BY JUSTIN RAY
Follow NASA's Deep Impact space probe as it hits Comet Tempel 1, giving humanity the first glimpse into the frozen heart of the rocky snowball. Reload this page for the very latest on the mission.
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SATURDAY, JULY 2, 2005
The action begins around 8:07 p.m. EDT (0007 GMT) with the final targeting maneuver by the mothership. This procedure is designed to target a window 9 miles wide for the impactor projectile to follow during its cruise to the comet.
The battery pack aboard the impactor will be activated at 9:57 p.m. EDT (0157 GMT) in preparation for separation from the mothership. In the final hours before release, the mothership adjusts its orientation, turns on heaters and arms separation actuators.
The actual deployment occurs at 2:07 a.m. EDT (0607 GMT) as electrical disconnect actuators and separation pyros fire. A spring-loaded split of the two spacecraft allows the mothership and impactor to move away from each other at 0.78 mph.
"Just after release, the impactor will use its thrusters to de-tumble itself, stabilize itself, and point its camera directly at the comet," said Dave Spencer, Deep Impact mission manager at JPL.
Twelve minutes after separation, the mothership re-orients itself and fires thrusters for the comet deflection maneuver. This 14-minute maneuver drives the craft away from the impactor's path, enabling it to safely fly past the comet on Monday morning. This will be the longest burn of the mothership's thrusters thus far in the mission.
The maneuver also serves to slow the craft's speed by about 227 miles per hour, creating a window of time to observe the impactor smashing into the comet. "This slow-down maneuver will allow it to image the impact event itself and the subsequent crater formation for about 13 minutes after impact," Spencer said.
"The time of comet encounter is near and the major mission milestones are getting closer and closer together," Rick Grammier, Deep Impact project manager said Friday. "After all the years of design, training and simulations, we are where we want to be. The flight and science teams are working the mission plan, and we are good to go for encounter."
"We've completed the final pre-release checkout of the impactor. The impactor probe will have a short, 24 hour life from release to impact, but an incredibly important role," Spencer said.
Watch this page for confirmation of impactor separation.
THURSDAY, JUNE 30, 2005
The washing machine-sized projectile will be released from its mothership spacecraft at 2:07 a.m. EDT (0607 GMT) Sunday for the day-long cruise to oblivion.
"We put the impactor in the comet's path so that the comet overtakes it. So it is like standing in the middle of the road with semi truck bearing down on you," said Rick Grammier, Deep Impact project manager at the Jet Propulsion Laboratory in Pasadena, Calif.
The impactor and comet collide at 1:52 a.m. EDT (0552 GMT) Monday, releasing the energy equivalent of 4.5 tons of exploding TNT as they smash together at 23,000 mph. The intense forces vaporize the projectile as the circular crater -- perhaps 300 feet in diameter and 100 feet deep -- is rapidly excavated.
"We expect it could put a crater about the size of a house up to the size of a football stadium and it could be anywhere from seven to 14 stories deep," Grammier said.
"As a result of forming the crater, it will throw out a bunch of the surface and interior material that is displaced. It will come up in a big cloud that will reflect the sunlight. So you will see a large brightening, and you will see that brightening from telescopes on Earth as well. Then you will see it slowly dissipate as the material either settles back down onto the comet itself or becomes part of its coma dust cloud. What we are hoping to see to from the flyby spacecraft viewpoint is being able to look all the way down into the interior of the crater and determine what its materials are made of," Grammier continued.
"Since these are the original remnants of the solar system formation, not knowing how the exterior of a comet relates to interior, what we are hoping to do is expose all of that fresh material and see the material that was actually present at the formation of the solar system."
"By understanding and watching how this crater develops, and then fine-tuning our computer models to reproduce what is actually observed, we can determine how the comet is put together," added Don Yeomans, Deep Impact project scientist from the Jet Propulsion Lab.
The impactor, a stubby-nosed bullet about two-and-a-half feet tall and three feet in diameter, sports a manhole cover-sized disc of copper with even more copper mass behind it to penetrate as deep into the comet as possible. A quarter of the impactor's launch weight is copper.
"We don't know what comets are made of, we don't know how strong they are. They could be weak and fluffy like a bowl of corn flakes, it could be like a concrete sidewalk that we are hitting. Part of the challenge in the design of the impactor was to take into account either possibility," Jay Melosh, Deep Impact co-investigator.
