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MISSION

Frequently Asked Questions (FAQs)

Frequently asked questions about the Dawn mission are grouped below by category. Click on a question below to find out the answer.

General

How did the Dawn mission get its name?

The mission was not named after a person, but rather it was so named to present a simple image of the purpose of the mission: to provide information on the dawn of the solar system.

Science

Why aren't there more images being taken or released now that the Dawn mission is at Vesta? (8/15/11)

Most likely, the mission is taking far fewer images at this point than you expect.

NASA missions have several phases. Until the beginning of the official "Science Operations," the spacecraft and the flight team's goals are to provide engineering data, including optical navigation images and calibration images. During those times, the number of images taken and their exposure times are decided in order to "check out" the instruments, operations plans, and determine how the spacecraft is flying. These initial images do not usually have anything of interest in them. Dawn, in fact, released its initial navigation image of Vesta and showed a white dot a few pixels across. Because Dawn approached Vesta very slowly, it took 2 months before Vesta began to look interesting and another 6 weeks before Dawn was close enough to take science images.

Just as we all do when taking pictures, Dawn takes many images hoping for one good shot. In many of the Dawn images, either there are several of essentially the same view, or sets of images where few details are visible because of exposure times, for example. In making color images or stereo images, multiple shots need to be taken of the same scene with only a change of filter or a few pixels change in view. These images are stored on the spacecraft to wait to be transmitted, and are often combined together before release in order to create the data product desired.

The Dawn mission is taking the maximum number of photos it can take, store, and send back from Vesta. Unfortunately, there is a limit to its telemetry rate (how much data it can send back) imposed by the distance it is away from Earth, the size of the antenna and the power available to transmit. Since it is farther away from the Sun than Mars missions, and has a smaller budget (resulting in a smaller rocket and smaller telemetry antenna) than missions like Cassini, Dawn can send back fewer images. Dawn is always in orbit at Vesta, but since its instruments are bolted to the spacecraft, when the spacecraft is turned for an eight-hour trasmission, that means that the instruments are pointed away from Vesta during that time.

Right now, in Survey orbit, (~2750 km altitude) and to some extent in HAMO (~700 km altitude), Dawn is very far away from Vesta acquiring "context" images. These are incredibly vital for the science. A way to think about this is that if you decide to take a cross-country car trip from California to Texas, you will need a map not only of the states so that you can see the whole route, but also you'll need city maps in order to stop and see the sites or find a good spot for lunch. You plan the route with the national map first, then the state map to figure out good cities to stop by, and then finally a city map to find a hotel or restaurant. It is the same thing for Dawn. In Survey orbit, we are taking a good look at the whole body. In HAMO, Dawn will get closer to Vesta, and thus be able to take higher resolution images to study regions of the surface in more detail, and finally in LAMO (~180 km altitude) it gets those great detailed images of surface features. But without the first two types of maps, it would be nearly impossible to know where the area is on the surface or what the feature is.

Another thing to keep in mind is that when Dawn is far away from Vesta, lots of images are taken but the orbital speed is very slow, so many of the images look the same. These images are good for making stereo images and tracking the spacecraft, but once you have seen one you have seen them all. The interesting pictures, like the global mosaic, that show new views of Vesta have all been released. Some of these products are composites of several images.

Eventually, all of the data from NASA missions are made public. NASA requires missions to release all of their data to the Planetary Data System, and the Dawn mission has agreed to do that within 6 months of the data being taken (some missions take years). So even all those repeat images will be out soon. Dawn plans to make even earlier releases, though we cannot be sure what the detailed schedule will be since the mission schedule is subject to change.

Answer provided by Britney Schmidt, Dawn E/PO Science Team Liaison

Why are there no close-up views of Vesta yet? Why can't Dawn "zoom in" on Vesta? (8/15/11)

Dawn's Framing Camera is a fixed focal length camera—almost all spacecraft cameras are. This means that the resolution of the pictures depends on Dawn's distance from Vesta. On your camera at home, when you "zoom in," you are changing the focal length of the camera by changing the relative position of the lenses in your camera. On Dawn, the lens positions are fixed. This is done for a reason—your backyard camera's focus can get "stuck," so scientists and engineers try to minimize the number of moving parts on the spacecraft so that the best science can always be done.

