The flying saucer is yet an unsolved mystery.

飞碟仍然是未解之谜。

And keeping the satellite's huge beam and the distant rectenna reliably synced up also stands as an unsolved technical issue.

而要使卫星发射出来巨大微波束与地面形天线一直保持同步，也是一个尚未解技术难题。

Another problem of minimality also remains unsolved—to wit, what is the smallest number of digits a puzzle maker can place in a starting grid and still guarantee a unique solution?

另外还有一类「极小」问题也还没有解：在保证有唯一解条件下，一个初盘至少需要几个数字？