Comets are wandering cosmic time capsules preserving 4.5-billion-year-old primordial material that holds the chemical records of the solar system's creation. Deep Impact's violent rendezvous with Tempel 1 is designed to burst through the crust coating the comet's nucleus, form a stadium-sized crater and offer an unprecedented glimpse at ancient ices packed beneath the surface.
"What we see coming out of comets as gas and dust is stuff that has been modified because it is very near the surface, and every time the comet goes around the sun the surface gets heated. So there have been changes in the surface layers... What I really want to do is figure out how different the surface is from what's inside," said Michael A'Hearn, astronomer from the University of Maryland and the Deep Impact principal investigator.
The pristine building blocks buried inside these rocky snowballs will tell astronomers what conditions were like when the solar system was spawning planets. Uncovering the compositional fingerprints of comets has become a priority for scientists because these objects peppered the young Earth, possibly delivering the organic materials needed for the rise of life, the water for our oceans and even playing a role in generating the atmosphere.
"Deep Impact is a bold, innovative and exciting mission which will attempt something never done before to try to uncover clues about our own origins," said Andy Dantzler, acting director of the Solar System Division at NASA Headquarters.
"Why understand comets? Why study comets at all? Comets are the most primitive bodies in our solar system and they are made up of the very material from which all of the planets and the sun, in fact, are made."
Discovered on April 3, 1867 by Ernst Wilhelm Leberecht Tempel in Marseilles, France, Comet 9P/Tempel 1 currently circles the sun every 5.5 years. Its orbit lies between Mars and Jupiter, providing the Deep Impact mission a perfect target for reaching with a modest launch vehicle, striking at high speed and being visible from Earth at impact.
The impactor is equipped with an autonomous navigation computer, cameras and a propulsion system to guide itself toward a suitable impact point that is well lit. After releasing the impactor, the mothership performs an evasive maneuver, plotting a trajectory to fly past the comet shortly after the impact.
"Early images from the impactor are mainly for navigation... to make sure that it hits in an illuminated area and not in a dark area. As we get closer, those images become important for science because as we get closer and closer we will get higher and higher spatial resolution. We will directly see the change in texture as you change spatial scale. Assuming the camera on the impactor survives until very shortly before impact, these will be the highest resolution pictures ever of a cometary nucleus, much higher than we will get from the flyby [spacecraft]," A'Hearn said.
Sophisticated instruments on the mothership will record the blast and peer into a crater that is formed. Meanwhile, observatories around the globe, plus the Hubble, Chandra and Spitzer space telescopes, will be watching the aftermath to collect crucial information about the dusts and gases blown out of Tempel 1.
Sky watchers in the western U.S., Hawaii, New Zealand, eastern Australia and the South Pacific could be able to see the impact, which happens 83 million miles away from Earth.
"We expect to provide some great fireworks for all our observatories," said Karen Meech, Deep Impact co-Investigator at the Institute for Astronomy in Hilo, Hawaii "That's exciting, to do it on July Fourth."
The flyby craft will be using its spectrometer to identify and quantify the materials across the comet's dust- and gas-filled coma head and taking images in a wide variety of different colors. Since the nucleus is believed to have a 41-hour rotational period, less than half will be seen at good resolution.
"Shortly before the time of impact, the flyby spacecraft determines how fast it is having to rotate to keep the high-resolution camera pointed at the nucleus. It uses that to calculate when it will be at closest approach and then knowing the difference velocity, when the impactor will impact. It sends that information up to the impactor so the impactor can optimize the imaging sequence at the expected time of impact. Flyby uses it internally, also, to optimize the imaging sequence for the time of closest approach," A'Hearn said.
Information from both craft is fed back to Earth in real-time in case comet shrapnel fatally wounds the mothership during the encounter.
The impact occurs with the mothership 5,400 miles away and closing fast. The medium-resolution camera will be taking pictures as swiftly as possible to capture the moment of impact. "We are hoping to catch a bright flash that will last less than a second by taking four or eight frames per second," A'Hearn said. The high-resolution camera snaps pictures at a slower pace.
Scientists expect the materials thrown out of the freshly bored hole will settle within a few minutes, permitting good visibility into the crater. The mothership has less than 14 minutes to make its observations while zooming toward the comet before passing by Tempel 1 at a distance of 300 miles. The craft enters a "shield mode" to protect itself from the powerful sandblasting during flight through the coma at closest approach.
