  We study a particle system naturally associated to the $2$-dimensional
Keller-Segel equation. It consists of $N$ Brownian particles in the plane,
interacting through a binary attraction in $\theta/(Nr)$, where $r$ stands for
the distance between two particles. When the intensity $\theta$ of this
attraction is greater than $2$, this particle system explodes in finite time.
We assume that $N>3\theta$ and study in details what happens near explosion.
There are two slightly different scenarios, depending on the values of $N$ and
$\theta$, here is one: at explosion, a cluster consisting of precisely $k_0$
particles emerges, for some deterministic $k_0\geq 7$ depending on $N$ and
$\theta$. Just before explosion, there are infinitely many $(k_0-1)$-ary
collisions. There are also infinitely many $(k_0-2)$-ary collisions before each
$(k_0-1)$-ary collision. And there are infinitely many binary collisions before
each $(k_0-2)$-ary collision. Finally, collisions of subsets of $3,\dots,k_0-3$
particles never occur. The other scenario is similar except that there are no
$(k_0-2)$-ary collisions.

Probability