Here is another way that Dawn is different than previous missions: because we use an Ion engine (which is more efficient and saves lots of money making it possible to visit two asteroids for the price of one), it takes about a month for us to get from Survey to the next orbit, HAMO and another month to get to the lowest orbit, LAMO, in December. Eventually, Dawn (and you) will see the whole surface in incredible detail, and several global maps will be made as we spiral down in the coming months.

Answer provided by Britney Schmidt, Dawn E/PO Science Team Liaison

Why would Dawn choose to use an ion engine, since it takes so much time to maneuver? (8/15/11)

That's simple: it may take longer to get the images and answers we want, but we get to go to two protoplanets for the price of one. Dawn has a small science team and a much smaller budget than most planetary missions. By using the ion engine, we can do more science and learn many more things with the same amount of money.

Answer provided by Britney Schmidt, Dawn E/PO Science Team Liaison

Why doesn't the Dawn mission just release raw images? (8/15/11)

Have you ever seen a television that has a bad screen--a corner that is redder than the rest of the screen, or a black streak down the image?

Camera CCDs can be the same way. Spacecraft Camera systems are very carefully engineered, but still there are some idiosyncrasies. Some pixels appear too bright, some are too dark, and because the spacecraft is taking pictures out in space, there are a lot of stray particles (called cosmic rays) that hit the CCD and affect the image. Thus, raw images could give the impression that Vesta looks much different than it actually is. Since brightness, darkness and surface patterns are very important in interpreting spacecraft data, we calibrate the images before they are released so that we do not send out bad information. It takes some time, but it's worth it to get the best pictures out. Remember, Dawn has a very small science team and support staff, being a Discovery class mission. Unfortunately, all of the downloading, processing and releasing takes an incredible amount of time. We are doing our best to release images as quickly as possible.

So in summary, Dawn cannot take as many images as you and I would like but Dawn will image all the sunlit surface and you will see it all with time. The Dawn Science Team understands your impatience. It feels the same impatience, having worked for decades to propose the mission, plan it, build it, launch it, fly it, and finally to get to Vesta.

Answer provided by Britney Schmidt, Dawn E/PO Science Team Liaison

Why aren't there more images of Vesta on the website? (6/22/11)

The spacecraft has to stop thrusting and turn to take a picture of Vesta and then turn again to send it back to Earth. We do not want to stop thrusting often or for long thus we only take "nav" pictures occasionally. These pictures are necessary to determine the exact position of Vesta using the location of the background stars. These are the pictures we are currently posting.

There is also latency from the time of taking the image to the time of putting it on the web because of processing of the image.

When we get into a mapping orbit, this will change to a daily image, but that does not happen until mid-August 2011. Until then, we will try to post about one picture each week.

How well does Ceres fit "Bode's Law?"

Ceres fits the Bode's law prediction as well as the rest of the planets do. This was the source of great satisfaction when Ceres was found and reinforced the credibility of the Bode's law.

If an astronaut could stand on Ceres, what percentage of Earth's gravity would she/he experience?

We'll first give the general solution and then apply it to Ceres. Assume first that all planetary bodies are made of the same material.

The volume of a planet is proportional to the cube of the radius and hence the mass [equal to the density times the volume] is also proportional to the cube of the radius. Surface gravity is proportional to the mass and inversely as the radius squared. Multiplying our formula for mass and surface gravity together we get that surface gravity increases as the radius of the planet. So if Ceres [ a little under 500 km radius] were made of the same material as Earth [ a little over 6000 km radius] then the astronaut would weigh [experience a force of gravity of] about 1/12 of that on Earth.

However, Ceres has a density of about 1/3 of that of Earth and there the astronaut would weigh about 3% of his Earth-bound weight.

Problem for reader: The radius of Vesta is close to half that of Ceres and its density close to twice that of Ceres. Follow the example above and apply it to Vesta. Will the astronaut on Vesta weigh much less, much more, or about the same as on Ceres?

Answer provided by Chris Russell, Dawn mission Principal Investigator

If an astronaut can run 0.1 mile in one minute in his space suit, what is the smallest asteroid on which the astronaut can land without having to be concerned about floating off when he or she runs? About 500 meter radius? About 2 km radius? About 8 km radius (the average size of the asteroid Eros orbited on the NEAR mission)?

