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The MIRI Medium Resolution Spectrograph is back in business -
that brings the James Webb Space Telescope back to full science capability.
And a special working group has figured out a way to minimize
the risk of micrometeoroid impacts.
Short version: Webb will now look behind instead of forward.
I'll explain that one in a bit,
but first, let's talk about MIRI coming back into full service.
In August, one of MIRI’s grating wheels started to register
some higher than expected friction whenever it was rotated.
These wheels are called Dichroic Grating Assemblies or DGAs.
They rotate to present different combinations of beam splitters
- the dichroics - and diffraction gratings that route the light
into the four different channels of MIRI’s Medium Resolution Spectrograph, or MRS.
I talked about how the MRS works
in some detail in my previous video about this problem.
But to recap, the DGA wheels rotate so the four channels
record one third of the total spectrum at a time.
With each rotation, the rest of the spectrum is filled out.
When one of the two wheels showed more friction than expected,
the MIRI team suspended the use of MRS while they analyzed the problem.
On November 8, NASA announced that the friction problem was resolved
and that MRS was returned to full service.
The official statement said that a review concluded that the friction occurred
under “certain conditions” and that the team developed
a new procedure for using the wheel during science operations.
The new procedure was tested on November 2nd
and the wheel performed exactly as predicted.
MRS mode was returned to full science observations ten days later,
and it's been running ever since.
In fact, the first spectrum that MRIs took
was of Saturn's polar regions.
This was possible thanks to a lucky combination of Saturn's axial tilt,
its orbital inclination, and Webb's
location near the top of its L2 orbit.
In fact, this particular alignment won't occur again for another 20 years.
So as an astronomer, I'm thrilled to know
that MRS is back in business and taking data.
But as an obsessively curious nerd, I’d
really like to know what those conditions are that causes friction
and what was done in terms of how MRS is now able to operate.
MIRI’s user documentation doesn't indicate
any changes in how MRS observations are implemented.
Still, the statement just seems a little vague,
and I can't seem to find anything that indicates
how the problem was actually resolved.
I don't know why somebody from Team MIRI hasn't called me up by now to tell me.
I mean, after all, I have the pin.
Meanwhile, a working group investigating that large micrometeoroid strike in
May announced a plan for how to further reduce the risk of future impacts.
I'll tell you more about the details in a moment.
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[Ad End] NASA announced its findings from a working group investigating
the Micrometeoroid impact on Webb's Mirror in May of 2022.
And this occurred while the science instruments were still being commissioned.
Even though micrometeoroids are just the size of a grain of sand,
they're traveling at tens of kilometers per second,
so they still pack a lot of kinetic energy.
At 25 square meters, JWST’s
mirror is the largest ever flown.
since it has to operate at cryogenic temperatures,
the mirror has to be exposed to open space so it can passively cool.
It was always understood that Webb would be hit by micrometeorites,
so they built the mirror to perform at a level much higher than is required
for science.
In this way, as the mirror accumulates impacts,
it will still deliver science-quality images over the course of its mission.
Even better, Webb's mirror segments can be adjusted
to cancel out some of the effects of those impacts.
And this mitigation strategy was based on modeling the Micrometeoroid
environment at L2 long before Webb was ever launched.
And they estimated that Webb's mirror should accumulate one or two hits a month,
with most of those hits being so tiny, they could effectively
be canceled out by routine mirror adjustments.
Now that's true of the tiny strikes, but maybe once every couple of years
or so, the mirror should be hit by something a little larger,
like the one that hit back in May.
To make matters worse, it
hit on a part of the segment that couldn't be fully corrected either.
Each segment has six actuators arranged
in a hexapod, with a seventh actuator at the center.
Had the impact occurred near one of these actuators,
the deformation would have been more correctable.
Instead, the impact occurred at a location where there wasn't as much control
authority available.
Still, after correcting for as much as they could, the mirror’s
performance is still twice as good as it needs to be to produce science.
This is the final selfie image of the mirrors
that were taken after the corrections were put into place.
It looks worse than it really is because this image isn't a picture of a crater.
Rather, it depicts the amount of wavefront
error across all of the mirror segments.
So if the telescope's optics were just exactly perfect,
then all of the light waves bouncing off every point on the mirror surface
would combine into a perfectly flat wavefront that goes into the telescope.
But there's no such thing as a perfect mirror, so there's always
going to be some tiny amount of imperfection in the wave’s flatness.
And this imperfection is the wavefront error.
After correcting for the May impact as much as possible, the telescope's
wavefront error was brought down to about 59 nanometers RMS.
And that's about nine nanometers greater than it was before the impact.
To be clear, the impact itself is probably just a tiny little crater
on the surface, maybe a couple of millimeters.
But it delivered enough energy such that the surrounding surface
was deformed ever so slightly from its optimal shape.
