Deep Impact (spacecraft)

Deep Impact is a NASA space probe designed to study the composition of the interior of a comet. On July 4, 2005, the impactor portion of the Deep-Impact probe sucessfuly struck the comet at about 7:00UTC.

Previous space missions to comets, such as Giotto and Stardust, were merely fly-by missions, only able to photograph and examine the surfaces of cometary nuclei. The Deep Impact mission will be the first to examine a cometary interior, and thus, scientists hope, reveal new secrets about these small frozen bodies.

Mission profile

Following its launch on January 12, 2005, the Deep Impact spacecraft is currently en-route to Comet Tempel 1. The journey will take 174 days at a cruising speed of 103,000 kilometers per hour (64,000 miles per hour). Once the spacecraft reaches the vicinity of the comet on July 3, 2005, it will separate into two portions, an impactor and a flyby probe. The impactor will use its thrusters to move into the path of the comet, impacting 24 hours later at a relative speed of 37,000 kilometers per hour. The impactor has a mass of 370 kilograms and will approach Tempel 1 with a relative speed of 10.30 kilometers per second (6.3 miles per second), thus delivering 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 will be sufficient to excavate a crater up to 100 meters wide (larger than the bowl of the Roman Colosseum), although the exact size is difficult to predict.

Just minutes after the impact, the flyby probe will pass by the nucleus at a close distance of 500 km, taking pictures of the crater, the ejecta plume, and the entire cometary nucleus. The entire event will also be photographed by Earth-based telescopes and orbital observatories, such as the Hubble, Chandra, Spitzer and XMM-Newton. In addition, the impact will be observed by cameras and spectroscopes on board Europe's Rosetta spacecraft, which will be 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. [1]

The total Deep Impact mission cost is US$330 million.

Scientific goals

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 "Smart Impactor" which will impact the comet, and the "Flyby" section, which will image 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 will primarily be used for navigation during the final 10-day approach.

The impactor section of the spacecraft contains an instrument that is nearly identical to the MRI. As the impactor nears the comet's surface, this camera will take high-resolution pictures of the nucleus (as good as 0.2 meters per pixel) to be transmitted in real-time to the flyby spacecraft before the impactor is destroyed. [2]

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. [3] 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 name of the mission is shared with the Deep Impact movie, in which a comet strikes the Earth, but this is coincidental, as both the scientists behind the mission and the creators of the movie devised the name independently of each other, at around the same time. [4]

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 emergency backup mode. The exact cause and extent of the problem is not yet known, but NASA indicated that the on-board monitoring system detected an overheat problem. NASA subsequently announced that the probe was out of safe mode and healthy. [5]

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 cancelled. During the "commissioning phase" all instruments were activated and checked out. During these tests it was found that the HRI images are not in focus after it underwent a bake-out period [6]. 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. [7]

Cruise phase

Comet Tempel 1 imaged on 25 April by the Deep Impact spacecraft

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 [8].

On June 23, the third trajectory correct maneuver was successfully executed. A 6 m/s (13.4 mph) velocity change was needed to adjust the flight path towards the comet.

May 30, 2005, 35 days from impact
June 15, 19 days from impact

June 15, 19 days from impact
June 21, 13 days from impact

June 21, 13 days from impact
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 (according to [9],[10]). The first images from the instrumented Impactor are expected 2 hours after separation. 22 hours after separation it will then begin autonavigation, which involves 3-4 correction maneuvers in the final 2 hours before impact.

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 is functioning as expected. [11]

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.

Impact occurs at 05:42 UTC

Comet Tempel 1, imaged from 4.2 million km at the start of Impact phase

Comet Tempel 1, imaged from 4.2 million km at the start of Impact phase
Impactor imaged by Flyby spacecraft shortly after separation

Impact Phase Timeline (NASA)

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. [12]

The comet is currently too dim to be seen with anything smaller than a large backyard telescope, but it is possible that the impact on July 4th will brighten the comet substantially, making it visible through binoculars toward the star Spica (possible even to the naked eye in areas with low light pollution). [13]

Controversy

The mission has raised concerns among a small minority that believe that comets are made of antimatter and that the mission would cause a massive explosion, and among astrologers, who believe that damaging a comet would disrupt their "system of spiritual values". One astrologer has even sued NASA over the mission. NASA has responded to the antimatter issue, which it does not find credible[14], but has not yet responded to the lawsuit, which was dismissed and then appealed in Russian courts.[15]

The Planetary Society, a space advocacy group, has responded to the astrologers by pointing out that the mission will merely "put a fresh hole on a comet with very many old holes from prior impacts." Overall, NASA spokeswoman Dolores Beasley says that the mission is "very popular".[16]

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. [17]
"Russian Astrologist Plans to Crash NASA’s Independence Day." Moscow News, April 4, 2005[18]
"NASA mission aims to explore a comet's core." Pittsburgh Post-Gazette. June 24, 2005. [19]

External links