A helpful rule that can be applied to this problem is that given material of the same density for each asteroid the orbit that skims the surface has the same orbital period. The derivation of this rule involves balancing gravitational force with centrifugal force and remembering that the orbital period is once around the body with the two forces in balance.

This period is about 90 minutes at Earth but for the less dense asteroids is closer to 120 minutes. The circumference of Ceres is about 3000 km so that an astronaut would have to be able to run about 25 km per minute to just orbit above the surface. The velocity needed to escape is 40% greater than this or over 35 km per minute [close to 25 miles per minute or 1400 miles per hour. Convert .1 mile/min to 6 miles/hr and you can see that the astronaut has no worries.

Scaling to the 8, 2, and 0.5 km asteroids, with the same density as Ceres, you can determine that she/he is safe on the 8, and 2 km. asteroids but not the 0.5 km one. Using a spread sheet, and substituting values, you can determine that with a radius of 1.35 km, the escape velocity will be about 6 miles/hr, assuming the density is the same as Ceres and escape velocity is 40% than the orbital velocity at the surface of the asteroid.

Answer provided by Chris Russell, Dawn mission Principal Investigator

Does Ceres have a firm surface upon which a spacecraft might land? How do you know that?

The asteroids in the main belt must consist principally of rocks with subsurface water possible on the larger asteroids. Thus, we expect every asteroid has a surface on which spacecraft can land.

Answer provided by Chris Russell, Dawn mission Principal Investigator

Why were these two asteroids in particular, Ceres and Vesta, chosen as targets?

These two bodies are the most massive of the minor planets; they straddle avery important "boundary" in the asteroid belt where the bodies change from being almost devoid of the effects of water to those that show hydration effects. In fact we believe Vesta is very very dry and Ceres may have a layer of 100 km of water ice or even liquid water beneath its crust, even though Vesta is on average 2.34 AU from the Sun and Ceres is only a little farther out at 2.77 AU. It is also very important to note that these two bodies are accessible to a space mission of moderate cost. All asteroids such as 2 Pallas are out of contention because matching orbits with them is not possible with a modest mission such as Dawn.

What is the average distance between individual asteroids? (6/13/10)

To answer this question requires making some assumptions and approximations. The asteroid belt is populated by objects ranging in size from dwarf planet Ceres (Dawn's second destination) at nearly 600 miles in diameter to microscopic particles. The smallest ones that are detectable depend on the distance and reflectivity of the bodies (that is, how bright they are), but a reasonable estimate is about 1 mile in diameter. Most such objects have not been discovered, but based on both observations and mathematical models, an astronomer recently estimated there may be 2 million residents of the asteroid belt that size and larger.

Asteroids are not distributed uniformly in the asteroid belt, but could be approximated to be evenly spaced in a region from 2.2 AU (1 AU is 93 million miles, or the average distance between Earth and the Sun) to 3.2 AU from the Sun and extending 0.5 AU above and below the ecliptic (the plane of Earth's orbit, which is a convenient reference for the solar system). That yields a volume of roughly 16 cubic AU, or about 13 trillion trillion cubic miles. (Note: space is big!)

If there were 2 million asteroids 1 mile or larger in that volume, each asteroid would have 6.7 million trillion cubic miles to itself, so the average distance between individual asteroids 1 mile in diameter or larger would be about 1.9 million miles. That is nearly 8 times the distance between Earth and the Moon.

Asteroid belts are often depicted in science fiction as boulders ranging in size up to many miles, jostling for space, nearly bumping into each other, often as ships dodge and swerve around them. As you can see, that is not the case at all. Although I have made some simplifying assumptions here, and different choices would yield a somewhat different answer, the conclusion is the same: the asteroid belt is mostly empty space, and the asteroids orbit the Sun largely in isolation. Encounters among them are rare.

The November 2009 Dawn Journal, just two weeks after the spacecraft entered the asteroid belt, gave a different illustration of the vast distances between asteroids, comparing the probe's trek through the asteroid belt with a cross-country drive. Once again, it shows that the distances are large. Still, the asteroid belt contains so many objects that the separations there are smaller than the separations between objects in other parts of the solar system, including where we spend most of our time.

Answer provided by Marc Rayman, Dawn Chief Engineer, JPL

Mission

Is the Dawn spacecraft going to land on Vesta or Ceres? (7/19/11)

Dawn is not engineered to land on any asteroid.

