Deep Impact (space mission)

Deep Impact is a NASA space probe designed to study the composition of the interior of the comet Tempel 1. At 5:52 UTC on July 4 2005, one section of the Deep Impact probe successfully impacted the comet's nucleus, excavating debris from the interior of the nucleus. Photographs of the impact showed the comet to be more dusty and less icy than expected. The impact generated a large, bright dust cloud that obscured the hoped-for view of the impact crater.

Previous space missions to comets, such as Giotto and Stardust, were fly-by missions, only able to photograph and examine the surfaces of cometary nuclei from a distance. The Deep Impact mission was the first to eject material from a comet's surface.

Mission profile

Following its launch on January 12 2005, the Deep Impact spacecraft traveled 429 million kilometers in 174 days to reach Comet Tempel 1 at a cruising speed of 28.6 km/s (103,000 km/h or 64,000 mph). Once the spacecraft reached the vicinity of the comet on July 3 2005, it separated into two portions, an impactor and a flyby probe. The impactor used its thrusters to move into the path of the comet, impacting 24 hours later at a relative speed of 10.3 km/s (37,000 km/h or 23,000 mi/h). The impactor, with its mass of 370 kilograms (814 pounds), delivered 1.96 × 10¹⁰ joules of kinetic energy- the equivalent of 4.5 tons of TNT. Scientists believe that the energy of this high-velocity collision was sufficient to excavate a crater up to 100 m wide (larger than the bowl of the Roman Colosseum), although the crater has not yet been spotted in post-impact images as the cloud of debris resulting from the impact is obscuring the view.

Just minutes after the impact, the flyby probe passed by the nucleus at a close distance of 500 km, taking pictures of the crater position, the ejecta plume, and the entire cometary nucleus. The entire event was photographed by Earth-based telescopes and orbital observatories, including the Hubble, Chandra, Spitzer and XMM-Newton. The impact was also observed by cameras and spectroscopes on board Europe's Rosetta spacecraft, which was about 80 million km from the comet at the time of impact. Rosetta should determine the composition of the gas and dust cloud kicked up by the impact.

After this flyby of Tempel 1, it could be possible to retarget Deep Impact to comet Boethin, depending on budget availability. On July 20 2005 a trajectory correction maneuver was performed to place the spacecraft on a trajectory to carry it to the Earth and use a gravitational slingshot to target another comet.

Scientific goals

The mission's Principal Investigator is Michael A'hearn, an astronomer at the University of Maryland.

The Deep Impact mission will help answer fundamental questions about comets, such as:

Is the composition of a cometary nucleus the same throughout, or has some physical process caused the interior to become differentiated from the surface? In other words, is the nucleus layered?
Are cometary nuclei highly cohesive and tightly-packed, or porous conglomerates?
Do any parts of a cometary nucleus contain pristine material that have been untouched since the creation of the comet during the Solar System's early history?

Scientists hope that these questions will be answered, at least in part, by data from the Deep Impact mission. For example, the size and shape of the crater produced by the impact will tell scientists how well-packed the cometary material is.

Spacecraft design and instrumentation

The spacecraft consists of two main sections, the 370 kg copper-core "Smart Impactor" which impacted the comet, and the "Flyby" section, which imaged the crater created by the impactor.

The flyby section carries two cameras, the High Resolution Imager (HRI) and the Medium Resolution Imager (MRI). The HRI is an imaging device that combines a visible-light camera, infrared spectrometer, and an imaging module. It has been optimized for observing the comet's nucleus. The MRI is the backup device, and was primarily used for navigation during the final 10-day approach.

The impactor section of the spacecraft contains an instrument that is optically identical to the MRI, called the Impactor Targeting Sensor (ITS). Its dual purpose was to sense the Impactor's trajectory, which could then be trimmed (adjusted) up to four times, and to image the comet from close range. As the impactor neared the comet's surface, this camera took high-resolution pictures of the nucleus (as good as 0.2 meters per pixel) that were transmitted in real-time to the flyby spacecraft before it and the Impactor were destroyed. The final image taken by the impactor was snapped only 3.7 seconds before impact.

The Impactor's payload, dubbed the "Cratering Mass" was 49% copper to reduce debris interfering with scientific measurements of the impact. Since copper was not expected to be found on a comet, scientists can eliminate copper from the spectrometer reading. If the impactor was loaded with other materials such as explosives, it would create a significant amount of organic vapor.

Mission events

Before launch

A comet-impact mission was first proposed to NASA in 1996. However, NASA engineers were skeptical that the target could be hit. In 1999, a revised and technologically-upgraded mission proposal, dubbed Deep Impact, was accepted and funded as part of NASA's Discovery Program of low-cost spacecraft. The two spacecraft (Impactor and Flyby) and the three main instruments were built and integrated by Ball Aerospace & Technologies Corp. in Boulder, Colorado, USA. The name of the mission is shared with the Deep Impact movie, in which a comet strikes the Earth; but this is coincidental, as the scientists behind the mission and the creators of the movie devised the name independently of each other, at around the same time.ABC News article - link broken as of 2006-03-02

Launch and commissioning phase

The probe was originally scheduled for launch on December 30 2004, but NASA officials delayed its launch, in order to allow more time for testing the software. It was successfully launched from Cape Canaveral on January 12 2005 at 1:47 p.m. EST (1847 UTC) by a Delta 2 rocket.

