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JAXA's Akatsuki was supposed to enter orbit around Venus in 2010, but the insertion failed. Its orbital maneuvering engine was supposed to burn for 12 minutes to get it into position, but it stopped after about three minutes. Following the failure, JAXA put it into orbit around the Sun, just inside the orbit of Venus. In December 2015 they want to try to put it into orbit around Venus again.

The confusion I have with this is that when the first attempt failed, part of the combustion system was destroyed and the oxidizer ended up being dumped. How are they going to get Akatsuki into orbit around Venus without its engine?

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They will use the attitude control engines instead of the main engine.

JAXA decided to carry out orbit control of the AKATSUKI using its liquid-fuel thrusters for altitude control (or the reaction control subsystem, RCS.) Based on this decision, almost all of the unnecessary oxidizer was discarded in Oct. 2011. As a result, the satellite became lighter, and remaining fuel can be more efficiently used for orbit control.

Akatsuki's orbit seems to have been a fortunate accident. At least, I could not find a statement that this was planned beforehand.

On its way to Venus, Akatsuki was in a heliocentric orbit that intersects Venus'orbit. And with limited delta-V available for insertion, this orbit must have been reasonably similar to Venus' own orbit.

When orbital insertion failed, it was inevitable that Akatsuki would cross Venus' orbit again. JAXA calculated it would be possible to aim Akatsuki to meet up with Venus again in 2015. To achieve this, Akatsuki performed several orbital maneuvers in 2011 with a delta-V of 243.8 m/s that would lead to it meeting Venus in 2015.

By december 2015, Akatsuki will have made 9 orbits around the Sun, while Venus has made 8 orbits in the same time. JAXA got lucky in the sense that Akatsuki's orbit was close enough to an 8:9 resonance that they could achieve 8:9 exactly with the limited delta-V available to them.

Akatsuki's orbit

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  • $\begingroup$ I think that is a good partial answer to the question. But how come they can miss Venus, which is 108,000,000 km from the Sun, then go around the Sun and come back to Venus after it has made about 8 orbits??? It's not like missing a crossroad in a town and then drive around the block to fix it. Was the trajectory from launch designed for this contingency? Are there orbital harmonies involved? $\endgroup$ – LocalFluff Jul 18 '15 at 6:10
  • $\begingroup$ I can understand that ISEE-3 had a chance to pass by Earth again after 36 years, since its orbit was close to that of Earth at between 0.93 and 1.03 AU from the Sun. But how could it happen by chance for interplanetary probes? Some probe has missed Mars orbit insertion, why doesn't it return? $\endgroup$ – LocalFluff Jul 18 '15 at 6:21
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    $\begingroup$ After the missed insertion, since Akatsuki's orbit was mostly inside that of Venus, it completed orbits faster than Venus, so it was always changing phase relative to Venus, "gaining on it" by a small fraction of an orbit each time around. So at some point, there must come an orbit where it will be just behind Venus, and about to lap it on the next orbit. At that point, a relatively small nudge in the trajectory, changing the orbital period, is enough to sync it up. So it's neither just coincidence nor a pre-planned contingency. $\endgroup$ – Russell Borogove Jul 18 '15 at 15:28
  • $\begingroup$ @LocalFluff: Small adjustments early on mean huge trajectory changes much later. There are a lot of orbital harmonics possible, actually AKATSUKI would meet Venus... eventually, in a couple centuries without any adjustments. Finding one that is close enough both in time and delta-V takes some luck and some knowledge. Then there's matter of orbital insertion, but Obereth effect to the rescue, make a flyby close enough and you can get captured by the body (on a very elongated orbit) at a small fraction of delta-V required to match the speed. $\endgroup$ – SF. Oct 23 '15 at 15:45

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