Aviation Week & Space Technology, 07/02/2007, page 56

Craig Covault, Cape Canaveral

Dawn ready to turn science fiction into reality on mission to orbit two infant planets

In the classic television series Star Trek, the Starship Enterprise speeds around the galaxy on blue light propulsion beams, then maneuvers into orbit around many different worlds before zooming off again to do more exploration.

Such science fiction becomes fact here as the $446-million NASA Dawn mission is readied for liftoff on an eight-year, 3-billion-mi. journey to the protoplanets Vesta and Ceres (see cover).

Powered by glowing blue beams from its own revolutionary solar electric ion propulsion system (IPS), Dawn is to fly to, then orbit, these two separate bodies hundreds of millions of miles apart. Only science fiction spacecraft have done such things before; Star Treks Enterprise did it using antimatter propulsion.

Like Star Trek come true, the NASA Dawn spacecraft and its Dutch Space solar arrays spanning nearly 65 ft. (left) will accelerate by about 7 mi. per sec. on blue beams of ion propulsion to orbit two different bodies.Credit: (LEFT SIDE OF SPREAD) NASA/JPL, (RIGHT) STAR TREK BETHSOFT.COM

"That is huge for a planetary mission, it is really incredible velocity capability," says Mike Mook, the Dawn Orbital Sciences project manager.

Before it powers up its IPS, Dawn must first use a United Launch Alliance Delta II Heavy booster to power out of Earth orbit at 6.8 mi. per sec. Liftoff from Pad17B is set for July 7 at 4:09 p.m. EDT at the start of a 27-min. launch window.

Managers will decide July 3 if the flight can hold to the July 7 target. Dawn must launch by July 11 or stand down until September so preparations can proceed on Pad 17A for the Mars Phoenix Delta II launch set for Aug. 3 with a window through August. Shuttle mission STS-118 is also scheduled for liftoff Aug. 9.

The three ion engines that power the Jet Propulsion Laboratory/Orbital Sciences Dawn spacecraft were developed by JPL in connection with Boeing Electron Dynamic Devices Inc. and L-3 Communications. Dutch Space, an EADS Astrium company, developed the 10,000-watt solar arrays that span nearly 65 ft. and feed the engines energy. Dawn has the largest solar array span of any U.S. planetary spacecraft launched.

The spacecraft is a high-risk mission that already has had a brush with disaster when it was first canceled, then reinstated by NASA after a 20% cost overrun and hardware concerns (AW&ST Apr. 3, 2006, p. 15). The initial cancellation was not just peripheral to the NASA budget, it caused a major space science policy flap.

Dawn is the first planetary spacecraft built by Orbital Sciences Corp. and the second recent planetary space technology development from the Washington area, following the Johns Hopkins Applied Physics Laboratorys New Horizons mission to Pluto.

Dawn also owes its survival for a 2007 launch attempt to the Naval Research Laboratory (NRL), which sits directly across from Ronald Reagan Washington National Airport on the Potomac River. When Dawns schedule was delayed by several weeks during the cancellation controversy, it lost its reservation to use the Goddard Space Flight Center altitude chamber for critical ion engine tests. This almost forced another launch slip. "But the NRL in Washington saved us," JPL Dawn Project Manager Keyur C. Patel tells Aviation Week & Space Technology.

Dawns ion engines were fired in NRLs massive altitude chamber normally used for secret military spacecraft testing. The final integration of Dawn and its shipment to Cape Canaveral also took place out of NRL, not Orbital Sciences, Patel says.

The Dawn mission has taken 15 years for approval and development, says Christopher T. Russell, the mission principal investigator from the University of California, Los Angeles.

Like the international crew on Star Trek, Dawn carries advanced hardware from different countries-in this case the U.S., the Netherlands, Germany and Italy.

The mission is to study how such different bodies formed out of the planetary nebula orbiting the Sun at the "dawn" of the Solar System. The flight will also be the first mission to stay within and explore the asteroid belt between Mars and Jupiter.

Since the 1960s, planetary spacecraft have flown by multiple bodies or orbited the Moon and individual planets. But no spacecraft except Dawn has ever had the propulsion capability to fly into orbit around one body, study it, then blaze out of orbit to fly a billion miles to another body and drop into orbit around it for more analysis. The operational use of solar electric ion propulsion now makes that possible (see p. 59).

Five years of IPS thrusting across the eight-year mission will involve:

March 2009, Mars Gravity assist toward Vesta.

September 2011, Vesta orbit arrival after 1.9 billion miles curving trajectory.

April 2010, Vesta orbit departure.

February 2015, Ceres orbit arrival after 1 billion miles curving trajectory from Vesta.

July 2015, end of primary mission in Ceres orbit.

