Remember that Y2K thing a few years ago? Where everyone was afraid the world was going to end
because computer programmers saved space by putting dates as… 77 for 1977, 85 for 1985,
Or 90 for 1990? But then it became clear that when the year 2000 finally rolled
around all of the computers would think it was actually 00. Or the year 1900.
Well, it turns out Hubble has something similar, only Hubble’s clock restarts every 6,213 days,
18 hours, 48 minutes, and 31.875 seconds. Or roughly every 17
years for those of you who like counting. That’s because Hubble’s computers have a
different way of tracking time than we have here on the ground. You’d think it would be as simple
as synching our ground clocks with Hubble’s personal timepiece, but you’d be surprised.
Hubble’s onboard clock is a 32 bit elapsed time counter. So instead of Hubble counting up seconds,
days, months, and years like a calendar as we know it, Hubble counts every 125 milliseconds, or 1/8th
of a second for those of you who like counting. Using 32 bits, up to 4,294,967,295 increments
can be stored. Once that final bit is hit however, everything restarts again from the top.
So to clarify; when Hubble was first deployed on April 24th, 1990. Its clock began counting up
every 1/8th of a second from April 24, 1990. And for every one of those 1/8ths of a second Hubble’s
binary counter got one step closer to its maximum of 4,294,967,295 before restarting back to zero.
This actually happened already, Hubble had its first clock rollover back on April 29,
2007. So from 1990 to 2007 is 17 years. And then from 2007 to 2024 is another 17 years.
So this will be Hubble’s second clock rollover. When you do the math, 4,294,967,295 multiplied
by 0.125 (or 1/8th of a second) divided by 60 seconds, divided by 60 minutes,
divided by 24 hours equals 6,213.78 days! Our magic number of around 17 years!
That’s all because Hubble’s initial requirements, back when it was being designed and built in
the 1970’s and 80’s, specified a base mission period of 10 years with a 15 year operational
goal. Therefore, the 32-bit clock design with rollovers every 17 years met those requirements.
But why does this even matter? Why not just let the numbers roll over on their own every 17 years
and start things from the top again? Well, if we tried that things would get pretty weird!
That’s because Hubble would suddenly think it was 17 years younger. It would think it
was back at that first clock rollover we just talked about that happened on April 29, 2007.
If Hubble thought it was 17 years ago, it would be very confusing. Hubble’s antennas would point in
the wrong direction when trying to send data or receive commands from a communication satellite
that was somewhere else. Because Hubble would think everything in space was in the same place
it was 17 years prior. Hubble would even think it was in a different place. And even more dangerous,
Hubble would think that the Sun was in a different place, and if Hubble looks at the Sun, that could
be very, very bad for its delicate instruments! The fact that the second clock rollover 34 years
after Hubble’s initial deployment is now upon us is yet another testament to the
dedication and ingenuity of everyone who built, upgraded, and continues to
maintain Hubble as the premier astronomical observatory we have come to know and love.
Back before the new millennium, computer engineers around the world updated things
to have all four numbers of the year instead of just the last two, so we managed to keep that
Y2K bug from actually destroying everything. And just like we managed to avoid any mishaps
with that issue, our team of Hubble engineers on the ground have sent up new routines to ensure
that our trusty space telescope could successfully rollover its clock without dropping back 17 years.
That way Hubble can keep doing its amazing work until the next clock rollover in 2041!
"Follow us on social media @NASAHubble"