# Presentation: 487683

## Dawn Mission

## How Do We Get There?

## DAWNA Journey to the Beginning of the Solar System

**Vesta**

**Travel Plans: Dawn’s Itinerary**
**The Dawn Spacecraft will travel**
** ****6.3 billion kilometers**
** ****Almost ten years**
** ****To the asteroid  4 Vesta and Dwarf planet **
**  ****1 Ceres.**

**Ceres**

**Notes:**
Slide Two: Any successful journey requires careful route planning and efficient packing and the Dawn mission is no exception.  The 6.3 billion-kilometer journey will take the Dawn spacecraft eight years, with a major layover at the asteroid, 4 Vesta (planned orbit at three altitudes) and then on to dwarf planet 1 Ceres (planned orbit at three altitudes).

## Travel Plans: Cross-Country Trip

**A 3000 mile trip from Los Angeles to New York City requires:**
**120 gallons of fuel**
**16 cubic feet storage**
**adds 685 pounds weight**
**4 days travel time**

**Notes:**
Slide Three: Imagine that you are planning a cross-country trip from Los Angeles to New York and there are no gas stations along the way.  You will have to carry enough gasoline with you to make the trip.  
It is 4800 km (3000 miles) from Los Angeles to New York.  If your car’s gas mileage averages 40 km/gallon (25 miles/gal), your car will need 120 gallons of fuel to make the trip.  It will require 0.5 cubic meter (or 16 cubic feet) of space to store your fuel and, at the start of the trip, the fuel will weigh 308 kg (685 pounds). 
Could you store this much gasoline safely in your car?  How would the additional weight of the fuel effect your gas mileage?  
If you travel 1200 km (750 miles) a day, the trip will take you four days.       

## A Trip to Mars

**A 35 million mile trip from Earth to Mars requires:**
**900 metric tons of rocket fuel**
**6 months travel time**

**Notes:**
Slide Four: Now, a LONGER DISTANCE trip...to Mars, it will take a little more planning. At its closest, Mars is about 35 million miles (about 56 million km) away from Earth. Dawn’s actual travel time will be about 1 billion km (620 million miles) from Earth to Mars. 
Even using a minimum-energy trajectory, sending a manned spacecraft from Earth to Mars, with chemical rockets alone, would require a total launch mass of greater than 1,000 metric tons—of which some 90 percent would be fuel.  The fuel alone would weigh twice as much as the completed International Space Station.
Chemical rockets burn their propellant at the beginning of a flight so, once the spacecraft is launched into trajectory, it would just coast the rest of the way to Mars.

## Interplanetary Trajectory

**Notes:**
Slide Five: Launching a spacecraft and directing it toward distant objects is a precise and challenging operation because the earth is moving in its orbit and the objects (Vesta and Ceres in this case) are moving as well. It is very much like a quarterback (the earth) moving toward the sidelines while successfully throwing a football (the spacecraft) and completing a pass to a wide receiver (an asteroid) who is also moving.  Practice and coordination are required on the football field. Careful planning and precision are required on the part of the mission planning team. 

At launch atop a Delta II rocket on September 27, 2007, the Dawn spacecraft will be sent on a trajectory that will take it close to Mars.  The launch will be timed so that, after traveling about 17 months, the spacecraft will arrive at the outer end of its orbit just as Mars is passing by.  Dawn will get a gravity assist from Mars to send it on its way to Vesta. This means that Dawn will use the planetary motion of Mars increase its speed and to change its direction.  Note that the times that the spacecraft is thrusting (accelerating) shown with solid dark line verses coasting shown with a dotted line. 

When Dawn gets to Vesta in August of 2011, it will be traveling at about the same speed as Vesta and as it passes by it will suffer gravitational capture and go into orbit around the asteroid. The spacecraft will orbit the asteroid for ten months so that the instruments in the payload can map the asteroid's elemental composition, its gravity field, mineralogy, and thermal emissions, and compile global color and topographic images. Then it is on to Ceres, another three-year journey to take an “up close and personal” look at the first asteroid to be discovered by astronomers on Earth! 

Although you can see quite a bit of “coast time” (dotted lines) on the trajectory diagram, Dawn’s Ion Propulsion System will be thrusting more than 55,000 hours (that’s more than six years).  This is three times the longest powered flight by any spacecraft. 

## Ion Propulsion System

- Hydrazine tank
- 45 kg capacity

- Xenon tank
- 450 kg capacity

- Folded solar arrays

- Ion thruster

- 2.3 meters

**Notes:**
Slide Six: And Dawn will pack only 450 kg of Xenon fuel for this almost ten-year journey!  The spacecraft itself has a mass of 745 kg and the whole propellant system fits into less than 2 cubic meters of space!  Dawn will use hydrazine thrusters (chemical propulsion) for trajectory correction maneuvers. 

In comparison, a trip to just Vesta, without an ion propulsion system, would require a higher-energy rocket than the Delta II and it would require 2500 kg of propellants aboard the spacecraft just to get to the asteroid.  So the Dawn mission, which includes a trip to Ceres, would not be possible without utilizing an ion propulsion system. 

The solar array panels, shown folded in this image, will extend almost 10 m on either side of the propellant and instrument package.  They will produce 10.3 kW of power at 1 AU from the sun and 1.3 kW of power at 3 AU.  Some of this solar power will be used to produce the Xenon ions that, in turn, will provide the thrust for the Dawn spacecraft. 

## Thrust vs. Time

**Notes:**
Slide Seven: 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 time is well worth it because of the great savings in fuel.  Dawn’s ion engines will require only 28 kg of Xenon fuel to reach Vesta and another 89 kg to travel to Ceres.
How can Dawn travel so far on so little fuel?  It is a matter of time vs. acceleration.

At maximum thrust, Dawn’s ion engine will expend only about 0.25 kg of Xenon per day, producing a thrust of 92 milliNewtons, about the same amount of force you feel when a piece of paper is placed on your open hand. Since this small thrust changes the spacecraft’s velocity only 10 to the negative 5 m/s every second, it will require a little over 1000 days to achieve a velocity of 1000 m/s. 

Compare this with the 20 minutes that it takes for 300 kg of chemical propellants to achieve this same 1000 m/s velocity by producing a thrust of 500 Newtons.  
 In this example, the change of velocity is an increase in speed necessary for Dawn to “catch up” to its targets (first Vesta then Ceres).

Dawn is trading off time for fuel efficiency...and since Dawn has plenty of time, ion propulsion is the propulsion system of choice for this mission.