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Remember that Y2K thing a few years ago? Where 
everyone was afraid the world was going to end  

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because computer programmers saved space by 
putting dates as… 77 for 1977, 85 for 1985,  

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Or 90 for 1990? But then it became clear 
that when the year 2000 finally rolled  

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around all of the computers would think 
it was actually 00. Or the year 1900. 

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Well, it turns out Hubble has something similar, 
only Hubble’s clock restarts every 6,213 days,  

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18 hours, 48 minutes, and 31.875 
seconds. Or roughly every 17  

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years for those of you who like counting.
That’s because Hubble’s computers have a  

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different way of tracking time than we have here 
on the ground. You’d think it would be as simple  

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as synching our ground clocks with Hubble’s 
personal timepiece, but you’d be surprised. 

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Hubble’s onboard clock is a 32 bit elapsed time 
counter. So instead of Hubble counting up seconds,  

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days, months, and years like a calendar as we know 
it, Hubble counts every 125 milliseconds, or 1/8th  

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of a second for those of you who like counting.
Using 32 bits, up to 4,294,967,295 increments  

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can be stored. Once that final bit is hit 
however, everything restarts again from the top. 

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So to clarify; when Hubble was first deployed 
on April 24th, 1990. Its clock began counting up  

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every 1/8th of a second from April 24, 1990. And 
for every one of those 1/8ths of a second Hubble’s  

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binary counter got one step closer to its maximum 
of 4,294,967,295 before restarting back to zero. 

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This actually happened already, Hubble had 
its first clock rollover back on April 29,  

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2007. So from 1990 to 2007 is 17 years. And 
then from 2007 to 2024 is another 17 years.  

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So this will be Hubble’s second clock rollover.
When you do the math, 4,294,967,295 multiplied  

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by 0.125 (or 1/8th of a second) divided 
by 60 seconds, divided by 60 minutes,  

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divided by 24 hours equals 6,213.78 days! 
Our magic number of around 17 years! 

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That’s all because Hubble’s initial requirements, 
back when it was being designed and built in  

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the 1970’s and 80’s, specified a base mission 
period of 10 years with a 15 year operational  

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goal. Therefore, the 32-bit clock design with 
rollovers every 17 years met those requirements. 

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But why does this even matter? Why not just let 
the numbers roll over on their own every 17 years  

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and start things from the top again? Well, if 
we tried that things would get pretty weird! 

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That’s because Hubble would suddenly think 
it was 17 years younger. It would think it  

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was back at that first clock rollover we just 
talked about that happened on April 29, 2007. 

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If Hubble thought it was 17 years ago, it would be 
very confusing. Hubble’s antennas would point in  

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the wrong direction when trying to send data or 
receive commands from a communication satellite  

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that was somewhere else. Because Hubble would 
think everything in space was in the same place  

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it was 17 years prior. Hubble would even think it 
was in a different place. And even more dangerous,  

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Hubble would think that the Sun was in a different 
place, and if Hubble looks at the Sun, that could  

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be very, very bad for its delicate instruments!
The fact that the second clock rollover 34 years  

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after Hubble’s initial deployment is now 
upon us is yet another testament to the  

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dedication and ingenuity of everyone 
who built, upgraded, and continues to  

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maintain Hubble as the premier astronomical 
observatory we have come to know and love. 

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Back before the new millennium, computer 
engineers around the world updated things  

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to have all four numbers of the year instead of 
just the last two, so we managed to keep that  

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Y2K bug from actually destroying everything.
And just like we managed to avoid any mishaps  

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with that issue, our team of Hubble engineers on 
the ground have sent up new routines to ensure  

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that our trusty space telescope could successfully 
rollover its clock without dropping back 17 years. 

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That way Hubble can keep doing its amazing 
work until the next clock rollover in 2041!

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