The Hayabusa2 probe swung by Earth on Thursday as part of a 5.2 billion-kilometer-journey to the Ryugu asteroid.
The Japanese spacecraft used the Earth's gravity to slingshot out of its original orbit around the sun and toward its target. This required exact calculations and precision control from the ground. NEC, Japan's leading computer maker, has developed technology to perform those tasks, enabling the probe's long-distance mission.
Hayabusa2 was in an orbit similar to that of the Earth's around the sun since its launch on Dec. 3, 2014. On Thursday, the probe made its closest pass toward Earth exactly a year after its launch and is believed to have increased its velocity relative to the sun from 30.3km per second to 31.9km per second, with the help of the Earth's gravity.
A little fling
This swing-by will push it into an orbit that will take it to the Ryugu asteroid. A swing-by is a navigational method that uses the gravitational pull of a planet or moon to accelerate and change the course of a space probe. The method saves fuel as it does not require the probe to fire its engine.
"It would take three ion engines operating at full blast for a year to gain the same speed" as the slingshot method, said Yuichi Tsuda, project manager at the Japan Aerospace Exploration Agency.
A successful swing-by requires computation of a probe's speed and altitude, the Earth's gravity, and other variables. In addition, it is necessary to change the probe's course at precisely the right time.
This is where NEC's technology comes in. The computer company has come up with a manual of commands and ways to repair the probe in case something goes wrong. The manual amounts to a plan to navigate Hayabusa2 all the way to Ryugu.
To measure the craft's location and velocity more accurately, JAXA and NEC have used a method called "Delta Differential One-way Range." Hayabusa2 was Japan's first probe to use this method.
Previously, JAXA used antennas at the Usuda Deep Space Center in Nagano Prefecture and the Uchinoura Space Center in Kagoshima Prefecture to measure the positions of its space probes. For Hayabusa2, it uses the new method, which allows simultaneous reception of radio waves from the probe at ground stations in Japan, the U.S. and Europe.
By measuring and analyzing trillionth-of-a-second time differences between the radio waves, the method can pinpoint, within a mere 180 meters, the position of the probe as it races through space. In the past, JAXA's positioning was accurate only to within about 2km.
Attitude adjustment
If Hayabusa2 fails to achieve the planned orbit, JAXA will need to use commands -- adjusting amount and direction of engine thrust to alter the probe's attitude, for example, or tweaking the antennas.
"We think of dozens of scenarios in which the probe fails to function properly and make commands accordingly," said NEC's Takeshi Oshima.
The company devises two weeks' worth of commands once a week and provides these to JAXA. Hayabusa2's command system is much more advanced than the one used in the first-generation Hayabusa probe. Previously, commands were created for unexpected situations as they occurred.
Based on its earlier experience, NEC has compiled likely commands into a database for Hayabusa2.
Since its launch a year ago, Hayabusa2's ion engines have been firing continuously. The probe is scheduled to reach Ryugu sometime between June and July of 2018.
When it arrives, the probe will use its impactor to create an artificial crater and collect material from beneath the surface. To complete this part of the mission, JAXA will need to perform a number of delicate tasks -- controlling the thrust of the engines, changing the probe's attitude and switching antennas.
The mission plan calls for Hayabusa2 to stay on Ryugu for a year and a half to collect materials, then bring them back to Earth. Because the probe may have to burn a lot of fuel on the asteroid, it will try to save as much as possible during its three-year flight.
The probe also uses a so-called solar sail that uses the minute pressure from the light of the sun to control the probe's attitude. NEC developed the technology in response to problems encountered in controlling the first Hayabusa's attitude.
Because space exploration is a national project for Japan, the technology developed for the probe is not a major source of revenue for NEC. But the company seeks to gain know-how in space probe operations, which it will use in its private satellite business, such as GPS, communications and broadcasting satellites.
