The Earth swing-by for Hayabusa2 occurred on December 3, 2015. The spacecraft’s path was then carefully observed and analyzed until December 11 to confirm that the planned orbit had been precisely followed. From now on, Hayabusa2 will travel to Asteroid Ryugu propelled by its ion engines, where it is due to arrive between June and July in 2018.
Using the tracking data collected during the swing-by (range and Doppler data *), we estimated that the closest approach to the Earth occurred on December 3, 2015 at 19:08:07 JST (UT 10:08:07). At this point, Hayabusa2 was at an altitude of 3090 km. Both the time and altitude matched their predicted values.
On December 8, 2015 at 16:00 JST, Hayabusa2 was travelling at 32.069057 km/s around the sun. Prior to the swing-by, the predicted value was 32.069489 km/s, giving an off-set between the planned and actual velocities of a tiny 0.000432km/s (43 cm/s). This difference is too small to create any issues for the spacecraft’s trajectory in the future and Hayabusa2 can continue as planned. Therefore, we are delighted to confirm that the Earth swing-by was a success!
In the month before the Earth swing-by, Hayabusa2 made two orbit correction maneuvers (TCM). The procedure for the TCM is shown in Figure 1, where we consider a plane passing through the centre of the Earth. This plane forms the target region during Hayabusa2's swing-by. (More technically, this is called the ‘B-plane’, but we omit a detailed explanation here.)
For a successful swing-by, the spacecraft must pass through a specific point on the target plane. On November 3, TCM1 reduced the distance to the desired point from 400 km to 11 km. Then on November 26, TCM2 reduced the distance again to 3 km. A third manoeuvre, TCM3, was planned but the 3 km distance was acceptable for the swing-by, so this last adjustment was not performed.
[*] The range data is the distance between the spacecraft and the ground station on Earth. It is measured using the time a radiowave takes to make the round trip between these two locations. The Doppler data is the change in the radiowave's frequency upon its return. This variation gives the speed of Hayabusa2 relative to the Earth along the line-of-sight. The Doppler data is also called the range rate data.