A $100 million Japanese space explorer parked in the vicinity of an enigmatic asteroid this week, allowing scientists to get a first glimpse of the mid-sized rock that will become the source of the first samples of such an object to ever be returned to Earth.

An artist's concept shows the Hayabusa nearing its asteroid target. Credit: JAXA

After methodically tweaking its course - first by electrical ion propulsion, then by conventional chemical thrusters - toward its target over the past few months, the Hayabusa probe finally arrived at its station keeping position some 12 miles from the asteroid early Monday.

Officials timed the arrival by when the 1,000-pound craft's closure rate relative to the object reached zero, indicating the probe was now essentially anchored in the "gate position" located about 12 miles from the space rock. That moment occurred as the spacecraft commanded its maneuvering jets to fire one last time at about 0117 GMT Monday, or in the late-morning hours in Japan.

Hayabusa's ion drive propulsion system took a the leading role for the rendezvous up until August 28, when control switched to the liquid-fueled chemical thrusters. That milestone left the four ion engines with a cumulative burn time of almost 26,000 hours, and the system will be re-started once the probe embarks on the last leg of its journey back to Earth.

The delicate space ballet took place almost 200 million miles from controllers on Earth, who left responsibility for the rendezvous to an on-board navigation system that is designed to operate without ground intervention.

The goal of the mission is to study asteroid 1998 SF36 - later named Itokawa in honor of an early Japanese pioneer in rocketry. The object was discovered in September 1998 by a joint team consisting of scientists from the U.S. Air Force, NASA, and the Massachusetts Institute of Technology.

Details about Itokawa have largely remained elusive in the seven years since its discovery, but scientists now have a much clearer picture of the potato-shaped asteroid estimated to measure 2,000 feet by around 900 feet. Its orbit stretches from inside Earth's out to a distance of 157 million miles from the Sun, making it a member of the Apollo class of near-Earth asteroids that pose potential impact threats to our planet.

Astronomers studied Itokawa via several ground-based telescopes during its last Earth fly-by in 2001, and they found evidence that the asteroid was brighter and more reflective than first expected. Scientists had a difficult time predicting what Hayabusa would find as it closed in on the object, and many questions were left unanswered.

However, many of those issues can now be thoroughly remedied with the new higher resolution images Hayabusa's optical asteroid multi-band imaging camera has captured. The pictures show a contrast of rocky and hilly terrain with smooth regions, but detailed analysis and sample retrieval will be conducted before scientists can announce the results of their detailed study.

A first look at the new images appears to show a loose layer of dust and dirt-like material covering the smooth surfaces of parts of the asteroid, which is a surprise to some project officials.

"According to a certain theory, small objects do not have regolith," said Hayabusa project manager Jun Kawaguchi. "But this asteroid seems to have smooth portions that appear (to have) some regolith."

Images from Hayabusa are revealing what the asteroid Itokawa looks like. Credit: JAXA

Work with Hayabusa's science payload has already begun, with the near-infrared and X-ray spectrometers now gathering measurements. The craft's laser altimeter is also conducting observations to learn more details about Itokawa's terrain.

The pair of spectrometers will labor to determine the elemental and mineral composition of Itokawa to help astronomers in their quest to link asteroids and comets to meteorites that have fallen to Earth and been recovered.

"The analysis has just started and no conclusive results (are) given until the measurements are detailed and analyzed," Kawaguchi told Spaceflight Now.

"When we talk about the shape just from impression and not from a scientific point of view, the shape apparently looks like an object (with many) smaller objects united. And also apparently, a small (object) and some other objects are accreted with a larger one. These are not interpreted at all and definitely have to wait for detailed study."

Hayabusa will have to occasionally refine its position relative to the asteroid because the object's gravity is too weak to keep the craft in a stable stationary position.

Controllers plan to order Hayabusa to close further toward Itokawa at the end of September, when more precise science activities are scheduled. Once in the "home position" under five miles from the asteroid, remote sensing observations will continue as the object completes one rotation about every 12 hours.

A goal of the remote sensing phase will also serve to choose up to three sites that are scientifically interesting and safe enough to bring the fragile spaceship to the surface for sample retrieval passes beginning in November. During each approach, a 16-inch funnel aboard Hayabusa will make contact with the asteroid, followed by the firing of a small metal pellet into the rocky crust at hundreds of feet per second.

Rubble and dust ejected from the high-speed impact will make their way through the funnel and into a chamber designed to carry the samples on the return trip to Earth and through the fiery re-entry into the atmosphere. Officials expect up to one gram - or two one-thousandths of a pound - of material will be gathered.

It is expected that the ground team will eventually choose two sites for sample runs, leaving a third as a rehearsal to test the operation and feasibility of the plan.

Also during the first pass, Hayabusa will deploy a tiny 1.3-pound rover known as MINERVA, which will leap, hop, and jump across the surface in the extreme low-gravity environment. MINERVA carries three stereo cameras for pictures, and a suite of six thermometers to measure temperatures.

Hayabusa - formerly known as MUSES-C - was launched on May 9, 2003, aboard an M-5 rocket from Japan's Kagoshima launch site. The spacecraft flew past Earth a year later to receive a gravity boost that aimed Hayabusa toward this year's encounter with Itokawa.

Time spent in the vicinity of asteroid Itokawa was reduced by a 2003 solar flare that slightly damaged the ability of Hayabusa's two solar panels to produce electricity, which is vital to the operation of the craft's primary ion propulsion system. The degradation caused the four ion drive engines to produce less total thrust with less operating time, delaying arrival from mid-summer until now.

Plans call for Hayabusa to depart Itokawa in early December to begin its trek back to Earth, culminating with a homecoming in June of 2007 by way of parachuted landing at Woomera in the Australian outback.