A new video from NASA Jet Propulsion Laboratory (spotted by Skarredghost) shows what could be the next HoloLens. As you can see from the image above, the device shares a design language with the current model, but it's different.
For one thing, there's no headband across the top. Presumably, this means that it's lighter and less likely to fall off of your head, although it does have a knob on the back of it to tighten it. The front is clearly different too, as it seemingly has a much more comfortable fit.
You can check out the video here, where at the end it shows the original HoloLens being used:
It's more likely that this is just a prototype device. All signs point to the next HoloLens being released late next year, so the design probably isn't finalized just yet.
Microsoft first showed off HoloLens at an event in January 2015, which is almost four years ago now. The developer kit started shipping in March 2016, and there hasn't been a new model since. The firm has, however, been developing the next one fairly openly.
For example, at this year's Build developer conference, Microsoft announced Project Kinect for Azure, its next-generation sensor that it even said will be in the next HoloLens. Since the company has been clear that it's working on a new iteration, it's entirely feasible that NASA is testing out the new model.
Update: Alex Kipman tweeted that this is indeed a prototype of the first HoloLens.