Deep Impact's state of health was uncertain during the first day after launch. Shortly after entering orbit around the Sun and deploying its solar panels, the probe switched itself to safe mode. The cause of the problem was simply an incorrect temperature limit in the fault protection logic for the spacecraft's RCS thrusters. The spacecraft's thrusters were used to detumble the spacecraft following third stage separation. NASA subsequently announced that the probe was out of safe mode and healthy.

On February 11, Deep Impact's rockets were fired as planned to correct the spacecraft's course. This correction was so precise that the next planned maneuver for March 31 was canceled. During the "commissioning phase" all instruments were activated and checked out. During these tests it was found that the HRI images were not in focus after it underwent a bake-out period. Mission members are investigating the problem. On June 9, as part of a mission briefing, it was announced that by using image processing software and the mathematical technique of deconvolution, the HRI images could be corrected to provide the resolution anticipated.

Cruise phase

The "cruise phase" began on March 25, immediately after the commissioning phase was completed. This phase continued until about 60 days before the encounter with comet Tempel 1. On April 25 the probe acquired the first image of its target at a distance of 64 million km (39.7 million miles).

On May 4 it executed its second trajectory correction maneuver. Burning its rocket engine for 95 seconds the spacecraft speed was changed by 18.2 kilometers per hour (11.3 miles per hour).

Approach phase

The approach phase extends from 60 days before encounter (May 5) until five days before encounter. Sixty days out was about the earliest time that the Deep Impact spacecraft was expected to detect the comet with its MRI camera. In fact, the comet was spotted ahead of schedule, sixty-nine days before impact (see Cruise phase above). This milestone marks the beginning of an intensive period of observations to refine knowledge of the comet's orbit and study the comet's rotation, activity and dust environment.

On June 14 and June 22 Deep Impact observed two outbursts of activity from the comet, the latter being six times larger than the former.

On June 23, the first of the two final trajectory correct maneuvers (targeting maneuver) was successfully executed. A 6 m/s (13.4 mph) velocity change was needed to adjust the flight path towards the comet and target the impactor at a window in space about 100 kilometers wide.

Image:DI_MRI_T1_doy150.jpg|May 30, 2005, 35 days from impact Image:DI_MRI_T1_doy166_log.jpg|June 15, 19 days from impact Image:DI_MRI_log_T1_doy172.jpg|June 21, 13 days from impact Image:DI_MRI_log_T1_doy178.jpg|June 27, 7 days from impact, near end of Approach Phase

More images from Deep Impact can be seen on the Wikimedia Commons' Deep Impact page.

Impact phase

Impact phase began nominally on June 29, five days before impact. The impactor successfully separated from the Flyby spacecraft at 6:00 (6:07 Ground UTC) July 3 UTC. The first images from the instrumented Impactor were expected 2 hours after separation.

The Flyby spacecraft performed one of two divert maneuvers to avoid damage. A 14 minute burn was executed and slowed down the spacecraft. It was also reported that the communication link between the flyby and the impactor was functioning as expected.

The Impactor spacecraft executed 3 correction maneuvers in the final 2 hours before impact.

Impact occurred at 05:45 UTC (05:52 Ground UTC, +/- up to 3 minutes, One-Way Light Time = 7m 26s) on the morning of July 4, within one second of the expected time for impact.

The Impactor returned images as late as three seconds before impact. Most of the data captured was stored on board the Flyby spacecraft, which radioed approximately 4500 images from the HRI, MRI, and ITS cameras to earth over the next few days.

Image:DI_MRI_log_T1_doy180.jpg|Comet Tempel 1, imaged from 4.2 million km at the start of Impact phase. Image:DI_MRI_Impactor_-24hr.jpg|Impactor imaged by Flyby spacecraft shortly after separation. Image:Deep_Impact_Impactor_1.jpg|Nucleus imaged by the Impactor. Image:Tempel_Impactor_150Km.jpg|Image from Impactor. Image:Deep_Impact_Impactor_3.jpg|Impactor close-up image, taken shortly before impact. Image:Deep Impact approach 2.jpg|The moment of impact, as shown on NASA TV. Image:Tempel1-impact.jpg|Full scope of the impact plume. Image:121520main_HRI-Movie.gif|HRI movie of impact.

Impact Phase Timeline (NASA)

Results

In the post-impact briefing at 0100 Pacific Daylight Time (08:00 UTC) on July 4 2005, the first processed images revealed existing craters on the comet. NASA scientists stated they could not see the new crater that had formed from the impactor. The only models of cometary structure they could positively rule out were the very porous models which had comets as loose aggregates of material.

