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A special mission, called Deep Impact, was designed to have a small probe, with camera, strike comet Tempel-1, at high velocity. The amount of material ejected, and the size of the crater, will give good clues as to the nature of the comet's material, and possibly internal structure. This impact occurred on the Fourth of July 2005 and was observed by cameras in the mothership and by many observatories on Earth.

The Deep Impact Mission

After the great excitement and beneficial knowledge from the Shoemaker-Levy event, NASA decided to stage an impact of its own, this time on a comet rather than a gasball planet. The comet chosen is Tempel-1, discovered in 1867 by George Tempel. Here is a telescope view of Tempel-1 when it was close enough to the Sun in August of 2001 to develop a tail; the bright circular area is the comet's coma.

Tempel 1 seen through an Earth telescope.

The rationale behind the mission, named Deep Impact, is simple: A probe sent from the mother ship onto the comet's surface should produce an impact crater, whose size, for the velocity and mass parameters chosen, can tell much about the nature of the material making up the surface and into the interior. A nose-mounted camera in the impactor will show progressive closeups of the target area, much like the Rangers did for the early days of lunar exploration by crashing onto the Moon. Instruments on the surviving mother ship can observe the impact and make measurements of the debris thrown off the comet for days thereafter. This mission, the 12th in the Discovery series, has a price tag of $333 million dollars.

Here is the Deep Impact spacecraft as it rests completed at its fabrication site:

The Deep Impact spacecraft.

Launch date was January 12, 2005 from Cape Canaveral. The transit time was deliberately set for arrival, impactor separation, and impact itself on July 4, 2005 as part of the U.S.'s "birthday" celebration. Here is the orbital history.

Orbital parameters for the Deep Impact Mission.

In the weeks prior to the impact telescopes trained on Tempel-1 observed events that proved to look very much like the July 4th impact. These have been noted on comets many times in the past. Two explanations seem most likely: 1) there is an internal gas release which ejects material outward; or 2) some object in space collides with the comet causing a fireball and ejecta. The first illustration below was made through the Hubble Space Telescope on June 14, 2005:

HST image of a fireball-like event on Tempel-1

The second explosive event was on June 22 and was observed directly by the video camera on Deep Impact. The escaping plume lasted well beyond 5 hours.

Sequence of images made by the Deep Impact camera showing a burstlike brightening and ejection of material from Tempel-1.

A spectrometer on board were able to determine some of the constituents in the plume:

Spectral curve for light emanating from the June 22 event on Tempel-1, as determined from a Deep Impact instrument.

On July 3, 2005 Deep Impact approached Tempel-1. Technicians and scientists at JPL awaited anxiously the final minutes. Some 12 hours before impact the impactor was successfully released from the mother ship and hurtled its way towards the advancing comet. The collision was designed to combine the orbital velocity of the comet with the opposing velocity of the impactor to attain a net impact velocity of 10.2 km per second (6.2 miles per second) or 37360 km per hour (23000 mph). For a mass of the impactor at 330 kg (820 lbs), the K.E. generated at this speed is equivalent to 4.5 tons of TNT. At the time of impact, 1.52 AM EDT, the comet was 134,700 km (83000 miles) from Earth.

As it closed on target, this view of Tempel-1 was obtained.

Tempel-1 viewed from Deep Impact 6 minutes prior to the actual collision.

The next three scenes show part of the comet's surface from two different angles and then close-up moments before the impactor struck about 60 meters from its pre-selected target.

Tempel-1 in the last minute before impact.

Similar view, different angle.

The surface imaged by the camera on the impactor, about 30 seconds before the collision.

At almost the exact instant of collision, this image was acquired by the mothership.

First image of the fireball made by the Impactor striking Tempel-1.

Just seconds thereafter this image was obtained:

The growing fireball and beginning of ejection of material from within the forming crater.

The moment immediately at impact, as imaged from the Deep Impact mother spacecraft, shows the luminous fireball at its maximum extent; the dark area is the comet, the bright is the fireball.

The moment of impact on Tempel-1

This fireball contained two sources of light: 1) superheated materials from the comet; and 2) light from the Sun reflecting from particles. As the ejecta plume continues to be maintained, it will lose the luminosity of the heated cometary substances but will continue to reflect sunlight. The plume was still glowing 5 hours after impact and will continue to be monitored from the mothership and telescopes over succeeding days. The crater itself was not visible during the first few days, attesting to the masking ability of the still dense debris in the plume.

Telescopes worldwide were trained on Tempel-1. The best view from a distance was obtained through the Hubble Space Telescope:

The impact on Tempel-1 as observed by the HST.

European Space Agency telescopes produced some of the most informative observations. The 8.3 m telescope at La Silla captured this visible light image that shows the dispersal of gas and particulate debris soon after the impact.

Optical photo of the debris from the Tempel-1 impact.

An NTT telescope yielded an image in which the ejecta plume could be roughly density-sliced to indicate variations in the amount of light-scattering debris. The coma of the comet is within the square.

Variations in light scattering density in the Tempel-1 ejecta plume.

The Optical Ground Telescope in the Canary Islands examined the ejecta using a red filter and a C-2 filter, with these results.

Red filter images of Tempel-1 before and after the Deep Impact event.

Same as above but using a C-2 filter.

An image from the mothership taken 55 minutes after the impact showed the extent of the still expanding ejecta curtain beyond the comet:

Density-sliced image of the ejecta from the Tempel-1 impact 55 minutes after the event.

Interpretation of this material has led to the conclusion that most of this curtain is composed of fine dust (face powder consistency), with some water vapor/water. The picture being derived from Tempel-1's surface is that it contains more solids, and less ice, than expected.

Calculation of the amount of dust ejected helps to estimate the size of the new crater as between 100 to 200 meters (up to 650+ feet in diameter).

So far, only a little specific science results have been released to the scientific community and the public. What is shown above is mainly observational. Unfortunately, the mothership is a flyby configuration and is receding from the comet. The dust may not clear enough to get a good look at the formed crater, as had been hoped. Whether any system will detect the crater at a later time is still uncertain.

But, in September 2005 scientists made this statement: Analysis of the dust made from the Spitzer spacecraft show the presence of silicates (including Olivine), as expected, but also small amounts of materials that appear to be clays and carbonates (unexpected). Aromatic hydrocarbons were also detected.

What is really wanted are actual samples of asteroid or comet nucleus materials. Plans for such missions are being worked out. However, on May 9, 2003, the Japanese Space Agency actually launched the asteroid probe Hayabusa towards a very small asteroid, Itokawa, which is only about 3 km (9000 ft) in long dimension and almost 0.7 km (2000 ft) in cross-sectional dimension. Here it is:

The asteroid Itokawa, shown as the Hayabusa spacecraft is approaching.

Itokawa will be reached in late September. Later, a tiny rover, MINERVA, will be sent to its surface to observe with its cameras and temperature probes. But more ambitious (and risky) will be the three attempts to set the mother ship down, sample the surface materials, and have Hayabusa take off for a return (with samples) to Earth for a parachute landing in Australia in June, 2007. Good luck!

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Primary Author: Nicholas M. Short, Sr. email: [email protected]