The flying saucer is yet an unsolved mystery.

飞碟仍然是未之谜。

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的巨大微波束与地面的网格形天线直保持同步，也是个尚未决的技术难题。

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?

另外还有类「极小」的问题也还没有决：在保证有的条件下，个初盘至少需要几个数字？