The term ‘T-minus’ is generally used during countdowns to space launches. During a NASA countdown to a rocket launch, ‘T-minus’ translates to ‘Time minus’; the ‘T’ stands for the exact time at which the rocket is scheduled to be launched.

I watch and listen intently to everything the ground staff and the astronauts (if it’s a spacecraft) do and say during the entire lead-up to the rocket’s liftoff.

However, when I first started watching rocket launch videos, I was often confused when I heard the news guy counting down to the liftoff as “T-minus 10, 9, 8….”.

I understood that it was the countdown to the launch of the rocket, but it took me quite a while to understand why they called it ‘T-minus’ or even ‘L-minus’ in some cases. Perhaps you readers already know the difference between ‘T-minus’ and ‘L-minus’, and have accepted the fact that these are both the same thing—countdowns to a rocket launch.

Indeed, T-minus, L-minus and certain other ‘minuses’ are all countdowns, but the difference between the first two is rather interesting, something that you clever readers might *not* know.

## What does T minus mean?

Whether they are sending humans to space or just an unmanned satellite, NASA commonly employs the term ‘T-minus’ during the preparation and lead-up to a space launch. It’s essentially a countdown, i.e., a sequence of reverse counting that signifies the ‘time remaining’ before the *exact moment* when a scheduled event will occur.

Sometimes, instead of ‘T-minus’, you may hear the term ‘L-minus’; that is also a countdown to a rocket launch, but there is a difference between the two, which will be explained below.

Imagine that NASA is going to launch a rocket in 10 hours. Thus, the ‘T-minus’ countdown will read ‘T-minus 10 hours’. As the time of the launch gets closer, the countdown will read ‘T-minus 9 hours’, ‘T-minus 5 hours’, ‘T-minus 55 minutes’ and so on until it reaches the most iconic part of the launch countdown – the final 10 seconds prior to the launch. At that time, the announcer says, ‘T-minus 10, 9, 8….3, 2, 1 and liftoff!’

There’s also ‘E-minus’, a term commonly used in mission planning. It also consists of a countdown to a certain event *during* the mission.

## What does the ‘T’ in ‘T-minus’ stand for?

In a NASA countdown to a rocket launch, ‘T-minus’ translates to ‘Time minus’; the ‘T’ stands for the exact time at which the rocket is scheduled to be launched. In more technical terms, the ‘T’ in ‘T-minus’ is a designator for the main sequence countdown time that serves as the synchronization device for most of the equipment and procedures that must be completed before, during and after the launch.

However, the ‘T’ in the ‘T-minus’ can also stand for ‘test’. The ‘T’ might not *always* be related to time, depending on the setting/situation in which it’s being used.

Apart from ‘T-minus’, another term that’s used a lot is ‘L-minus’, which is also a countdown to the launch of a rocket. However, as mentioned above, there’s a small difference between ‘T-minus’ and ‘L-minus’.

The ‘E’ in ‘E-minus’ stands for ‘encounter’ or ‘event’. This term is used during space missions, i.e., once a satellite is already in space. For instance, if a satellite is going to collide with a comet in 5 hours, NASA ground staff would phrase the countdown to the encounter as ‘E-minus 5 hours’.

## What’s the difference between ‘T-minus’ and ‘L-minus’?

‘T-minus’ refers to the time remaining (until the launch) on the official countdown clock. There are pre-planned holds in the countdown process, during which time the T-time is also stopped. Thus, the T-time can be stopped according to pre-planned holds in the launch procedure.

‘L-minus’ refers to the *actual* days, hours and minutes remaining in a scheduled launch, wherein the launch occurs exactly at L-0. This is the natural countdown, and as such, it is never stopped (unlike the T time, which can be stopped).

Under normal conditions, however, both of these countdowns are in perfect sync. The launch occurs when both the countdowns read 0.

So, the difference between the *real* time and the launch time is the only difference between the ‘T-minus’ and ‘L-minus’ countdowns.

### References

**About the Author**:

Ashish is a Science graduate (Bachelor of Science) from Punjabi University (India). He spends a lot of time watching movies, and an awful lot more time discussing them. He likes Harry Potter and the Avengers, and obsesses over how thoroughly Science dictates every aspect of life… in this universe, at least.

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