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Planetary News: Asteroids and Comets (2004)

Japan's Hayabusa [MUSES-C] Swings By Earth on Way to Asteroid Itokawa

By A.J.S. Rayl
20 May 2004

On its 1 billion kilometer mission to land on the asteroid Itokawa and return home with a sample of its surface, Japan Aerospace Exploration Agency's Hayabusa swung by the Earth yesterday, at an altitude of about 2300 miles [3700 kilometers] from the surface of the planet, completing the maneuver at 3:22 p.m., Japan Standard Time (JST). Currently, the spacecraft "is functioning almost perfectly," reports Jun'ichiro Kawaguchi, Hayabusa Project Manager, ISAS/JAXA.

Along the way, the little asteroid explorer acquired a number of images of Earth and the Moon, and sent home a few postcards suitable for framing.

Formerly known as MUSES-C, Hayabusa is scheduled to arrive at Itokawa in June 2005. After its initial rendezvous, the spacecraft will spend five months near the asteroid named after the late rocketeer Hideo Itokawa, father of Japan's space program. The spacecraft will not go into orbit around the asteroid, but will remain in a station keeping, heliocentric orbit close by.

During that time, Hayabusa will conduct detailed observations of Itokawa with various instruments onboard and capture more precise images of the surface using its high-performance camera. The mission team will then select target areas and dispatch a robot to touch down on the near Earth asteroid's surface to collect one gram's worth of dust samples from three sites on the big rock, which it will return in June 2007.

Hayabusa is powered by a system of four ion engines designed to be activated throughout the flight to and from the asteroid. During the swing-by yesterday, the spacecraft's accumulated velocity vector by its ion-engine acceleration was diverted in the direction of Itokawa and the spacecraft will now be accelerated continuously as it heads on a straight shot course to the asteroid.

Yesterday's event marks the first time any spacecraft has performed a swing-by maneuver using an ion engine as the main thruster. "The combination of ion engines propulsion with Earth gravity assist is the world's first attempt and we think doing this is thrilling and exciting," enthused Kawaguchi.

"This is an audacious and exciting mission, to bring back a sample of near-Earth object is significant scientifically, and as an engineering achievement," noted Planetary Society Executive Director Louis Friedman. "Such objects have profoundly influenced the evolution of Earth."

Hayabusa's journey, from beginning to end, will take about four and a half years, and is estimated to cost the equivalent of $170 million.

In addition to its somewhat daunting scientific objective, the mission also has some prime space engineering and technology directives -- to test a few new technologies developed in Japan for this and future sample return missions:

  • The high performance electric propulsion system - comprised of four microwave-discharge-type ion engines, known in America as ion drives;
  • An autonomous navigation system;
  • An asteroid-landing robot that can touch down, seize samples, and hop off; and
  • A spacecraft/re-entry capsule system for returning the samples to Earth.

"Use of these technologies is very important -- both for solar system exploration generally, and for Japan as a major player in space," Friedman said. "No one country can do everything, and this is the great advantage of international cooperation -- to share the work and the rewards of exploration."

Like most space missions, Hayabusa has had its share of tense moments. It was originally slated to investigate asteroid 1989 ML, but the project's original launch date in July 2002 on an M-5 three-stage solid-fuel rocket was delayed when a similar M-5 rocket failed to deliver Japan's Astro Ex-ray observatory into orbit. That delay meant that 1989 ML would be beyond the mission's grasp by the time the spacecraft could launch again. So, a new target -- asteroid 1998 SF36 -- was selected, and shortly thereafter the International Astronomical Union (IAU) bestowed it with its permanent name -- Itokawa.

The asteroid explorer was finally ferried aloft by Japan's M-V-5 rocket from the Uchinoura Launch Center at Kagoshima on Kyushu Island last year, on May 9, in what controllers described as a picture perfect launch, into "blue skies." When the spacecraft's radio signal was received at Kagoshima station after the Earth had rotated beneath it, and telemetry data at the Goldstone Tracking Station showed that the rocket had pretty accurately put it into the planned orbit, Japan christened MUSES-C with its new name, Hayabusa, and sent it on its way.

By July last year, the Hayabusa team had completed engine tests and adjustments were completed, and the spacecraft transitioned into its cruising mode of continuous acceleration. It has, for the most part, been cruising steadily through space since then with its four ion-drive electric propulsion engines propelling it at a slow and steady 12 meters per second.

