.

Lyman-alpha Spectrum of Comet Hyakutake (1996 B2)

A key part of the investigation was the measurement of the expansion speeds of the hydrogen atoms, taking special advantage of the Goddard High Resolution Spectrometer (HRS), one of the Hubble instruments. Spectrometers separate light (ultraviolet light from the comet in this case) into its constituent colors. Speeds can be measured using the same Doppler principle which enables police radar to measure a car's speed. Observations in this program were designed to improve and test our understanding of water photochemistry which is important for the interpretation of a variety of observations of comets.

Shown above is the spectrum of the region around the ultraviolet emission of cometary Lyman-alpha taken with HRS. It was taken at a point located 111,000 km sunward of the nucleus where the comet's Lyman-alpha emission is optically thin. The comet's Lyman-alpha emission (marked "Comet" to the right) is centered at the comet's redshifted velocity of about 53 kilometers per second, showing the comet's speed relative to the Earth on that day. The thinner line to the left (marked "Geocorona") is produced by the Earth's hydrogen geocorona which is the most extended part of the Earth's upper atmosphere. The width of the comet line is indicative of the velocities of hydrogen atoms in the coma, some of which are in excess of 20 kilometers per second. The solid curved line is the result of a model for the distribution of hydrogen in the coma which accounts for the detailed physics and chemistry of the photochemical destruction of water and the production cometary H and OH. The importance of such a detailed model is that is permits us to calculate an accurate production rate of water from observations of H and OH.

Image of the Hydrogen Lyman-alpha Emission of Comet Hyakutake

WFPC2 was also used with the far ultraviolet Woods filter in order to provide a two-dimensional image of the hydrogen Lyman-alpha coma of comet Hyakutake. See the blue image to the left below.

Model-Data Comparison for the Hydrogen Lyman-alpha Distribution

The blue WFPC2 Lyman-alpha image on the left gives the two-dimensional distribution of Lyman-alpha emission by hydrogen in the comet. It is compared with a numerical model to the right shown to the same scale. The actual hydrogen distribution in the model is spherically symmetric, however, the distribution of light is not because the hydrogen coma is optically thick to the scattered solar Lyman-alpha UV radiation which prevents it from reaching the innermost coma. The model contours are in units of kilorayleighs and the dot marks the position of the nucleus in the model.

The Numbers

An analysis of the hydrogen Lyman-alpha data indicates that the water production rate of the comet was about 8 tons per second on April 4. A complementary set of observations of the OH radical using the Hubble Faint Object Spectrograph was analyzed in a self-consistent manner with the hydrogen observations and implies a water production rate of 7 tons per second. This represents good agreement given the expected uncertainties in the calibration of the two instruments and in model analysis parameters.

The Team

The team of scientists consists of Principal Investigator Dr. Michael Combi of the Space Physics Research Laboratory (SPRL) , at the University of Michigan , and Co-Investigators Dr. Michael Brown of the California Institute of Technology, in Pasadena, California, Dr. Paul Feldman of Johns Hopkins University, in Baltimore, Maryland, Dr. H. Uwe Keller of the Max Planck Institute in Lindau, Germany, Dr. Robert Meier of the Naval Research Laboratory, in Washington DC, and Dr. William Smyth of Atmospheric and Environmental Research, Inc. in Cambridge, Massachusetts.