February 14, 2011 Leave a comment
A bonus round is something one usually associates with the likes of a TV game show, not a pioneering deep space mission. “We are definitely in the bonus round,” said Stardust-NExT Project Manager Tim Larson of NASA’s Jet Propulsion Laboratory in Pasadena, Calif. “This spacecraft has already flown by an asteroid and a comet, returned comet dust samples to Earth, and now has almost doubled its originally planned mission life. Now it is poised to perform one more comet flyby.”
Could comets have brought water to Earth?
Comets preserve important clues to the early history of the solar system. They are believed to have contributed some of the volatiles that make up our oceans and atmosphere. They may even have brought to Earth the complex molecules from which life arose. For these reasons, the Committee on Planetary and Lunar Exploration (COMPLEX) has emphasized the direct exploration of comets by spacecraft. The investigation of comets also addresses each of the three strategic objectives for solar system exploration enunciated in NASA’s Space Science Enterprise Strategy (SSES) 2003.
- To learn how the solar system originated and evolved to its current state.
- To understand how life begins and determine the characteristics of the solar system that led to the origin of life.
- To catalog and understand the potential impact hazard to Earth from space.
The Stardust-NExT mission will contribute significantly to the first and last of these objectives by obtaining essential new data on Tempel 1 and capitalize on the discoveries of earlier missions such as Deep Impact to determine how cometary nuclei were constructed at the birth of the solar system and increase our understanding of how they have evolved since then. The Stardust-NExT mission provides NASA with the unique opportunity to study two entirely different comets with the same instrument. By doing this scientist will be able to more accurately compare its existing data set.
The primary science objectives of the mission are as follows:
- To extend our understanding of the processes that affect the surfaces of comet nuclei by documenting the changes that have occurred on comet Tempel 1 between two successive perihelion passages.
- To extend the geologic mapping of the nucleus of Tempel 1 to elucidate the extent and nature of layering and help models of the formation and structure of comet nuclei.
- To extend the study of smooth flow deposits, active areas, and known exposure of water ice.
- On February 14, 2011, at a projected distance of 200 km, the Stardust-NExT spacecraft will obtain high-resolution images of the coma and nucleus, as well as measurements of the composition, size distribution and flux of dust emitted into the coma. Additionally, Stardust-NExT will update the data gathered in 2005 by the Deep Impact mission on the rotational phase of the comet.
- If possible, to characterize the crater produced by Deep Impact in July 2005 to better understand the structure and mechanical properties of cometary nuclei and elucidate crater formation processes on them.
- Measure the flux and mass distribution of dust particles within the coma using the DFMI instrument.
- Analyze the composition of dust particles within the coma using the CIDA instrument.
- Monitor comet activity over 60 days on approach using imaging.
A Successful Prime Mission
NASA’s Stardust spacecraft was launched on Feb. 7, 1999, on a mission that would explore a comet as no previous mission had. Before Stardust, seven spacecraft from NASA, Russia, Japan and the European Space Agency had visited comets – they had flight profiles that allowed them to perform brief encounters, collecting data and sometimes images of the nuclei during the flyby.
Like those comet hunters before it, Stardust was tasked to pass closely by a comet, collecting data and snapping images. It also had the ability to come home again, carrying with it an out-of -this-world gift for cometary scientists – particles of the comet itself. Along the way, the telephone booth-sized comet hunter racked up numerous milestones and more than a few “space firsts.”
In the first round of its prime mission, Stardust performed observations of asteroid Annefrank, only the sixth asteroid in history to be imaged close up. After that, Stardust racked up more points of space exploration firsts. It became the first spacecraft to capture particles of interstellar dust for Earth return. It was first to fly past a comet and collect data and particles of comet dust (hurtling past it at almost four miles per second) for later analysis. Then, it was first to make the trip back to Earth after traveling beyond the orbit of Mars (a two-year trip of 1.2 billion kilometers, or 752 million miles). When Stardust dropped off its sample return capsule from comet Wild 2, the capsule became the fastest human-made object to enter Earth’s atmosphere. The mission was also the first to provide a capsule containing cometary dust specimens, speciments that will have scientists uncovering secrets of comets for years to come.
With such a high tally of “firsts” on its scoreboard, you’d think Stardust could receive a few parting gifts and leave the game. And an important part of the original spacecraft is currently enjoying retirement – albeit a high-profile one: Stardust’s 100-pound sample return capsule is on display in the main hall (Milestones of Flight) of the Smithsonian’s National Air and Space Museum in Washington. But the rest of NASA’s most-seasoned comet hunter is still up there – and there is work still to be done.
“We placed Stardust in a parking orbit that would carry it back by Earth in a couple of years, and then asked the science community for proposals on what could be done with a spacecraft that had a lot of zeros on its odometer, but also had some fuel and good miles left in it,” said Jim Green, director of NASA’s Planetary Science Division.
