Time in Space: A Review of Typical Milestones and NET/NLT Designations in Space Projects

It’s All About Timing

Timing is paramount, particularly in the sphere of space exploration. Each mission can be likened to a complex puzzle, requiring each piece to slot perfectly into place for the broader picture to emerge. To ensure all parts align just right, there are two significant types of checkpoints used: ‘No Earlier Than’ (NET) and ‘No Later Than’ (NLT). These terms effectively bracket the ‘earliest’ and ‘latest’ dates for task completion. This article reviews these critical markers relative to their importance in maintaining the timely progression of space missions and ensuring the successful attainment of their objectives.

Typical Milestones

Milestones in space projects can vary depending on the nature of the mission, but they often include the following key phases:

Milestone Description
Preliminary Design Review (PDR) This is a comprehensive examination of the design concept to ensure it meets the mission’s requirements. The milestone is usually marked by an official review meeting.
Critical Design Review (CDR) This review comes later in the project, once the design has been finalized. It verifies that the system design is capable of meeting all performance requirements, and that project risks have been appropriately mitigated.
System Integration This milestone signifies the point at which all major components and subsystems are assembled, integrated, and tested to ensure they work together properly.
System Verification/Test This milestone indicates that the system has been thoroughly tested and verified to meet all functional and performance requirements.
Launch Readiness Review (LRR) This review is conducted just prior to launch, ensuring that the spacecraft, the launch vehicle, and all ground systems are ready for the mission.
Launch This milestone marks the moment the spacecraft is launched from Earth, beginning its journey into space.
In-Orbit Checkout Once the spacecraft is in orbit, a series of tests are conducted to ensure all systems are functioning correctly.
Mission Operations This marks the beginning of the mission’s primary operations, such as data collection or exploration.
End of Mission This milestone signifies the successful completion of the mission’s objectives.
Deorbit and Reentry (if applicable) For some missions, the end involves safely deorbiting the spacecraft and bringing it back to Earth.
Post-Mission Analysis After the mission, data is analyzed and lessons are learned to improve future missions.

Each of these milestones could potentially have associated NET or NLT dates, depending on the requirements of the project.


NET in project management or space launch contexts usually stands for “No Earlier Than.” It is used to indicate the earliest possible date that a particular milestone or event, such as the launch of a spacecraft, can occur. This means that while the event could occur later, it should not happen before the NET date. This designation is commonly used when the exact timing of an event is uncertain but can be confined within a specific timeframe.


NLT stands for “No Later Than.” This term is used to specify the latest possible date by which a milestone, task, or project should be completed. It’s a deadline that sets an outer limit for the timing of a task.

In project management or planning, setting a NLT date is critical to maintain the project timeline and ensure that subsequent tasks or phases aren’t delayed. Similarly, in the context of space missions, a NLT date could be used to denote the last possible launch window. This could be influenced by various factors including planetary alignment, weather, and technical readiness.


Here are specific examples of both types of milestones:

NET (No Earlier Than) Milestone

Let’s imagine that SpaceX is planning a mission to Mars. For such a mission, numerous preparatory tasks need to be completed, such as spacecraft assembly, testing, payload integration, etc. Given the complexities of these processes, the actual launch date can be hard to pin down precisely.

A possible milestone could be: “NET Milestone – SpaceX Mars Mission Launch: NET January 10, 2024.” This means that the Mars mission will not launch before January 10, 2024. However, it may launch later if, for example, there are last-minute technical issues, unfavorable weather conditions, or regulatory delays.

NLT (No Later Than) Milestone

Now, let’s consider a software development project. The team is working on a critical update that needs to be rolled out because of a major regulatory change. If they fail to implement the update by the time the new regulation comes into effect, the company could face severe penalties.

A potential milestone could be: “NLT Milestone – Regulatory Software Update: NLT June 1, 2023.” This means that the software update must be completed and implemented no later than June 1, 2023. If it isn’t, the company could be non-compliant with regulations and subject to penalties. This date is the last possible date the project can be completed without negatively affecting the company’s compliance status.

Going Deeper

Simplified Project Lifecycle
NASA Program/Project Life Cycle Process Flow