Pallas is the second biggest astroid at 608 km. in diameter as opposed to Vesta`s 538 km, yet there is no mention of visiting it. Is the fact that its perihelion 2.11 AU and aphelion 3.42 AU is too eliptical for a visit to take place, or will you try to fit a visit in after you visit Ceres if Dawn is still working? (3/19/11)

There is some debate on the exact size of Pallas. My student and I used HST to examine Pallas and we got a smaller diameter than yours, much closer to that of Vesta and possibly smaller than it. But we do not ignore Pallas because of its size. It is impossible to reach with a mission in the same class as Dawn because it takes too much thrust to reach Pallas.

Pallas is highly inclined to the ecliptic plane. A lot of energy is needed to climb out of the ecliptic plane especially as far out of the plane as Pallas is. I DID try to design a mission to reach Pallas and it was impossible with the Dawn spacecraft even if we went nowhere else than Pallas.

What is the reason for going to Vesta before Ceres?

We are going to Vesta first because it is closer to the Sun than Ceres and we pass it on the way to Ceres. This possibility also depends on Vesta and Ceres being both in the right part of the sky (nearly aligned with the Sun). This in turn happens every 17 years so we are very fortunate to be able to do this dual asteroid mission right now.

Answer provided by Chris Russell, Dawn mission Principal Investigator

Will there be opportunities to visit other asteroids, either en route to Ceres or as part of an extended mission?

Unlikely, because there is greater return by spending more of our resources on Vesta and Ceres.

Why does it take so long to get to Vesta and then from Vesta to Ceres? The travel time scheduled appears to be significantly longer than would be required by a Hohmann transfer orbit.

The reason that Dawn has a longer trip time than might be required by conventional means is that Dawn uses an ion propulsion system which precludes achieving a Hohmann transfer orbit. Hohmann transfer orbits are the most propellant-efficient means of moving between two circular coplanar orbits. Hohmann transfers are certainly not the fastest route between orbits; however, they are used frequently because most missions are tightly constrained in mass, so propellant is a very precious resource.

To accomplish a Hohmann transfer, two propulsive maneuvers are required. The first one breaks the spacecraft out of the initial orbit and puts it in an orbit that intersects the desired final orbit. The spacecraft is then in the Hohmann transfer orbit, which is an ellipse tangent to both circular orbits. After the spacecraft has coasted to the point that connects the transfer orbit to the desired final orbit, it fires its engine a second time, now to circularize its orbit, thus matching the target orbit.

Dawn's ion propulsion system is far more efficient than a chemical propulsion system would be, but it produces much less thrust. In other words, it takes significantly less xenon propellant for Dawn to change its velocity by a given amount than it would if it used chemical propellants, but it also takes longer. Ultimately ion propulsion can allow a spacecraft to achieve a higher speed than one with chemical propulsion could. (Ion propulsion provides what I always like to call acceleration with patience.)

Dawn cannot provide a sufficiently large acceleration to follow a Hohmann transfer orbit -- the thrust is simply too gentle. As a result, after receiving its initial boost out of Earth orbit from the Delta rocket, it spirals away from the Sun until it reaches Vesta's orbit. It thrusts most of the way to Vesta, very gradually adding energy to its orbit around the Sun, rather than beginning with a huge burn and coasting to Vesta. This gentle reshaping of the orbit, in contrast to the more abrupt changes typical of chemical propulsion, is a characteristic of ion propulsion and other so-called low-thrust propulsion systems, such as solar sails. As low-thrust propulsion is just beginning to be used for reaching destinations, some of our standard conceptions for how spacecraft move around the solar system may need to be revised.

An example might help illustrate the difference between using chemical and ion propulsion. The engine on a conventional interplanetary spacecraft may burn roughly 300 kilograms of propellants in around 20 minutes of operation, achieving a velocity change of perhaps 1000 meters/second. At its maximum thrust, Dawn's ion engine can expend only about 0.25 kg of xenon per day, changing the spacecraft's velocity by 10 m/s. To achieve that 1000 m/s thus would require only 25 kg of xenon -- a tremendous savings given the high cost of launching spacecraft from Earth -- but it would take 100 days. As the spacecraft recedes from the Sun, its solar arrays produce less power, so it operates at a lower throttle level, using still less propellant and taking still longer to achieve these velocity changes.

