PASADENA, Calif. (CBS) - NASA's Deep Impact probe released a compact, instrumented smart bomb Sunday, a copper-clad robotic kamikaze programmed to place itself in the path of a speeding comet early Monday for a scientifically spectacular 23,000-mph Fourth of July collision.

An artist's illustration depicts the impactor separating from the main spacecraft. Credit: Maas Digital

Releasing the energy equivalent of 4.5 tons of TNT, the 820-pound "impactor" spacecraft, blasting through comet Tempel 1's dark crust and into its icy interior, is expected to blow out a crater that could be as small as a house or as large and deep as a football stadium.

Either way, the crater and the cloud of debris blown back out into space will give scientists their first glimpse into a comet's hidden interior and in so doing, open a new window on the birth of the solar system.

"Deep Impact will provide the most detailed pictures we've ever seen of a comet," said Lindley Johnson, Deep Impact program executive at NASA headquarters. "But instead of just looking at the surface of the comet, it's actually going to plunge in and blow out the material so we can see what's inside."

Said Jessica Sunshine, a co-investigator with Science Applications International Corp.: "Like any good geologist, we take our hammer and we hit it and find out how strong it is and what it's made out of on the inside. ... Literally, these materials have not seen the light of day in 4.6 billion years."


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Launched Jan. 12, the $333 million Deep Impact mission entered its final stages at 2:07 a.m. Sunday (11:07 p.m. PDT Saturday) when radio signals confirmed release of the impactor spacecraft some 500,000 miles from Tempel 1.

Twelve minutes after release, the Deep Impact mothership, known as the flyby spacecraft, carried out a 14-minute thruster firing to change its velocity by nearly 230 mph, putting it on a course that ultimately will carry it below and then behind the comet.

Both spacecraft were reported to be in good health after the make-or-break maneuvers, setting the stage for impact at 1:52 a.m. Monday (10:52 p.m. PDT Sunday).

Instruments aboard the flyby spacecraft will watch the crash from a distance of some 5,348 miles - far enough to eliminate any threat of comet dust or crater debris. It also will relay pictures and telemetry from the impactor back to the Jet Propulsion Laboratory before skimming just 300 miles or so below the comet at closest approach 14 minutes after impact.

"If all goes well, the impactor spacecraft will be taking images all the way in to the point of impact, sending those images back to the flyby spacecraft, providing us with some of the most astonishing pictures of a comet ever taken," said mission manager Dave Spencer."

But getting those pictures will be challenging to say the least.

Tempel 1 is barreling through space at some 66,880 mph. Released almost directly in front of the comet on a near-miss trajectory, the impactor was moving at roughly 49,000 mph. Navigating autonomously, the impactor was programmed to make up to three maneuvers during its final two hours of life to position itself so the comet runs it down at a relative velocity of 23,000 mph.

"This is a tremendously exciting, daring first-of-a-kind mission to impact the nucleus of a comet," said project manager Rick Grammier. "What makes it daring and exciting also makes it extremely difficult and challenging from a technical and orchestration viewpoint. Let me give you an analogy of what this mission is like. It's a bullet trying to hit a second bullet with a third bullet at the right place and the right time watching the first two bullets and gathering the scientific data from that impact. That is extremely challenging."

Monte Henderson, deputy program manager with spacecraft-builder Ball Aerospace and Technologies Corp., said not even a right-stuff fighter pilot could pull off the flying challenge faced by the two Deep Impact spacecraft. As a result, both were programmed in advance to collect their own navigation data and to maneuver autonomously as required.

An artist's concept shows the Deep Impact mothership releasing its copper laden impactor at Comet Tempel 1. Credit: NASA/JPL

"These two spacecraft have been designed to accomplish something the best fighter pilots in our country would not be able to do," Henderson said. "The primary challenge of this mission - two spacecraft independently locating an illuminated area on the comet, the impactor flying itself into the lit area and impacting the comet while the flyby diverts itself out of the path of the comet and images that same lit area to watch the encounter and the resulting crater formation and debris field - is something that could not be accomplished using human reflexes and human vision. So we had to design a navigation and control system that would allow the flyby and the impactor spacecraft to perform this very intricate ballet."

The collision will change the comet's velocity by a barely noticeable 014 inches per hour. It will change the time needed for one orbit by less than one second and raise the low-point of the comet's orbit by about 33 feet. A NASA press kit says the collision will be similar to that of a 767 jet airliner running into a mosquito.