Vesta is somewhat misshapen, with a diameter of 326 mi. It has the physical characteristics of inner planets, whereas Ceres is round, with a nearly 600-mi. diameter, and resembles the icy moons of the outer planets (see p. 60). By comparing these two protoplanets, scientists hope to develop a better understanding of the transition from Mercury, Venus, Earth and Mars, which have somewhat similar solid surfaces, to the gas giants, Jupiter, Saturn, Neptune and Uranus, which have icy moons unlike anything in the inner Solar System. The area between the inner and outer planets is the asteroid belt, where Vesta has a terrestrial-like surface and Ceres is more like the icy moons of the outer Solar System. Dawn can study them both on the same flight to see why they are different-and hopefully find out new information on how the Solar System formed.

The Naval Research Laboratory came to the rescue of Dawns solar electric propulsion firing tests.Credit: NASA/JPL

While Vesta has dry basaltic rock and is the source of many meteorites found on Earth, Ceres has a more differentiated surface including perhaps a 60-mi. deep outer shell of water ice, maybe even flowing water or seas underneath the ice. If Dawn finds suitable water evidence, Ceres could become another candidate for primitive life-form development.

The three primary instruments on the Dawn spacecraft are: twin German framing cameras that will be used to map the surfaces; an Italian Space Agency visible/infrared spectrometer to map mineral distribution; and Los Alamos National Laboratory gamma ray and neutron detectors to help map mineralogy.

At both Vesta and Ceres, Dawn will first be placed in about a 1,550-mi. orbit to survey the surface. This will then be reduced to a 500-mi. mapping altitude, then to under 125 mi. for precise gravity measurements, Russell says.

Both objects are only about 300 million miles from Earth, but with ion propulsion Dawn can trade time and travel distance for the launch of a much larger spacecraft with dual-orbit injection capability.

First, the powerful new Delta II Heavy will thunder aloft on 1.1 million pounds of thrust-equal to the initial Atlas V and Delta IV Evolved Expendable Launch Vehicles. This will be only the fourth flight to use such a launcher.

Each solar electric propulsion (SEP) engine on Dawn will combine xenon with electricity to create a 78,000-mph. flow of ions to propel the spacecraft.Credit: CARLETON BAILIE/AW&ST

The Delta II Heavy uses nine Alliant graphite epoxy solid rocket boosters that are 4 ft. longer and 6 in. wider than the normal Delta II solids. Each provides about 140,000 lb. of thrust, compared with 97,000 lb. for the more traditional individual Delta solid motor. Each also carries 37,500 lb. of propellant-enough for each solid to provide this higher thrust for 13 sec. longer than normal Delta motors.

Overall, this is 18 tons more solid propellant than the standard Delta II usually carries, an increase that will push the liftoff weight of the vehicle to 50 tons more than a standard Delta II.

The vehicle will lift off on six of its nine solids and 200,000 lb. of thrust from its RS-27 oxygen/kerosene engine. Its 1.1 million pounds lb.of thrust will be 318,000 lb. more than a standard Delta II. Three additional solids will be ignited at Mach 3 as the first six burn out. This will place the second and third stages and Dawn in an initial 102-naut.-mi. parking orbit.

After another second stage firing, the Thiokol Star 48 solid upper stage with 15,000 lb. thrust will be fired to accelerate Dawn to Earth escape velocity.

Three ion engines (arrows) will be used separately. The center tank holds 937 lb. of xenon fuel for solar electric propulsion.Credit: NASA/JPL

The JPL Deep Space 1, launched in 1998 and operational through 2000, tested ion propulsion on a technology mission that flew by the comet Borrelly. DS1 will be "dwarfed by Dawns flight plan," says Marc Rayman, Dawn project systems engineer at JPL. "DS1 accomplished 688 days of thrusting. Dawn will thrust for more than 2,000 days."

That will allow enough time to stop once each week to turn to point the high-gain antenna to Earth for a communications session and also to conduct engineering activities where the ion propulsion must be turned off. Attitude control is by hydrazine thrusters.

Dawns orbit of Vesta could be perturbed by the asteroids different gravity fields.Credit: ORBITAL SCIENCES CORP.

"To initiate thrusting, the xenon feed system has to be pressurized. The flow rates are so low that the team built its own sophisticated pressure regulation system (again using the same design as DS1s) in which the main tank feeds two smaller plenum tanks. "One plenum supplies xenon for thrust and the other supplies xenon for cathodes that produce electrons to ionize the xenon or to keep the spacecraft neutral," says Rayman (see p. 59).

Software controls the process and Dawn often will not even be communicating with Earth when it powers up the propulsion system. However, he says, "Engineering telemetry on the currents and voltages required to sustain thrusting and on the number of recycles-events in which the thruster shorts out and restarts itself-will be monitored."

Rayman adds, "We have the capability to tune it by changing the rate at which xenon flows through the thruster and by adjusting electrical parameters. We do not expect to have to do that, although we are prepared to do so."

Dawn is folded at Astrotech for launch on Delta II Heavy. Sensors sprout from behind the 5-ft.-dia. Earth communications dish.Credit: CARLETON BAILIE/AW&ST