Data from the mission are still being analyzed, but initial results were surprising. The material excavated by the impact contained more dust and less ice than had been expected. In addition, the material was finer than expected; scientists likened it to talcum powder rather than sand.

One disappointment was that the impact does not seem to have penetrated below the upper layer of material in the comet; according to some scientists, it only "scratched the surface".

Analysis of data from the Swift X-ray telescope showed that the comet continued outgassing from the impact for 13 days, with a peak five days after impact. A total of 250,000 tonnes of water was lost ^*.

Future activities

NASA is considering whether to continue the Deep Impact mission by having the spacecraft fly by another comet. Comet Boethin has been mentioned as a possible target.

On July 21 2005 Deep Impact executed a trajectory correction maneuver that placed the spacecraft on course to fly past Earth on December 31 2007. This maneuver will keep the options for future use of the spacecraft open. NASA will entertain requests for future use of Deep Impact through its Discovery Program.

Public interest

Media coverage

The impact was a substantial news event reported and discussed online, in print and on television. There was a genuine suspense because experts held widely differing opinions over the result of the impact. For example, would the impactor go straight through and out the other side, would it create an impact crater, would it open up a hole into the interior of the comet, etc.

Experts came up with a range of soundbites to summarize the mission to the public. Iwan Williams of Queen Mary, University of London, said "It was like a mosquito hitting a 747. What we've found is that the mosquito didn't splat on the surface; it's actually gone through the windscreen." One of the NASA investigators, Dr Sunshine, explained the mission by analogy with how a geologist examines a rock: "He doesn't just look at it, he gets his hammer out and hits it, to find out about what it's like inside and how it's put together: is it a loose association of particles or is it solid?"

Send Your Name To A Comet!

The mission was notable for one of its promotional activities, "Send Your Name To A Comet!". Visitors to the Jet Propulsion Laboratory's website were invited to submit their name between May 2003 and January 2004, and the names gathered - some 625,000 in all - were then burnt onto a mini-CD, which was attached to the impacter. This gimmick was credited with driving interest in the mission.

Reaction from China

Chinese researchers used the Deep Impact mission as an opportunity to highlight the efficiency of American science because public support ensured the possibility of funding long-term research. By contrast, “in China, the public usually has no idea what our scientists are doing, and limited funding for the promotion of science weakens people’s enthusiasm for research."

After the U.S. mission succeeded, China revealed a plan for what it called a "more clever" version of the mission: landing a probe on a small comet or asteroid to push it off course.India Times article link broken as of 2006-03-02

Contributions from amateur astronomers

Since observing time on large, professional telescopes such as Keck or Hubble is always scarce, the Deep Impact scientists have called on "advanced amateur, student, and professional astronomers" to use small telescopes to make long-term observations of the target comet before and after impact. The purpose of these observations is to look for "volatile outgassing, dust coma development and dust production rates, dust tail development, and jet activity and outbursts." Since 2000, amateur astronomers have submitted over a thousand CCD images of the comet.

The comet is currently too dim to be seen with anything smaller than a large backyard telescope, but it was thought possible that the impact on July 4 could brighten the comet substantially, making it visible through binoculars toward the star Spica (visible even to the naked eye in areas with low light pollution).Griffithobs.org link broken as of 2006-03-02

One Notable Amateur observation was the students of the King's School Canterbury, Kent, UK who during a press conference took images live using the Faulks Automatic Telescope in Hawaii (the students operate the telescope over the internet and were in the UK) they were one of the first groups to get images of the impact.

One amateur astronomer reported seeing a structureless bright cloud around the comet, and an estimated 2 magnitude increase in brightness after the impact.Deep Impact / Tempel 1 Observation Google Groups thread

Another amateur published a map of the crash area from NASA images.

Musical tribute

The Deep Impact mission coincided with celebrations in the Los Angeles area marking the 50th anniversary of "Rock Around the Clock" by Bill Haley and His Comets becoming the first rock and roll single to reach No. 1 on the recording sales charts. Within 24 hours of the mission's success, a two-minute music video had been created using images of the impact itself combined with computer animation of the Deep Impact probe in flight, interspersed with footage of Bill Haley and His Comets performing in 1955 and the surviving original members of The Comets performing in March 2005. The video was posted to NASA's website for a couple of weeks afterwards.

On July 5, the surviving original members of The Comets (ranging in age from 71 to 84) performed a free concert for hundreds of employees of the Jet Propulsion Laboratory to help them celebrate the mission's success. This event received worldwide press attention.Later, in February 2006, the International Astronomical Union citation officially naming asteroid 79896 Billhaley included a reference to the JPL concert [http://www.klet.org/names/view.php3?astnum=79896.

References

"Deep Impact: Our First Look Inside a Comet." June 2005 issue of Sky and Telescope magazine, pp. 40-44.
"Deep Impact encounter press kit." PDF file.
"Deep Impact: Mission Science Q&A." NASA. ^*