An artist's concept of Stardust and Comet Wild 2 making their encounter. Credit: NASA

On Jan. 2, 2004, at about 11:41 a.m. PST (2:41 p.m. EST), after traveling the solar system for almost four years -- and 3.22 billion kilometers (2 billion miles) -- the Stardust spacecraft will make its closest approach to comet Wild 2 as the comet saila past Stardust at a relative speed of 21,960 kilometers per hour (13,650 miles per hour). In terrestrial terms such velocity is guaranteed to smoke any police officer's radar gun, but in cosmic terms such relative speed between spacecraft and comet is relatively leisurely, allowing Stardust to "soft-catch" samples of comet dust without changing them greatly.

The passage through the most intensive rain of cometary particles within the coma is expected to last about 8 minutes. Within minutes after the closest approach, Stardust will begin to transmit images and other scientific data collected and stored in its computer memory during the flyby.

Flyby miss distance
Planning for the near-miss of a 5.4-kilometer-wide (3.4-mile) comet and a 4.8-meterlong (15.9-foot) spacecraft closing on each other at a velocity much faster than any rifle bullet -- all while 389 million kilometers (242 million miles) away from home -- is a fulltime job. The flight team was tasked to come up with a flight plan that balances the maximum opportunity for collecting the freshest cometary particles off the comet's nucleus while providing the greatest likelihood of keeping the spacecraft out of harm's way.

Defining "harm's way" in an environment as astronomically foreign as the coma of a comet is a challenge, to say the least. To help understand the hazards posed by the flyby, scientists and engineeers developed a dust model for Wild 2. This model spells out the team's best estimates for the location, quantity and size of the dust surrounding the comet's nucleus. Stardust's managers have been carefully weighing this information as they fine-tune the "miss distance" for the spacecraft's pass by the comet.

Based on the that analysis, mission managers established a miss distance of 300 kilometers (about 186 miles). That distance, however, can be tweaked as late as Dec. 31, 2003 -- just two days before the flyby -- when the spacecraft fires its thrusters to finetune its trajectory for the encounter. The team will gather images taken by Stardust's navigation camera, as well as data from ground-based observatories, to help make a final decision on the comet flyby distance.

Encounter sequences
For planning purposes, the Stardust mission team has defined the spacecraft's encounter of comet Wild 2 as a 119-day period spanning from 88 days prior to comet flyby to 31 days after. This encounter is further divided into five time segments during which various mission activities are planned:

• Far Encounter: Minus 88 days to minus 1 day
• Near Encounter: Minus 1 day to minus 5 hours
• Close Encounter: Minus 5 hours to plus 5 hours
• Closest Encounter: Minus 360 seconds to plus 360 seconds
• Post Encounter: Encounter day to plus 31 days

Far encounter: Minus 88 Days to Minus 1 Day
During this phase, the Stardust team has been using the spacecraft's navigation camera to scour the skies in the area where they expected to find Wild 2. Acquiring the comet visually is considered of paramount importance for a successfully executed flyby, as it will further refine knowledge of the exact location and orbit of the comet.

On Nov. 13, 2003, some 50 days before the encounter, an important milestone was passed when Wild 2 was detected by the spacecraft's navigation camera, several weeks earlier than anticipated. These initial images and later images leading up to encounter are used by navigators to help plan thruster firings to fine-tune Stardust's flight path.

During the final month of the spacecraft's approach to the comet, thruster firings have been scheduled to take place Dec. 3, Dec. 23 and Dec. 31, 2003, as well as on Jan. 2, 2004 (Jan. 1 PST). The Dec. 31 maneuver will set Stardust's final targeted miss distance, and also change the spacecraft's orientation, or "attitude," so that its protective shields will safeguard Stardust from the onslaught of cometary particles. The Jan. 2 thruster firing will clean up any execution errors from the previous maneuver and improve targeting precision.

On Dec. 24, when it is 9 days out, Stardust's schedule calls for it to deploy its aerogelladen collector grid with the "A side" facing the direction of incoming comet particles. After a heat shield is removed from the sample return capsule's opening, the collector emerges from the capsule and is extended fully sticking above the spacecraft's shielding exposed to the stream of comet particles. The sequence will take about 30 minutes to complete.

