US officials are under pressure to explain more about a series of objects shot out of the sky
An object downed yesterday over Michigan was the fourth this month; the first was a suspected spy balloon from China
It is still not clear what the three most recent objects to be shot down over North America were
The White House says they posed a threat to commercial flights and were downed out of an abundance of caution
China has a balloon programme connected to its army but it is of limited benefit to its surveillance efforts, the US says
Beijing has accused the US of flying its own balloons over China - the White House denies this