"Our baseline is it will take 200 seconds to form the crater, but uncertainties in the density of the nucleus - something that we just don't know - the crater could take as long as 600 seconds to form. This was one of our mission design problems, making sure we had long enough interval to observe so that we make sure the crater finished forming before we flew by but keeping the interval small enough that we weren't so far away at the time of impact that we had no resolution. This what led us to the 800-second window between impact and the going into our shield mode through the innermost coma," A'Hearn said.
"The biggest uncertainty in the mission is what the phenomena will be at the time of impact. 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."
Deep Impact has just one shot at grabbing scientific data on the primordial material packed inside the comet.
"We do maps across the nucleus after the impact to try and get spectra of the crater floor, see how different it is from the neighboring terrain that is undisturbed," A'Hearn said. "We take some spectra off the limb to look at the gases that are coming out of the crater. As we get very close, we actually have to let the camera drift a little and take a couple of images to make sure we get crater in the high-resolution camera."
What might the craft see down in 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."
About 50 seconds before closest approach, the flyby craft orients itself with protective shielding guarding against a destructive hit by comet dust.
"We've designed extra shielding on certain parts of the spacecraft. So when I say it turns to shield mode, what that means is it actually places those shields in the direction of the cometary dust and debris. That is meant to protect the spacecraft itself from any particle hits. That shielding was designed based on what we know today of probable particle sizes, distribution and density at that distance from the comet," Grammier said.
Despite the added protection, the mothership will be relaying its pictures and information to Earth live in case the craft doesn't survive the encounter to tell the tale afterward.
"There are worries, that is why we are transmitting as much as we can in real-time, as much as the communications system will allow us to," A'Hearn said. "The engineers have predicted that the probability of a fatal hit is down at the one or a couple percent level, given the amount of shielding we have."
Once through the dangerous region, the departing mothership maneuvers to observe the comet's back side a quarter-hour after closest approach.
"We fly through the innermost coma, fly through the orbital plane and then turn around and look back... to take images of the other side. When we take pictures of the other side, the crater itself will be hidden, but we will still be looking to see if we can see ejecta from the crater. A likely scenario is that after we make the crater, there will be a lot spontaneous outgasing from the floor of the crater because there is very volatile ice near the surface that used to be buried deeply. There is a reasonable chance that we would see a new jet in the coma coming from the crater - and we would see it where it comes out from behind the limb of the nucleus," A'Hearn said.
"We also use these look-back images to figure out the three-dimensional shape of the nucleus since we don't get to see a full rotation. We do the look-back monitoring for up to a day after impact."
Ground-based telescopes in Hawaii will have prime viewing with the comet high in the sky at the time of impact, while the southwestern U.S. and Baja California will have Tempel 1 low in the sky. But a global campaign is underway to provide thorough monitoring of the comet before and after the collision with special imaging techniques.
"We may create this new jet that may persist for hours or days or weeks or even months. So we are looking for observations afterwards," A'Hearn said.
"We are trying to get complete longitude coverage so we can monitor the comet continuously from something like four days before the impact - two rotation periods - until a week after the impact."
"At the time of encounter, we may be able to see a bright flash of light momentarily," Meech said. "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."
The impact will have no detectable influence on the comet's orbit around the sun, scientists say.
"In the world of science, this is the astronomical equivalent of a 767 airliner running into a mosquito," Yeomans said. "The impact simply will not appreciably modify the comet's orbital path. Comet Tempel 1 poses no threat to the Earth now or in the foreseeable future."
The mission follows NASA's Stardust project that flew past Comet Wild 2 in 2004, catching dust particles for return to Earth next year. The European Rosetta mission is currently flying to Comet Churyumov-Gerasimenko where a tiny lander will be dispatched to the frigid nucleus.
"The last 24 hours of the impactor's life should provide the most spectacular data in the history of cometary science," A'Hearn said. "With the information we receive after the impact, it will be a whole new ballgame. We know so little about the structure of cometary nuclei that almost every moment we expect to learn something new."
TUESDAY, JUNE 28, 2005
The Submillimeter Wave Astronomy Satellite has been asleep on orbit for the past 11 months. SWAS operators placed it into hibernation after a highly successful 5.5-year mission highlighted by the discovery of a swarm of comets evaporating around an aging red giant star. Now, they have awakened SWAS again for the first-ever opportunity to study a comet on a collision course with a U.S. space probe. Read our full story.
MONDAY, JUNE 27, 2005
FRIDAY, JUNE 17, 2005
WEDNESDAY, JUNE 15, 2005
THURSDAY, JUNE 2, 2005
Read our earlier launch coverage.