Near encounter: Minus 1 day to Minus 5 hours
As the spacecraft nears the comet, it will take pictures of the comet more frequently, and spend more time transmitting these images to Earth. To support the increased data traffic, ground stations of the Deep Space Network will be in touch with Stardust continuously during these hours.

Around 5:18 p.m. PST on Jan. 1, the spacecraft will fire its thrusters for a final time before encountering the comet. After this maneuver, the spacecraft will roll back to its protective "encounter attitude" or orientation.

Close encounter: Minus 5 hours to Plus 5 hours
In all of Stardust's journey spanning almost seven years, the majority of scientific data will be collected in a period of just 10 hours -- what mission managers call the "close encounter." As this time window opens, when the spacecraft is about 100,000 kilometers (60,000 miles) away from the nucleus, Stardust will begin to enter the comet's coma. Because of its flight path, it will enter the coma in front of the nucleus as it flies through the halo of gases and dust at the head of comet Wild 2.

During this phase the comet's nucleus should begin to emerge in the navigation camera's field-of-view as an extended dark body. At 30 minutes out, the navigation camera will begin taking images at the rate of nearly one every 30 seconds. Over the next 38 minutes, 72 images will be stored in the spacecraft's computer memory. Fifteen minutes before closest approach, the spacecraft will turn on its dust flux monitor.

Closest encounter: Minus 360 seconds to Plus 360 seconds
Stardust will be flying through the most concentrated region of cometary particles at this point. At 5 minutes out, the navigation camera will increase its rate of picture-taking to one every 10 seconds. This rate will continue for the next 10 minutes.

Comet Wild 2 is flying through space faster than the spacecraft is. To pull off the encounter, navigators had to calculate a location in time and space where the spacecraft needed to be so that the comet could essentially "run over" it. Navigators were further tasked to have this brief meeting of spacecraft and comet occur on the sunlit side of the comet, so that the Sun could act as a natural flashbulb for picture-taking and because that is where the majority of comet particle are expected to be found.

The comet will approach Stardust from outside and below the spacecraft's orbital plane. At this point the spacecraft will be flying "backwards" with its nose pointed down so its defensive shielding can protect it from the expected hailstorm of particles. The comet will overtake the spacecraft and continue on its orbital path, which will carry it above and inside the trajectory of Stardust.

One major challenge for Stardust's mission designers was to figure out a way to keep the comet nucleus within the navigation camera's field-of-view during encounter. To accomplish this, they programmed the navigation camera's scanning mirror to gradually pan as the comet passes by. During closest encounter, the spacecraft will perform a roll to keep the camera pointed at the comet.

This roll, lasting about 30 seconds, could result in loss of the signal from the spacecraft's high-gain antenna. To minimize the chance of loss of signal during this maneuver, mission planners will command the spacecraft's medium-gain antenna to take over from the high-gain dish during the 12 minutes surrounding closest encounter. The spacecraft will stop sending scientific or engineering data and instead will transmit a simple carrier signal during this closest passage. After the 12 minutes of closest encounter are over, the spacecraft will roll to point its high-gain antenna at Earth once again, and will resume transmitting encoded data.

As Stardust flies through the most concentrated region of cometary particles, its dust flux monitor instrument will be logging into the spacecraft's computer memory measures of the size and frequency of dust particle hits. At the same time, the comet and interstellar dust analyzer will be performing instantaneous compositional analysis of material and also storing its results in the spacecraft's onboard memory.

Post-encounter: Flyby day to Plus 31 days
After pointing its high-gain antenna at Earth once again, Stardust will use its navigation camera will take one parting snapshot of Wild 2. Selected images and other science and engineering data recorded onboard during the coma fly-through will begin to be transmitted to Earth.

About five hours after closest approach, the sample collector with its cometary pickings will begin a 30-minute process of stowing itself. After the sample return capsule's lid closes, it will remain sealed until Earth return in January 2006.