Of a route longer than ten billion kilometres, 3.3 million remained when the module let go [5][7]. That works out at 0.033 percent [8], and it is a fair measure of how little the leftover distance has to do with the leftover difficulty. Mercury is only slightly bigger than the moon, and its grip is feeble beside the sun's, which is what makes an approach easy to get wrong in either direction: incinerated, or gone [4].
Distance also sets the operating rules. Ignacio Tanco, ESA's head of inner solar system mission operations, put the lag between a check made on the ground and a command executed at the spacecraft at roughly 30 minutes, which takes real-time control off the table [10]. ESA's published timeline placed separation at 14:00 CEST and the earliest possible signal at 15:53 [13], a gap of 113 minutes [14]. That is far longer than any signal travel time at this range. Elsa Montagnon, who runs module operations, reported afterwards that the spacecraft had completed its reconfiguration as expected [15]; the reconfiguration is what the interval was budgeted for.
ESA's own wording about the difficulty is not consistent. The agency's briefing called this the most operationally challenging planetary arrival sequence it has ever attempted [3], while the page hosting its livestream says one of the most complex [2]. Neither formulation is a probability. What can be said flatly is that the step does not reverse: the four struts were cut rather than unlatched, and the module that had done the propelling is now flying separately [11][30]. Tanco's framing beforehand was that this amounted to launching a new spacecraft, one that happens already to be at another planet [16].
Two manoeuvres stand between the current configuration and any measurements: capture 79 days after separation, then the orbiters' own parting 18 days later, 97 days from Thursday [19]. What Thursday's telemetry does not tell you is how the hardware behaves once it is working in close. Tanco described the design problem as the equivalent of a very hot pizza oven running on your back [26], on a planet whose surface can pass 427 degrees Celsius while ice sits in permanently shadowed polar craters [25].
The reason for two spacecraft shows up in the altitudes. ESA's Mercury Planetary Orbiter will pass 480 kilometres above the surface, mapping topography and interior structure and putting general relativity to a test, while JAXA's Mio swings out to 11,000 kilometres to work the magnetosphere and magnetic field [20][21]. Sixteen instruments and 1.7 billion euros, most of it ESA's, ride on those two events [22][23]. Whether Mercury's hollows are really being eaten away by minerals vaporising out of the rock [27] is a question for an instrument at 480 kilometres, which is the reason to care how the November burn goes.