And this is why you can't even tell there's a problem
by looking at these beautiful images, and there isn't nearly
enough degradation to impact the science quality either.
In fact, the error introduced by the impact
is much less than the inherent errors introduced
by the rest of the observatory and the instruments themselves.
However, there is another problem of sort in that Webb
seems to have enough fuel on board to last for at least the next 20 years.
And that's the official statement from NASA.
Unofficially, though, there's now some talk about 25-plus years
of fuel remaining.
So given that this impact happened earlier than expected
and given that Webb could be around for a lot longer than planned, NASA's
decided to convene a working group to think about the problem a little more.
They announced their results on the November 15th entry on Webb's blog.
As of publication, Webb accumulated 14 impacts since launch.
And that works out to about 1 to 2 impacts per month,
which is exactly what the predictions were before launch.
This led the working group to conclude that the May impact was very unlucky,
but nevertheless statistically consistent.
In other words, yeah, it was a fluke.
But they also found that on average micrometeoroids striking the mirror
head on are moving twice as fast as those
striking the mirror in the same direction as the telescope's motion.
Now, since kinetic energy goes as the velocity squared, an impact
in the forward “ram” direction will generate four times
the energy as an impact in the rear-facing “wake” direction of the telescope.
Now that's an average, of course,
because the individual velocities of the micrometeorites will vary.
Still, if Webb could avoid pointing in the ram direction as much as possible,
it should reduce the collision energy by a factor of four.
Now Webb is already constrained on where in the sky can safely point to
because it has to keep its shield facing the Sun at all times.
And this sets up a circular region called the Field of Regard.
As Webb orbits the Sun, different parts of the sky enter into this field.
Over a six month period, Webb will see everything in the entire sky.
So a fixed target in roughly the same plane as Webb's orbit enters
the field of regard twice a year, once as Webb is moving toward the target,
and then again six months later, as Webb is moving away.
So NASA has imposed a new Micrometeoroid Avoidance Zone, or MAZ.
The objective is simple: point the other way.
And to help astronomers plan their observations a little better,
the Proposal Tool will give them a little bit of assistance.
This tool helps astronomers to define their observations
on both Hubble and Webb.
It allows them to specify what targets they want to point to, what instruments
they want to use, observing modes, exposure times, filters...
And it tells them in turn when their target is visible
and when they should expect their observations to be executed.
This way, astronomers can hand off a complete observation
spec to the Program Coordinators at the Space Telescope Science Institute.
And there they actually get the observations scheduled and executed.
The great thing about the Planning Tool is that it'll tell you
if your observations are violating any constraints on things like
Sun angle, guide star availability, and so on.
But starting next year, the tool will also advise
when your observations can be scheduled in the Meteoroid Safe Zone.
But there are two reasons why
it may only be possible to be done while pointing into the MAZ.
One is that the observation is time-sensitive
and it just happens to work out that the target
is only visible during the MAZ during those opportunities.
Another is if the telescope
has to be oriented in a particular way to observe the target,
and the only time those orientations can be achieved while keeping the shield
facing the Sun happens to be while the target is in the MAZ.
In those situations, you'll get a warning message in the Planning Tool.
Now, if you can replan your observations to avoid the MAZ
entirely, then great, do it, make it happen.
But if not, then you just enter an explanation explaining why
you can't do it any other way, and the warning clears.
Then you can submit your proposal and you're good to go.
And that's the thing about this new policy.
It's not that they're going to prohibit observations in the MAZ, because,
let's face it, the chances of a severe impact
at any given time are still quite low.
But they'll otherwise avoid the MAZ as much as possible.
And by the way, not all
targets are going to be equally affected by the MAZ constraint.
Targets at high ecliptic latitudes, that is, targets that are very high above
the plain of Webb's orbit around the Sun, will never enter into the MAZ at all.
Targets at latitudes closer to the upper and lower edges of the MAZ
will find their visibility windows affected
less than 50% of the time.
It's only those targets that lie
close to the ecliptic that are going to be the most affected.
In retrospect, it seems like this
was something NASA's should have thought of before launch.
But the fact is, is that you just can't think
of every single possible scenario, all the time.
NASA's is considering a large optical telescope at L2,
which won't have to be kept as cold as Webb.
So it's possible that this new telescope would be fitted with a baffle
to help protect against strikes hitting the mirror at an angle.
For now, though, a little adjustment as to how Webb is used
will go a long way toward extending its useful mission life.
And by the way, huge thanks to my patrons for helping to extend
Launch Pad Astronomy’s mission, and I'd like to welcome Nick Borquez,
Rick Moore, Emily O'Brien, Alexander Tuzhikov, Andy Schleiss, Erik
Vermaat...I hope I got your pronunciations right, I apologize, I know I didn't.
But if you'd like to join me on this journey
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Until next time, stay curious, my friend.