New Delhi, Jan. 20 -- In back-to-back failures, two of Isro's PSLV missions have malfunctioned during the third of four stages. In May last year, PSLV-61 experienced a drop in pressure in the propulsion chamber during the third stage, leading to a deviation from the intended course. And last week, PSLV-62 too deviated off course towards the end of the same stage. The third stage can be challenging because it uses a solid propellant, which does not allow the precise control that liquid propellants provide in the second and fourth stages. And although the first stage too uses a solid propellant, the third is fired high in space, in conditions very different from the near-Earth environment where the first one ignites. The failures should be seen less as a trend than as an aberration, because PSLV's track record has been outstanding for decades. Isro's workhorse had succeeded in most of its previous missions, demonstrating that all four stages can operate reliably. It is not clear if a new factor has introduced uncertainty into the third stage.P4...