Data from the impactor and the mothership will be transmitted to Earth in realtime. For redundancy, a complete set of data also will be recorded on board the mothership for later transmission.

Eighty-three million miles away, NASA's Hubble Space Telescope, the Spitzer infrared telescope, the Chandra X-ray Observatory and some 60 ground-based telescopes in 20 countries will study the impact in a variety of wavelengths. The giant 33-foot Keck telescopes and others in Hawaii will have ringside seats for the blast, which will occur in darkness over the south Pacific Ocean.

While visible, in theory, to the unaided eye from the southwestern United States, "you won't be able to see it with the naked eye, most likely," Grammier said. Even with a telescope, "we expect that the flash of the impact will be very momentary so you need to be trained on the comet at the time. Obviously, we expect the observatories ... to see it because they'll be watching for it."

Here is a timeline of upcoming events (all times in EDT):

TIME...........EVENT
   
July 3
02:00 p.m......Pre-impact news briefing/update
03:00 p.m......Flyby diversion trim maneuver
05:45 p.m......Ephemeris update
11:30 p.m......NASA television coverage begins
11:53 p.m......Start impactor auto-navigation imaging
   
July 4
12:21 a.m......Impactor trajectory maneuver No. 1
01:17 a.m......Impactor trajectory maneuver No. 2
01:39 a.m......Impactor trajectory maneuver No. 3
01:52 a.m......Impact
02:05 a.m......Flyby reorients to shield mode for close approach
02:06 a.m......Flyby closest approach
02:51 a.m......Flyby first post-impact image
04:00 a.m......Post-impact news briefing on NASA TV
02:00 p.m......Deep Impact news conference on NASA TV

Comet 9P/Tempel 1 was discovered in 1867 by Ernst Wilhelm Leberecht Tempel, a German-born astronomer who was searching for comets from Marseilles, France. Tempel 1 orbits the sun between Mars and Jupiter, taking 5.5 years to complete one revolution. Based on very limited observations, scientists believe the slightly elongated nucleus is roughly 3.7 miles across.

Deep Impact co-investigator Donald Yeomans, a comet expert at the Jet Propulsion Laboratory, described the comet as a "jet-black pickle-shaped icy dirtball the size of Washington DC."

Tempel 1 was chosen for the Deep Impact mission because it is reachable in the mission timeframe, the nucleus is fairly large and thus relatively easy to hit and its orbit allows the impact to occur on the sunlit side.

Tempel 1 originated in the Kuiper Belt, a wide flattened disk of icy debris extending from the orbit of Neptune to well beyond Pluto. Perturbed by gravitational interactions, primarily involving Jupiter and Saturn, a Kuiper Belt comet can fall into the inner solar system and become captured in a so-called short-period orbit.

In the early solar system, gravitational encounters also threw large numbers of comets into a vast, spherical shell known as the Oort Cloud. Comets that eventually fall back into the inner solar system from the Oort Cloud typically have orbits measured in millions of years.

"Comets formed in the outer part of the solar system and preserve clues to its formation," said principal investigator Michael A'Hearn. "They formed from Jupiter on out to beyond Neptune four-and-a-half billion years ago, together with all the planets. The inner ones got ejected to the Oort Cloud, which extends halfway to the next star, whereas the ones that formed in the Kuiper Belt are probably still in the Kuiper Belt.

"We are examining comets that come in from the Kuiper Belt with Deep Impact. The problem in understanding the comets is, each time the comet goes close to the sun, the surface layer gets heated and this changes the surface layers. So it's only the interior that preserves the clues to the formation of the solar system."

Some models hold that cyclic solar heating affects only the top foot or so of a comet's crust. Other models hold that surface evolution extends to much greater depths.

"A related question is, we know comets eventually stop out-gassing," A'Hearn said. "Why is this? is this because they've exhausted all the gas in the interior, all of the ices have evaporated, all the gasses have flown out? Or is it because the surface layer has developed some sort of a crust of organic goo or just overbearing materials that prevents the ice inside from evaporating and escaping as gas."

By studying the shape of the cone of debris released by the impact, scientists will gain additional insights.

"The biggest uncertainty in the mission is what the phenomena will be at the time of impact," A'Hearn said. "And that is because there are many different ideas in the scientific community about the nature of the cometary nucleus.

"There are some people in the community who think the nuclei are strong and that we will have an ejecta cone that leaves the nucleus entirely. We think the cone will stay attached to the nucleus and the crater will be controlled by gravity.

