I would start by Identifying the critical tasks the missing team member was handling and redistributing them among the remaining team members based on their expertise. Bring in backup personnel or consultants who can temporarily fill the gap. Reevaluate and adjust the testing schedule to accommodate the changes in team dynamics and ensure no critical steps are overlooked.

First of all, create a team spirit that makes people want to stay in the team, or at least to give ample notice when they are leaving. Secondly, create a learning environment where team members teach each others about their work. I once worked in a team where we had regular discussions about each other's projects and we were capable of assisiting each other if someone temporarily needed extra resources. It was not uncommon in our team to pitch in at someone elses project to support a peak load. We all enjoyed working together. Thirdly, have a mitigation plan in place, in case you unexpectantly loose a team member. This mitigation plan is easier to implement if other team members already are familiar with the project in question.

I will first make a cluster of groups among the remaining maining group members by ensuring they have similar espertises or at least matches with their academic knowledge,(until that position is filled with a expertise) Then i will form a team lead among those small groups who will directly report to the project supervisors. Then the allocated tasks will be divided into more generalistics terms such as Physics, Chem, Maths, IT. Hopefully even though no one has specific knowledge regarding a sophisticated tasks but if we can disect the task into sub groups it would be perfect and easy .