It seems like months and months go by between Kepler reports these days, doesn't it?
Well, some of you have seen the Kepler stories in the news a few weeks back as we released the announcement of five newly discovered planets. There are many more on the way, but this is what we could put together from the first six weeks of flight data and a couple dozen nights of ground-based observing to eliminate the false positives, determine the mass of the planets and the detailed properties of the stars.
And much of the sorting through the data had to be done manually because our data processing software isn't fully functional here at Ames; some of the software development had to be deferred. The good news is that we finished development of a major software upgrade in late November, so we should be able to make up for lost time in the coming months. After all, we don't have just 6 weeks of data here, but almost a full year!
If you haven't done so recently, check out the Kepler website at http://kepler.nasa.gov. It's been updated and is much improved. Also, the official Kepler blog at http://blogs.nasa.gov/cm/blog/kepler. We are trying to get more people to participate in the blog, you know, to give you a wider range of experiences. The first two entries are mine, but I'm trying to get someone who was at the AAS conference to write about what it was like when the Kepler results were announced. Of course, that blog will never have the real inside scoop I have here (wink, wink).
Recently, Scottie sent me an email and asked what an exoplanet was.
Well, an exoplanet is any planet around another star. All the stars we are looking at are relatively nearby. Within our galaxy, between about 1,000 and 3,000 light years away (our galaxy is something like 250,000 light years across). Why do we only look at relatively nearby stars? Simple. We need the photons. Stars that are too far away are simply too faint to be accurately measured.
And what else has been happening?
We finally passed our post-launch assessment review and a management transition. The Kepler Mission is now being managed by Ames Research Center rather than the Jet Propulsion Lab. We had another Safemode, but we're getting pretty good at recovering from these. Never the less, I really am looking forward to seeing these things go away. In the mean time, I should probably point out that, though we take these things seriously, going to Safemode does no damage to the spacecraft, nor does it in any way shorten the mission life. It merely means that something unexpected happened, serious or confusing enough that the onboard computer decided it would be best to go to a safe state and wait for help from those of us on the ground. Safemode is the safest state of the spacecraft. Now, Emergency Mode would be a whole other story!
On the plus side, the changes we made back in September seem to be working great and our pointing accuracy has been superb, with no more incidents of the unexpected drops into our coarse-point mode. I did tell you about that, didn't I? We're continuing to make minor tweaks in the spacecraft operation to improve performance or reduce risks, and looking forward to the day when we can't find any more improvements to make, all to make the data better with less noise.
In December, we changed some settings on a heater that was cycling on and off every few hours and was somehow showing up in the stellar brightness measurements (somehow it seems to be affecting focus very slightly). Hours is about the duration of a planetary transit, so we really don't want things changing on that time scale. On the other hand, the heater is cycling to keep the reaction wheels warm, and we count on these wheels to keep us pointed accurately. So what we did is narrow the temperature limits, and now the heater cycles every 12 minutes or so, and isn't such a bother.
We are also getting ready to make some changes to the software running on the spacecraft. Updating the software requires us to reboot the processor, something you don't want to do very often, because there's always the chance it won't come back on! But we've been saving up a few minor bug fixes, and now have a more important bug to fix. It sent us into Safemode in November, and as long as we're going to fix that one, we're going to do the others, too. But we'll do a lot of testing first, and we'll make the update in April, during one of our regularly scheduled data downloads.
Otherwise, things are going rather well, so I don't have a lot to write about. Not that that seems to have inhibited me much.
Actually, there was a new development in January that had us hopping. In fact, it's delayed this update while we worked on it. It seems that one of our detector modules died.
Hopefully it's only dead, not dead dead, and maybe we can revive it. But it doesn't look good. This is the most serious issue we've had since we started gathering science data.
One of our modules (Module 3, on the edge of our field of view) stopped working on January 9. Since then it's been dark, with no star images.
The Kepler focal plane is approximately one foot square. It's composed of 25 individually mounted modules. The 4 corner modules are used for fine guiding and the other 21 modules are used for science observing. Attached are some pictures that show a single science module and the assembled focal plane with all 25 modules installed. Note that the fine guidance modules in the corners of the focal plane are very much smaller CCDs than the science modules.
On the left, a single science module with two CCDs and a single field flattening lens mounted onto an Invar carrier. On the right, a focal plane assembly with all 21 science modules and four fine-guidance sensors, one in each corner, installed.
Under normal operations, each module and its electronics convert light into digital numbers. For the darkest parts of the image between stars, we expect these numbers to be very small (but not zero). Correspondingly, for the brightest stars in the image, much larger numbers are expected creating an image of each observed star and its background neighborhood. The numbers we see coming out of failed module 3 are all very similar in size and considerably lower than the normal levels. These numbers produce an image that looks like the "snow" on a television that has very bad reception. There are no stars visible in these images.
There are several ways a module to go bad, such as a blown fuse, a shorted or broken wire, or the failure of individual parts. The Kepler detectors were fabricated to rigid standards and subjected to rigorous testing on the ground before launch. And the design contains several redundant features to prevent the failure of a module, but the space environment is unforgiving and failures do occur. The fact that this anomaly affects all four channels on one module will help determine the probable cause.
Depending upon what caused the module to stop working, we might be able to recover it, though the flexibility of the Kepler design is limited in this extent. Even if the module is recoverable, it will be several weeks before recovery actions are taken because we need to review the current performance and design to ensure that any recovery steps do no further damage to the focal plane or spacecraft. I mean, the last thing we want to do is lose more modules while we try to recover this one!
The module that has stopped working is on the periphery of the field of view. Since the Kepler spacecraft rotates by 90 degrees every three months, this module would normally observe a different portion of the Kepler field of view each season, as shown in the following image. Hence, no part of the Kepler field of view has been rendered unobservable it's just that four regions are only observable three out of four seasons, or 75% of the time, if the module is unrecoverable.
Kepler field of view showing position of the failed module for each season.
We specifically designed Kepler to preclude the propagation of failures such as this. The design is highly compartmentalized so that a failure in one area will not spread to another. There will be minor changes to the signals from the remaining science modules because the focal plane and its electronics have changed temperature somewhat since the bad module isn't drawing as much current anymore, but the current problem is not expected to spread beyond the one affected module.
As far as the affect on science goes, a rule of thumb would be that because only one of the 21 science modules is affected, that the reduction in science results will be about 5%. This shouldn't significantly affect the Kepler science performance.
So there you go. There's the latest, with an inside scoop.
Charlie Sobeck, Deputy Project Manager