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For the future planetary exploration of the world ---What "NOZOMI" left with us---

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"NOZOMI" that traveled in interplanetary space with the names of 270,000 people for a little over five years after departure from earth on July 4, 1998 had to give up its injection into orbit around Mars due to unrecoverable malfunction, which was confirmed at 8:30pm on December 9, 2003 (Tue), in spite of hard struggling efforts by the mission team. Following its final decision, ground control center sent a command, from 8:45pm to 9:23pm of the day, to change its trajectory to lower an impact percentage of colliding with Mars. As a result, "NOZOMI" passed by 1000km above the Martian surface on December 14 and escaped from the gravitational sphere of Mars on December 16 to continue on traveling along the orbit around the sun.

For past five years "NOZOMI" encountered with various kinds of difficulties for its first planetary exploration for Japan, but each time overcame troubles with wisdom and hard efforts by the team staffs almost to the brink of fulfilling its mission objective.

For "NOZOMI" as the very first mission of planetary exploration of Japan, it is a duty of the mission team, entrusted from people with development of science and technology, to apply to the maximum extent what the mission team so struggled to have learned from "NOZOMI" for past five years to the future planetary exploration. A future new way will be opened only by humbly and coolly reflecting on the past experiences. In order to contribute to the world planetary exploration as equal to our achievements of astrophysics, only the way to do it is but to learn from the past lessons including those of American and Russian experiences, which failed twenty times out of thirty trials of sending Mars exploration probes.

We must fairly sum up our conclusions by advancing the merits and solving the demerits and never by self-reproach nor self-condemnation.

(1) Achievements in space engineering

The four probes of Japan have ever got out of the gravitational sphere of earth into interplanetary space. The first two were "SAKIGAKE" and "SUISEI" launched in 1985 for the purpose of exploring HalleyÕs comet followed by "NOZOMI" and "HAYABUSA" (asteroid sample return mission) that sailed on space journey in May 2003, among which "NOZOMI" is the only probe to head for a planet for its final destination.

Challenging on exploring unknown world of planet within the limited budget and manpower under tight schedule was such an exciting job, whereas there were a number of technology objectives we needed to establish by proof actually on orbit.

The first to come was a mission analysis. "NOZOMI" experience gave us a strengthened power in producing the best scenario to accomplish the maximum achievements by comprehensively trading off combinations of technologies, even under the severe restricted conditions.

We could establish reliable technological foundation added on expertise from "HITEN" (lunar swingby mission) in 1990 as to the designing of trajectory and operational technology of probe that uses swing-by the bodies like earth, moon and others. Mission analysis team faced a difficult trouble of insufficiency in fuels at the time of earth swing-by in 1998, which finally solved this problem to find a new flight course to have rescheduled the original plan by their heroic dedications in giving up holiday season from the end of a year all through new-yearÕs. How encouraging their dedication and wisdom were to the science society of Japan at that time!

The second issue was how to make orbit determination precisely. We have learned and mastered the reliable technology of determining orbit of deep space probe by applying precise dynamic model through collecting data of velocity and range of the probe through waves transmitted from control center and responses reflected from the probe.

The third was a technology toward autonomous operation of spacecraft. The probe is so distant far. Sometimes it takes over ten minutes time difference even by waves in conversing with probe. Therefore, we need to substantially apply computerized technology of self-dependent intelligence to make judgment by its own. We have learned such technology to a certain extent from "NOZOMI", which was succeeded to "HAYABUSA" as being a highly automated self-dependent intelligence satellite.

We have also learned practical lessons of telemetry technology of communicating each other as far away as 250 million kilometers. Know-how of operating gigantic antenna of 64 meters diameter has been firmly established.

Also a great outcome was seen in saving weight of instruments onboard. As interplanetary exploration probe, as compared with earth orbiting satellites, needs enormous energy in launching, weight saving technology is essentially required as to equipments such as electronics, battery, antenna, solar battery, and all other boarded instruments including propellant mechanical system. There tend to be always many inadequacies in designing, however, "NOZOMI" was perfectly designed as far as weight saving is concerned.

As "NOZOMI" had to travel, unexpectedly, a long space journey under the complicated operational conditions required of its safe cruising, we had to develop self-dependent intelligence by improving many soft-wares to support from ground control center. This experience gifted us with invaluable lessons.

Although, as mentioned above, "NOZOMI" left with us a great deal of contributions to future planetary explorations, there are many things we have to severely reflect upon; above all, the valve in question. The beginning of troubles was a half opening of anti-back-flow valve, installed onboard as learned from US Mars Observer worked negatively, attached to the supply pipe of oxidizer to control engine. Drawing a lesson from others does not necessarily work well. Did a lesson we thought we learned from the States result in giving us an adverse effect? We must carefully examine this problem including the way of our operation at that time.

Next is the problem of short circuit, directly attacked by particles from solar flare. Malfunctions of satellite instruments caused by solar flare ceaselessly occur. Recent solar explosions more or less damaged ten probes of the world at least. Japan is courageously hoped to move ahead for its solution even alone, instead of leaving the matter as it is as a global matter.

On this matter there are two things to reflect upon as follows.

One is that electric system effected to break down power current at once by excessive flow of electricity when a command was sent to short circuit. However, this kind of breaking system was originally designed to protect current circuit, so what was thought as a good designing turned out to have worked negatively. It may be impossible to prepare 100% safe for the least expected things, so what was carefully implemented could possibly happen to turn out to the adverse effect like in a case of valve and breaking system. There has been seen so far quite a number of "adverse effect" in space missions of the world. What we have experienced from breaking system would not be only one lesson we learned, but surely would be one of the lessons to have made us wiser for future probe designing.

Another is that this electric circuit was designed responsible both for frequency modulation of telemetry and controlling fuel-heating system. Of course, top priority was placed on saving of the total weight but only if either of the above functions should have worked, we could have found a way out. We need a thorough examination on this point.

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