"Other people think we will fracture the nucleus into several pieces, other people think we may just compress material downward and not eject anything outward, or almost nothing outward," A'Hearn said. "It is this uncertainty in the predictions, or the wide range of predictions, that makes it particularly important to do this conceptually very simple experiment."

If all goes well, Deep Impact will answer those questions, and more.

Depending on the strength of the nucleus, A'Hearn expects the impactor to blast a crater at large as a football stadium or as small as an average home. If the cloud of dust normally surrounding the comet is relatively thin, the last image taken by the impactor's 4.7-inch telescope, at an altitude of 12 miles or so, will show details as small as 8 inches across. If the dust is thicker, the camera's optics will be sand-blasted earlier and the final image would be snapped from a higher altitude, reducing resolution to 10 feet or so.

For comparison, the European Space Agency's Giotto probe could not distinguish surface features on Halley's comet smaller than about 300 feet across - the size of a football field.

"Our mission, the Deep Impact mission, is going to resolve objects on the surface (of Tempel 1) about the size of the football," said Yeomans. "So we're making a huge leap forward in resolution."

The ejecta cone will be observed in detail by the mothership's 11.8-inch high-resolution instrument, or HRI, the large telescope that delivers magnified images to a multi-spectral camera and an infrared spectrometer. Once within 420 miles of Tempel 1, the HRI should achieve a resolution of six feet per pixel.

A medium-resolution instrument aboard the mothership features a 4.7-inch telescope, identical to the one on the impactor, to provide a wider field of view with a maximum resolution of about 33 feet per pixel. The MRI will observe the entire crater and surrounding territory while the HRI focuses on small areas of the crater's interior.

The instruments will photograph the collision, the resulting crater and analyze light reflected from the dust blown back into space to characterize its chemical composition.

"The ejecta in the case of a big crater, will be comparable to what the comet normally releases over the period of about five days," A'Hearn said. "And it will all come out in four or five minutes. An optimistic assumption is ... sunlight reflecting off that dust will be visible to the naked eye from Earth. The pessimistic assumption is the comet is very strong.

"When you get the small end of the cratering ... then you don't get nearly as much ejecta," he said. "The key observation is to look at the ejecta cone and see if it grows wider and wider and stays attached to the nucleus or does it lift off?"

The former implies a low-density, gravity-dominated nucleus while the latter implies a nucleus that includes some sort of bonding agent to give the material strength.

As for what the mothership's instruments might see peering down into the crater, "my guess is if we excavate more deeply, we will see more carbon dioxide and carbon monoxide, dry ice vaporizing instead of water ice vaporizing," A'Hearn said. "The more volatile ices have been depleted in the surface layers. That's the kind of signature that we're looking for, to see how that composition changes."

The mothership will have less than 14 minutes to collect the impact data before it turns away, re-orienting itself into a protective position to prevent damage as it flies through the thicker regions of the dust cloud surrounding the nucleus.

"These two spacecraft are the armored Humvees of deep space experiments," Henderson said. "They're smart and they're tough. We have the potential to encounter significant debris as we're flying into the coma and the tail of the comet. So we have installed debris shields on both the flyby and the impactor spacecraft to protect ourselves from debris.

"Let's keep in mind the size of the debris we can protect against. We're traveling at 23,000 miles per hour and these shields were designed to stop particles up to a quarter of a gram, or one fourth the size of an M&M. All of our models and the data we've received from previous flyby missions tells us we're going to be fine. But certainly for the impactor, it's going to be a very bumpy ride as it gets closer and closer to the comet."

Once safely past the nucleus, the flyby spacecraft will turn again to take a final few photographs of the crater and the ejecta cone and to transmit its stored data to Earth. The impact should make quite a splash, at least for space-based and large ground-based telescopes like those atop Mauna Kea, Hawaii.

"At the time of encounter, we may be able to see a bright flash of light momentarily," said Karen Meech, a Deep Impact science team member. "But the main part that we're going to be looking for from the ground will be some of the long-term effects. For example, as the dust from this newly excavated crater starts to flow away from the comet, it will take many days to spread ... and form a nice dust tail.

"Ground-based observations with a wide-angle field of view can best watch the tail develop. In addition, we will get to look at wavelength regions we won't have on the spacecraft and can look for molecules coming outside from the nucleus, different types of molecules. We're hoping to see a change in the chemistry after the impact as compared to pre impact.

"So there will be a lot of exciting science ... at various observatories all over the world," she said. "Basically, everybody's going to be able to participate."