The Official Website of Grand Ayatollah Makarem Shirazi

 If a disbeliever converts to Islam, it is not obligatory to observe the qada of missed prayers and fasts due from the past. But if a Muslim turns apostate and converts to Islam again, it is incumbent upon them to perform the qada' of the obligations missed at the time of the disbelief, and based on precaution, they must give the kaffārah of deliberately fast-breaking for the fast they have not observed. In the countries where there is no way of determining the exact direction of qiblah, it suffices to stand in the direction known as qiblah among Muslims. To observe the rule of qiblah in Polar Regions, one is required to stand in a direction that has the shortest distance from Mecca. Astronauts must pray in the direction of where the earth is located in the space. There is no problem in following Sunni Imams in congregational prayers in their regions and in places where you have communication with them for maintaining solidarity on the condition that they have no animosity against Shia Muslims. However, the rule pertaining to the connection between rows of prayer must be observed in any case.