Tempel’s New Tail

Hale Telescope
The 200-inch (5.1 m) Hale Telescope (f/3.3) was the world’s largest effective telescope for 45 years (1948 – 1993).
Credit: Caltech

Astronomers using the Palomar Observatory’s 200-inch Hale Telescope have been amazed by comet Tempel 1’s behavior during and after its collision with the Deep Impact space probe.

In the minutes just after the impact the comet was seen to increase its near-infrared brightness nearly fivefold. As the event progressed astronomers at Palomar were able to distinguish jets of material venting from the comet’s nucleus that have persisted for days.

Early results from the data, in images taken just minutes after impact, showed a possible plume of dust and gas extending outward some 320 km (200 miles) from the comet’s center, roughly coinciding with the site of the probe’s final demise.

This apparent dust plume has persisted for several nights, allowing astronomers to watch the comet’s slow rotation. The night after impact the plume was on the far side of the comet, but was visible again the next evening as the comet’s rotation brought it back into view. Two days after impact, the plume was seen again, this time extending about 200 km (124 miles) from the comet’s center. According to Bidushi Bhattacharya of the California Institute of Technology’s (Caltech) Spitzer Science Center, "This could be indicative of an outburst of gas and dust still taking place near the region of the impact."

"We are very excited by these results. It is a fabulous time to be studying comets," says James Bauer of the Jet Propulsion Laboratory (JPL). "It will be interesting to see how long the effects of the impact persist," he adds.

Tempel's impact plume
Night of the impact (left) and two nights after (right). Both show extentions to the south of the comet’s center likely originating from the site of the probe’s final demise. Credit: Caltech

The images of the comet, obtained by Bauer and Bhattacharya, were sharper than those from most ground-based telescopes because they used a technique known as adaptive optics. Adaptive optics allows astronomers to correct for the blurring of images caused by Earth’s turbulent atmosphere, giving them a view that often surpasses those of smaller telescopes based in space.

Using the adaptive-optics technique to improve an astronomer’s view is generally only possible when a bright star is located near the object they want to study. On the night of impact there was no bright star close enough to the comet to use. Mitchell Troy, the adaptive-optics group lead and Palomar adaptive-optics task manager at JPL, worked with his team to make adaptive optics corrections anyway. "Through the dedicated efforts of the JPL and Caltech teams we were able to deploy a new sensor that was 25 times more sensitive then our normal sensor. This new sensor allowed us to correct for some of the atmosphere’s distortions and significantly improve the view of the comet," says Troy. This improved view allowed astronomers to see the dust and ejected material moving out from the comet’s surface immediately following the impact event and again days later.

Earth-based observations from telescopes like the 200-inch at Palomar give astronomers an important perspective on how the comet is reacting to the impact, a perspective that cannot be achieved from the front-row seat of a fly-by spacecraft. Astronomers on the ground have the luxury of long-term observations that may continue to show changes in the comet for weeks to come.