Dawn will carry enough propellant to change its speed by more than 10 kilometers/s (or about 6 miles per second) over the course of the mission, far more than any spacecraft's propulsion system has ever accomplished, but it will require an accumulated thrust time of more than 6 years. Although it will take Dawn longer to go from Earth to Vesta and from Vesta to Ceres with ion propulsion than it would with chemical propulsion, the longer trip time is well worth it. Dawn will use a significantly less expensive rocket than it would if it had to carry the much more massive propellants required for a conventional chemical propulsion system. In fact, Dawn simply would be unaffordable without ion propulsion. Now, however, your tax dollars and mine can be used to accomplish a broad and exciting program of solar system exploration, including the acquisition of a wonderfully rich set of science data at Vesta and Ceres.

Answer provided by Marc Rayman, Dawn Chief Engineer, JPL

I see that a gravitational trajectory assist is scheduled for February 2009 (angular momentum transfer) with Mars. Normally, outward-bound probes pass by the planet while closer to the sun than the planet. The Dawn probe seems to be further away from the sun than Mars, so it would be traveling faster than Mars before the transfer. Wouldn't that slow the probe down, instead of speeding it up with respect to the sun? A more succinct question would be: How much delta V are you expecting from the Mars encounter (heliocentric velocity).

There is a wide range of geometries that can make planetary gravity assists effective, and while approaching from outside the orbit of the planet may appear unusual, Dawn is not unique in doing so. The specifics of the gravity assist include not only the relative speed between the probe and the planet but also the direction each one is moving at the time of the encounter.

In our case, the principal benefit of the gravity assist is to change the plane of Dawn's orbit around the Sun. Based on your choice of words, you seem to have some understanding already of the key principles, so you probably already know that most planets orbit the Sun close to the plane of Earth's orbit, also known as the ecliptic. You may also know that changing the plane of an orbit can be propulsively very expensive. Vesta and Ceres orbit farther from the ecliptic than most planets do.

If we had launched in 2006, the ion propulsion system could have achieved the plane change by itself. The mission is a little more difficult with a 2007 launch, because there is less time to complete the required ion thrusting before the relative alignment of Vesta and Ceres makes the trip between them inconveniently long. Therefore, we take advantage of the gravity of Mars to reduce the time Dawn needs to thrust, allowing it to reach Vesta at about the same time, even after launching a year later. The principal effect of the encounter is to change the plane of Dawn's orbit by about 5X, with most of the plane change being in inclination. That is equivalent to about 2.3 km/s, but it does not change Dawn's orbital energy. The gravity assist also provides about 1.1 km/s to raise the energy of Dawn's orbit around the Sun. The combined effect is to impart a delta-v of about 2.6 km/s.

Answer provided by Marc Rayman, Dawn Chief Engineer, JPL

Have any of the solar panels been damaged by Micrometeorites during the mission so far?

Not as far as we can tell. If there is any damage, it would be very minor and well below the threshold that we can measure.

Since the Dawn mission is to fly through the asteroid belt, is there any concern it will be hit and destroyed by micrometeorites?

More precisely, Dawn flies IN the asteroid belt, so it has a very similar speed to the material around it. So, the material is a little less dangerous that you might assume. But, most importantly, the small meteoroids are far between and the chance of hitting one if you are the size of Dawn is small (but not totally negligible). We, therefore, are concerned and will avoid any region where we think there might be higher than usual danger.

For more information about "flying through the asteroid belt," check out Dawn Chief Engineer, Marc Rayman's November 27, 2009 Dawn Journal.

Answer provided by Chris Russell, Principal Investigator for the Dawn mission and Marc Rayman, Dawn Chief Engineer, JPL

Why was a Ceres orbit chosen over having Dawn travel beside Ceres and allow the rotation to reveal the surface to the science instruments? (7/06/10)

A spacecraft that would be close enough to Ceres to have a good view of it will have to be in orbit around it. Ceres and Vesta are quite unlike most asteroids. They are massive bodies with significant gravity. Hayabusa was able to orbit the Sun near Itokawa because that asteroid's gravity is so weak that it did not pull the craft into orbit. That is not the case with Ceres and Vesta. Were Dawn to remain in orbit around the Sun and not be captured by Ceres' gravity, it would have to stay more than about 200,000 km from the dwarf planet.