Compared to the conventional chemical propulsion engines, electric propulsion engines are extremely efficient. On the other hand, their thrust levels are quite low; therefore, they must operate continuously for a longer period of time to perform the same level of orbital maneuvers as that of the chemical propulsion engines. Hence, ion drives propel spacecraft in a slow and steady manner -- the tortoise-to-the-hare fable is often noted as analogy.

Many space exploration scientists and engineers view the high efficiency of ion drives to be an essential technology for deep space cruising on future planetary missions.

Hayabusa's engines ionize the propellant, Xenon, by microwave, then accelerate the ions in a strong electric field and expel them at high speed. That provides the reactionary energy that propels the spacecraft. Although NASA's Deep Space 1 holds the distinction of being the first spacecraft to use ion drive, Hayabusa is the first Japanese craft to employ the technology, and the first to use microwaves to ionize xenon fuel.

Developed by the electric propulsion engineering section at the ISAS division of JAXA, Hyabusa's engines promise long life and high reliability. Since they do not use the discharge electrodes for plasma generation, deterioration is not an issue. The operation record of 8,000 hours per unit in space achieved last year is a world-class accomplishment.

For Hayabusa it was smooth sailing until last November when the spacecraft encountered the biggest solar flare in history. The little 1100-pound [500 kilogram] spacecraft, however, came through unscathed, and once it swings by Earth tomorrow, it will be on a trajectory that will take it straight to Itokawa.

By the time Hayabusa reaches the vicinity of the asteroid next summer, it will take radio waves 10 minutes to traverse the distance from the spacecraft back to Earth, so the designers have equipped the spacecraft with a highly autonomous navigation system that will enable it to decide each move on its own. It will do that by measuring the distance to and the shapes of the asteroid surface using an Optical Navigation Camera and Light Detection and Ranging instrument.

Although just getting to Itokawa will be a significant achievement, Hayabusa's journey really just begins with its arrival. Landing on top of the asteroid is going to be tricky. Current estimates measure Itokawa's size at 490 by 180 meters -- that's just 1600 feet by 590 feet. And that's why Hayabusa will spend five months intensely studying this rock with a host of instruments including, infrared and X-ray spectrometers, in addition to its cameras and light detection and ranging instrument.

After the mission team makes a final decision on where to gather its samples, Hayabusa will begin its descent to Itokawa, measuring the relative position and attitude to the surface with its Optical Navigation Camera and Light Detection and Ranging instrument, autonomously adjusting the propulsion engine as it transmits an image of the whole process to Earth. At about 100 meters altitude, the spacecraft will release Target Markers onto the asteroid.

Although Hayabusa's Mission Control cannot maneuver the spacecraft in real time, it can send an order to stop the descent when assessing a danger, but that is about all it can do. Most of the essential decisions have to be made by the spacecraft itself.

Prior to the final touch down, the spacecraft shuts down the propulsion engine and enters into a free fall descent. This is to prevent the jets from contaminating the asteroid surface by coming too close to it.

The next task -- collecting a sample -- may be an even greater challenge. The surface gravity of the asteroid is really small and is less than 1/100,000 of that of the Earth. Anything -- a drill or any other digging tool - that is not secured by a strong anchor into the surface may just be pushed away before the drill actually bores a hole.

Since we do not know what the asteroid is really made of, there is no way of knowing how hard the surface will be or how difficult this collection of surface dust will be. The Japanese are employing a simple, direct strategy -- to "break the surface" by using a tiny pyrotechnic device to fire 'sampler horns' into the three targeted sites on the asteroid.

Once the collections of surface dust are completed, the spacecraft will start the engine, lift off, and resume its hovering position at 100 meters altitude as it waits for the next order from Mission Control.

In the final phase of the mission, Hayabusa will send a small reentry capsule with the asteroid sample into the Earth's atmosphere at a velocity more than 7,500 miles per second [12 kilometers per second]. The maximum heating rate of the capsule in the reentry is several tens times higher than that of the Space Shuttle and several times higher than the Apollo recovery module. The heat shield material, therefore, is state of the art. After being fully decelerated by the atmosphere, the capsule is designed to deploy a parachute for soft-landing. On landing, the capsule will send out a beacon signal until it is retrieved in 2007.

For now, however, Hayabusa has passed a critical juncture in the mission and, as the spacecraft cruises on toward Itokawa, its three purple lights shining royally in the blackness of space, there are, no doubt, many proud and smiling faces in Japan.