26 November 2005
by Kate Melville
Since our last update on JAXA's (Japan Aerospace Exploration Agency) plan to collect samples from an asteroid and bring them back to Earth for analysis, the Hayabusa (peregrine falcon) spacecraft has been plagued by a series of setbacks. On Sunday, November 20, ground control lost contact with Hayabusa, and the craft failed in its first attempt to gather a sample from the 1,797-foot long asteroid, dubbed Itokawa. After some analysis, however, the JAXA team claims that Hayabusa did make a landing on Itokawa. Furthermore, during an alleged second landing, a sample may have been collected as planned. Interestingly, it seems that the much lauded autonomous capabilities of the Hayabusa craft are the source of much of the mission's woes.
The first bit of good news that Hayabusa's Project Manager, Junichiro Kawaguchi, and fellow Joint Science Team members celebrated was the deployment of a target marker onto Itokawa's surface, at 5.46am, Sunday, November 20 (JST). The target marker is comprised of an aluminum plate bearing the names of 880,000 people from 149 countries, including Steven Spielberg and Arthur C. Clarke, and was set down on a flat region of the asteroid the team called Muses Sea.
While the team celebrated the placement of the marker, it was first assumed that Hayabusa failed to land on, let alone collect a sample from Itokawa.
Instead, scientists and engineers in the Hayabusa mission control room lost contact with Hayabusa between the hours of 6 and 9am Sunday, November 20 (JST). During the blackout, however, Hayabusa continued to store data of its progress and the team was later able to assess what had happened.
Upon first analysis of the data, the team found that Hayabusa deployed the marker while approximately 130 feet above the asteroid after maintaining a slow fall, at 4.72inches/sec. Hayabusa then autonomously reduced its speed by 0.3inches/sec and continued to freefall toward the asteroid at 0.098inches/sec, while switching its range measurement mode from Laser Altimeter (LIDAR) to Laser Range Finder (LRF). At 82 feet, and for reasons unknown, Hayabusa went into what is known as "safe-hold mode", which involved autonomously reducing its speed to zero and hovering above the surface of the asteroid. It was then believed that at 5.40am, Hayabusa had a further freefall to within 55 feet of the Itokawa's surface. The spacecraft then autonomously switched to attitude control mode in order to analyse the surface contours of the asteroid. At this point, the spacecraft autonomously stopped telemetry transmission to Earth (as scheduled), and switched to beacon transmission mode. By using a low gain antenna (LGA) that covers a wider area, the beacon transmission mode has a greater efficiency at Doppler measurement.
During this time, real time analysis of onboard instruments was not possible (as scheduled). Further analysis of the recorded data, however, revealed that Hayabusa seemed to have autonomously aborted descent and attempted an emergency ascent because its obstacle checking Fan Beam sensors "detected some kind of catch-light," the Hayabusa team said. Although it was shown that Hayabusa's attitude was slightly off, a margin for error was factored in to accommodate just such a circumstance. Consequently, the spacecraft chose to continue its safe descent, but Hayabusa did not activate its Touch Down Sensor function. Out of all this confusion, it was assumed that Hayabusa did not land on Itokawa at all.
However, upon further analysis of data during the week, the team discovered that Hayabusa had actually made a soft landing on Itokawa, and remained there for 30 minutes. The team say this is verifiable by the data history of LRF and also by the attitude control record. This is a milestone for the Japanese space program, as it marks the first time a Japanese craft has landed on a celestial body. The craft seems to have suffered no serious damage during the landing, but it's claimed that some adjustment to its heat sensors is required as ground control were receiving some anomalously high temperature readings.
Perhaps because of the anomalous high temperature readings, ground control forced Hayabusa into an emergency takeoff at 6:58am (JST). However, because the team were unaware that Hayabusa had actually landed, projector firing was not implemented. Subsequently, celebrations of the landing and the world's first spacecraft to take-off from an asteroid were belated. "Really speaking, it is the world-first departure from a celestial body except the moon," said the team. Despite the jubilation of the team, the primary objective of acquiring a sample from the asteroid was not met, and Kawaguchi and his team immediately began planning for a second crack at Itokawa. The team rescheduled another touchdown and collection attempt at 7.00am, on the morning of the 26th (JST).
It seems that the second attempt may have paid off. Apparently, Hayabusa successfully landed on the asteroid Itokawa for a second time and has succeeded in collecting soil samples, the JAXA team said. The Hayabusa team has verified that the craft shot a small metal ball, measuring about 0.4 inches in diameter, into the surface of the asteroid to loosen surface material for collection. This operation was successfully synchronized with the crafts collection horn coming into contact with the surface of the asteroid. If true, more celebrations for the team are in order, as this too is a world first. However, these celebrations are going to be especially belated, as the team cannot confirm the collection until Hayabusa lands back on Earth, in one piece, in 2007. Currently, the craft is 180 million miles from Earth.
The current celestial high jinx come only a couple of weeks after a botched attempt at releasing Hayabusa's secondary scout craft, Minerva (MIcro/Nano Experimental Robot Vehicle for Asteroid). Minerva was designed to take advantage of Itokawa's low gravity environment by hopping or bouncing across the asteroid and taking pictures as it went. Unfortunately, during Minerva's deploy command, Hayabusa's necessary, but seemingly troublesome, autonomous controls kicked in at a crucial point during the procedure; in this instance the altitude controls. Consequently, Minerva was released from too high an altitude and escaped Itokawa's gravitational pull, and is now drifting through our solar system at the stately pace of 3.26 inches a second.