Being in orbit is not an obstacle to using the rotation to reveal the surface. The tentative design for the first science orbit at Ceres is at an altitude of about 5,900 kilometers. It will take Dawn about 112 hours to complete one such orbit, but Ceres rotates in about 9 hours. Therefore, it is almost as if Dawn hovers; it progresses only a short distance in its orbit as Ceres rotates beneath it. The spacecraft will be in a near-polar orbit, so with the combination of the rotation of its target and its own orbital motion from pole to pole, Dawn will be able to see the entire illuminated surface easily. (For the analogous orbit at Vesta and further details about observations from there, you might want to take a look at Dawn Chief Engineer, Marc Rayman's Dawn Journal from May 27, 2010.)

"Having Dawn travel besides Ceres" means having Dawn match Ceres' orbit around the Sun. To get into orbit around Ceres, Dawn will do that, thanks to its use of ion propulsion. Marc explains how we use ion propulsion to orbit another body in the April 28, 2010 Dawn Journal.

Answer provided by Marc Rayman, Dawn Chief Engineer, JPL

Ion Propulsion

Where does Dawn get xenon for the thrusters? (9/30/11)

As Xe has many applications, it is readily available for purchase from private companies. Dawn (and Deep Space 1, the first interplanetary mission to use ion propulsion) procured Xe from Spectra Gases, Inc. 

The companies extract Xe from the atmosphere. When atmospheric nitrogen is liquified, it contains a mixture of noble gases which are subsequently separated. Because there is such an enormous industry associated with nitrogen, the production of purified xenon is not as expensive as it would otherwise be.

The Xe is stored in the spacecraft under high pressure in a tank that's about 270 liters. To reach the operating thruster, it is fed through a very complicated system that ensures exactly the required flow rate is achieved. As mentioned in the May 25, 2009 Dawn Journal, the rate is only about 3 milligrams per second, and it needs to be controlled with great accuracy.

In the broadest sense, how does ion propulsion work?

Ion propulsion is accomplished by creating ions (charged particles) that are accelerated electrostatically through a potential difference. As these ions are accelerated, conservation of momentum requires that the spacecraft be accelerated in the opposite direction. This provides the thrust to change the speed of the spacecraft.

For more information, see Dawn Chief Engineer Marc Rayman's explanation in the Dec. 28, 2006 Dawn Journal.

Why did the engineers choose Xenon to propel Dawn's ion engines?

There are several advantages of xenon. The ion propulsion system uses electrical power to ionize and accelerate the propellant (as described in Dawn Chief Engineer Marc Rayman's Dec. 28, 2006 Dawn Journal). The action of the ions leaving the thruster causes a reaction that pushes the spacecraft in the other direction. Although this system is extremely efficient, the thrust is very low. But the more massive the ion, the greater the thrust. Xenon, being a relatively massive atom, yields a higher (but still low!) thrust than many other candidate propellants. It is the most massive nonradioactive noble gas (outweighing helium, neon, argon, and krypton). Being nontoxic, it is easy and safe for engineers to work with. Because it is inert, the xenon atoms and ions that happen to make their way to sensitive spacecraft surfaces do not react chemically to degrade thermal, electrical, or optical properties. The xenon is relatively easy to store onboard with conventional high pressure systems at typical spacecraft temperatures.

Answer provided by Marc Rayman, Dawn Chief Engineer, JPL

What makes ion propulsion advantageous for the Dawn mission?

Dawn's ion propulsion engine provides a very small acceleration (about the weight of a piece of paper in your hand). The engine is effective for our travel to the asteroid belt because of the long period of time over which the spacecraft accelerates. The efficiency of ion propulsion and the amount of time we have for travel makes the Dawn mission possible.

If we used conventional chemical propulsion, we would have had to carry greater quantities of fuel and the extra mass would have made the mission beyond our budget, as launching a heavier spacecraft requires a larger rocket which carries a higher cost.

- See additional NASA frequently asked questions about Ion Propulsion

Answers provided by Chris Russell - Dawn Principal Investigator, Lucy McFadden - Dawn Science Team, Britney Schmidt - E/PO Liason to the Science Team, Joe Wise - E/PO Manager, and Marc Rayman - Dawn Chief Engineer.