The understanding of this verse cannot be taken as literal read by many, that Christ saw Satan fall from heaven, like lightening. During the time that the disciples where travelling about, we are not told where Christ was. We assume that he remained in a village or town waiting for the disciples to return to report to him what had happened while they were travelling from village to village, town to town. Nor are we told how long their time away from Christ was or what Christ was doing during this time period. The Luke account is silent on these matters.
Was Christ in heaven or was he able to see into the heavens and observed Satan's rage, like lighting, falling. We are told in Isaiah 24:21-22 that Satan and the other fallen heavenly hosts will be judged in heaven at the same time as the kings of the earth are judged on the earth and that the judged heavenly hosts and earthly kings will be collectively imprisoned in a pit for many days to await the time of their punishment. The kings of the earth are going to be judged at Armageddon where they will be assembled in our near future, probably around 25 years, or so, time from about now.
In Revelation 12:7-11 we are told that there is a war going on in heaven and when this war of judgement is over, that Satan is thrown down to the earth, and in Revelation 20:1-3 we are also told that an angel comes and quickly takes hold of Satan and binds him for a 1,000 years and locks him in the Bottomless pit for a 1,000 years. When Satan is judged in heaven he no longer has access to heaven. In Luke 19, we are told that Satan knows that he will not be able to influence the people of the earth and so while he is away in the Bottomless pit, he organises and gives his servants the means to oppress the peoples of the earth such that they have no time to enjoy the Kingdom of God that comes when he is judged upon the whole earth.
We have to keep the whole End Times picture in view to have any understanding of what is to happen, and cannot simply look at small segments of the events that will occur and draw conclusions about the whole End Times from just these individual segments that we meditate on.
Shalom