Moving into the Bonus Round
In January 2007, from a stack of proposals with intriguing ideas, NASA chose Stardust-NExT (Stardust’s Next Exploration of Tempel). It was a plan to revisit comet Tempel 1 at a tenth of the cost of a new, from-the-ground-up mission. Comet Tempel 1 was of particular interest to NASA. It had been the target of a previous NASA spacecraft visit in July 2005. That mission, Deep Impact, placed a copper-infused, 800-pound impactor on a collision course with the comet and observed the results from the cosmic fender-bender via the telescopic cameras onboard the larger part of Deep Impact, a “flyby” spacecraft observing from a safe distance.
“The plan for our encounter is to be more hospitable to comet Tempel 1 than our predecessor,” said Joe Veverka, principal investigator of Stardust-NExT from Cornell University in Ithaca, N.Y. “We will come within about 200 kilometers [124 miles] of Tempel 1 and view the changes that took place over the past five-and-a-half years.”
That period of time is significant for Tempel 1 — it is the period of time it takes the comet to orbit the sun once. Not much happens during a comet’s transit through the chilly reaches of the outer solar system. But when it nears perihelion (the point in its orbit that an object, such as a planet or a comet, is closest to the sun), things begin to sizzle.
“Comets can be very spectacular when they come close to the sun, but we still don’t understand them as well as we should,” said Veverka. “They are also messengers from the past. They tell us how the solar system was formed long ago, and Stardust-NExT will help us understand how much they have changed since their formation.”
So the spacecraft that had traveled farther afield than any of its predecessors was being sent out again in the name of scientific opportunity. In between spacecraft and comet lay four-and-a-half years, over a billion kilometers (646 million miles), and more than a few hurdles along the way.
Your Mileage May Vary
“One of the challenges with reusing a spacecraft designed for a different prime mission is you don’t get to start out with a full tank of gas,” said Larson. “Just about every deep-space exploration spacecraft has a fuel supply customized to get the job done, with some held in reserve for contingency maneuvers and other uncertainties. Fortunately, the Stardust mission navigation team did a great job, the spacecraft operated extremely well, and there was an adequate amount of contingency fuel aboard after its prime mission to make this new comet flyby possible – but just barely.”
Just how much fuel is in Stardust’s tanks for its final act?
“We estimate we have a little under three percent of the fuel the mission launched with,” said Larson. “It is an estimate, because no one has invented an entirely reliable fuel gauge for spacecraft. There are some excellent techniques with which we have made these estimates, but they are still estimates.”
One of the ways mission planners can approximate fuel usage is to look at the history of the vehicle’s flight and how many times and for how long its rocket motors have fired. When that was done for Stardust, the team found their spacecraft’s attitude and translational thrusters had fired almost half-a-million times each over the past 12 years.
“There is always a little plus and minus with each burn. When you add them all up, that is how you get the range of possible answers on how much fuel was used,” said Larson.
Fuel is not the only question that needs to be addressed on the way to a second comet encounter. Added into the mix is the fact a comet near the sun can fire off jets of gas and dust that can cause a change in its orbit, sometimes in unexpected ways, potentially causing a precisely designed cometary approach to become less precise. Then there are the distances involved. Stardust will fly past comet Tempel 1 on almost the opposite side of the sun from Earth, making deep-space communication truly, well, deep space. Add into the mix the Stardust spacecraft itself. Launched when Bill Clinton was in the White House, Stardust has been cooked and frozen countless times during its trips from the inner to outer solar system. It has also weathered its fair share of radiation-packed solar storms. But while its fuel tank may be running near-empty, that doesn’t mean Stardust doesn’t have anything left in the tank.
“All this mission’s challenges are just that – challenges,” said Larson. “We believe our team and our spacecraft are up to meeting every one of them, and we’re looking forward to seeing what Tempel 1 looks like these days.”
The Final Payoff
Larson, Veverka and the world will get their chance beginning a few hours after the encounter on Monday, Feb. 14, at about 8:56 p.m. PST (11:56 p.m. EST), when the first of 72 bonus-round images of the nucleus of comet Tempel 1 are downlinked.
All images of the comet will be taken by the spacecraft’s navigation camera – an amalgam of spare flight-ready hardware left over from previous NASA missions: Voyager (launched in 1977), Galileo (launched in 1989), and Cassini (launched in 1997). Each image will take about 15 minutes to transmit. The first five images to be received and processed on the ground are expected to include a close up of Tempel 1′s nucleus. All data from the flyby (including the images and science data obtained by the spacecraft’s two onboard dust experiments) are expected to take about 10 hours to reach the ground. Stardust-NExT is a low-cost mission that will expand the investigation of comet Tempel 1 initiated by NASA’s Deep Impact spacecraft. JPL, a division of the California Institute of Technology in Pasadena, manages Stardust-NExT for the NASA Science Mission Directorate, Washington, D.C. Joe Veverka of Cornell University, Ithaca, N.Y., is the mission’s principal investigator. Lockheed Martin Space Systems, Denver Colo., built the spacecraft and manages day-to-day mission operations.
