６月末に参加者を募ったら１週間弱で約５００席が予約でいっぱいになった。
When participants were sought at the end of June all, about 500, seats were reserved in under a week.

もちろん、実際に「はやぶさ」が太陽に近づいているわけではなく、図のように地球から見て太陽の反対側に位置するだけですが、このような現象を「合」と呼びます。
Of course "Hayabusa" is not actually closing in on the Sun, it is just positioned as in the figure so that, seen from the Earth, it is on the opposite side of the Sun; this is called 'conjunction'.