Nasa's OnEarth server will be experiencing some downtime in the next few days. They are upgrading to a new server, so the good news is service will be improving. But the process of moving equipment, getting new addresses, et cetera takes time and downtime is hard to avoid.
We'll post any updates as we here them, so stay tuned. As of right now it is still up and serving.
Note: this only affects TopoFusion's Landsat maps and Elevation (3D) downloads. All other servers are behaving as normal.
Thanks to Lucian Plesea for his continued help and communication. He has recently added an elevation caching scheme that will lead to some cool new features in TopoFusion. Stay tuned on that too.