The Stardust-NExT will utilize the existing spacecraft to flyby comet Tempel 1 and observe changes since NASA’s Deep Impact mission visited it in 2005. Stardust-NExT will provide NASA with a first-time opportunity to compare observations of a single comet made at close range during two successive perihelion passages, at low risk and low cost.
In 2005, Tempel 1 made its closest approach to the sun, possibly changing the surface of the comet. With a 3-year trajectory, the mission flight plan is designed in a similar way to that of the original mission, with an Earth gravity assist (EGA) in 2009 to achieve the flyby of Tempel 1 in 2011. The original flight path of the Stardust spacecraft to Wild 2 included an EGA in 2001.
Mission Design and Navigation:
The Stardust spacecraft divert maneuver that followed the release of the sample return capsule (SRC) was intentionally designed to place the spacecraft in a trajectory that returns to Earth in case the SRC release that occurred January 15 had failed. Thus the current orbit intrinsically provides the Earth gravity-assist (EGA) flyby opportunity in 2009, which enables the Tempel 1 encounter. The mission duration, from the divert maneuver after SRC release (January 15, 2006) to the February 14, 2011 Tempel 1 encounter, is a little over 5 years. The date of encounter will be optimized during the mission to account for improved knowledge of the comet’s ephemeris during cruise, and to maximize the probability of viewing the Deep Impact impact crater. Table F-1 summarizes the principal characteristics of the comet encounter.
|Table F-1. Tempel 1 Encounter Characteristics|
|Flyby date||February 14, 2011|
|Approach Phase angle||81.6°|
|Closest Approach Point||200 km altitude, 40° south of direction to the Sun|
|Solar Distance||1.55 AU|
|Earth Distance||2.25 AU|
The trajectory consists of four loops of the sun in two separate orbits. Loops 1 and 2 represent the orbit the spacecraft bus was left in after the sample return on January 15, 2006. The EGA on January 14, 2009 places the spacecraft in the final heliocentric orbit (Loops 3 and 4) intercepting Tempel 1 on February 14, 2011 (39d after the comet’s perihelion). This profile is very similar to the launch-to-Wild 2 phase of the Stardust primary mission.
|Figure F-1. Stardust-NExT trajectory, with one EGA prior to Tempel 1 encounter, provides for an uncomplicated mission simpler than the Stardust prime mission.|
The maneuver plan is shown in Table F-2. Of the three deep space maneuvers (DSM’s), only the first is deterministic. This maneuver targets the EGA in January, 2009. Other DSM’s adjust the arrival time at Tempel 1. Ranges of favorable locations for DSMs (2 and 3) are indicated. Their exact location will be optimized during the mission.
Table F-2. Maneuver plan targets EGA and Tempel 1 encounter with few maneuvers
|Maneuver||Epoch||Comment||Execution Date (UTC)|
|DSM1||Entry + 603d||EGA Targeting||9/19/07|
|DSM1_CU||DSM1 + 30d||Cleanup||10/19/07|
|EGA2||E – 10d||EGA Targeting||1/5/09|
|EGA2a||E – 1d||EGA Targeting||1/13/09|
|DSM2||T1 – 1y||Arrival Time Adjust||2/12/10|
|DSM2_CU||DSM2 + 30d||Cleanup||3/14/10|
|T1 (DSM3)||T1-30d||T1 Targeting||1/15/11|
|T2||T1 – 10d||T1 Targeting||2/4/11|
|T3||T1 – 2d||T1 Targeting||2/12/11|
|T4||T1 – 18h||T1 Targeting||2/14/11|
|T5||T1 – 6h||T1 Targeting (Contingency)||2/14/11|
The Stardust navigation team has chosen to place the closest approach point 40° southward of the direction to the Sun, at a longitude that offers the most favorable viewing opportunity of the Deep Impact crater at closest approach. Periodically, reevaluate the aimpoint during the mission, taking into account the most recent information available about the predicted uncertainty of the comet’s rotation state at encounter, until the time shortly before DSM2 (deep space maneuver) at which a selection of a final aimpoint for targeting.
Controlling the arrival time to target our chosen aimpoint is the greatest mission design challenge of Stardust-NExT. In order to successfully control the arrival time as discussed above, two conditions must be met: (1) we must be able to predict the rotation rate and rotational state of the comet with sufficient accuracy to reliably compute the right arrival time, and (2) we must have sufficient DV onboard to change the arrival time as needed.