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So while everyone's been talking about #Apollo 50, I've been thinking of a more recent event. Tomorrow's the 25th anniversary of #ShoemakerLevy9's impact with Jupiter.
This was humanity's first close look at the effects of an impact event, and I think along with the growing scientific acceptance of the Chicxulub impact's relationship to the end-Cretaceous extinction, is largely responsible for our entire program of near-Earth asteroid research.
But back to July 1994. As Shoemaker-Levy 9 closed in on Jupiter, astronomers were getting ready to swing a year's worth of planning into action to study the impact in as many ways as possible. But there was one problem - the impacts wouldn't be directly observable from Earth.
You see, the calculated impact sites were just *slightly* over Jupiter's horizon. It would take 45 minutes for the impact site to rotate into view for the first collisions on July 16 (although this would shorten to ~20 minutes by the last collisions on July 22).
However, there was one spacecraft in position to view the event directly. NASA's Galileo spacecraft was en route to Jupiter and scheduled for arrival about a year and a half later. Its location in the Solar System gave it a more or less direct view of the impact.
There was one glaring problem, though. Galileo was a crippled spacecraft. Originally scheduled for launch on the Space Shuttle in May 1986, it was moved to storage after the Challenger disaster. It would sit there until 1989.
Several mission changes were made. First, NASA cancelled the liquid-fueled shuttle-capable Centaur booster that would send Galileo directly to Jupiter. Note: it insanely dangerous piece of hardware that was only acceptable in the irrational exuberance of the early shuttle era.
The loss of the Centaur booster forced NASA to redesign Galileo's flight profile. The best option was to use a VEEGA slingshot trajectory, using a flyby of Venus and two flybys of Earth to build up the momentum to make it to Jupiter.
This trajectory change made it necessary to alter some of the hardware on the spacecraft. Flying by Venus meant the spacecraft would get more solar heating than it was designed for. This meant adding extra coolers to an electrical system already trimmed to the minimum necessary.
To get the electrical power, several things would have to come off the spacecraft. One of them was an easy choice: the reverse motor for the drive mechanism to open and close Galileo's high-gain antenna. Once open, there wasn't a need to close it again.
Necessary modifications made, Galileo was packed up and launched on Atlantis in October 1989. It made a successful flyby of Venus in February 1990 and was scheduled to perform its first flyby of Earth in December 1990.
It was during this first flyby that Galileo was commanded to open its high-gain antenna for the first time. It hadn't been opened earlier due to the amount of heat it could collect during the Venus flyby. There was a problem.
During the refurbishment process for Galileo's flight, no one had checked the amount of lubrication available in the high-gain antenna deployment mechanism. Most of it had evaporated off during the time the spacecraft sat in storage.
Then, when Galileo was completing its Venus flyby, whatever was left was evaporated by solar heating. When it came time to deploy the antenna the deployment mechanism jammed, badly.
Now, if you've ever dealt with something that's jammed, the obvious solution is to rock it back and forth until whatever is stuck popped loose. But remember, the reverse motor was removed to make way for the coolers.
So the only thing to do was to try jolting the mechanism loose, using the motor as a hammer to try to get something to unstick. This helped open the antenna a little more, but progress was slow and there were concerns that using the motor in this way might cause a short circuit.
In addition, ground tests also found that as the mechanism deployed, it increased the amount of tension on the parts that were most stuck. This made it unlikely that they would ever deploy. With no benefit and a lot of downsides, the antenna was abandoned.
The solution to sending data back was to use the low-gain antenna, originally designed to send back a few bits of engineering telemetry. This was the equivalent of trying to using a firehose with a sprinkler - Galileo would never return data at the rate it was capable of.
So back to Shoemaker-Levy 9. Galileo was positioned to observe, but it was extremely limited in the amount of data it could store. A few full-resolution pictures and some spectra were all it had room for. Downlink would take weeks - much longer than the impact sequence.
The imaging solution was relatively straightforward. Jupiter was much smaller than the camera's field of view. So Galileo could use the digital equivalent of a double-exposure trick - imaging Jupiter many times in the same frame at different locations on the sensor.
But this technique had an issue: it was not a continuous recording process. At 2.5 seconds between exposures, it could miss the critical early moments of the impact. SL9 was travelling at over 60 km/s - in 2.5 seconds it would have traveled 150 km.
2.5 seconds were all it would take from the moment a fragment of SL9 reached the top of Jupiter's atmosphere to the moment it winked out of existence inside of the inferno of atmospheric gas piled up in front of it.
Alternative methods were devised. SL9 would impact the predawn skies of Jupiter, meaning that the fireball would stand out against the blackness of space. So why not create a trailed image to collect a nearly uninterrupted profile of the flash intensity?
Using the camera, Galileo would observe the impacts of the K, N, V, and W fragments. (Unfortunately Galileo used a less-sensitive observing mode for the V impact, which severely underperformed expectations as viewed from Earth. Images from this event was never sent back.)
Next up was the spectral data. Although these datasets did not have the same visual appeal, they would tell the most about the impact. These would tell how much energy was released and how hot the impact sites got.
The photopolarimeter was a small instrument that could observe the brightness of Jupiter at three different wavelengths. Most observations of the impacts were performed using the 945nm channel, which would detect the thermal flash from the fireball.
Initially, it was hoped that the instrument would resolve the initial flash traveling through the atmosphere, followed by a fireball when it detonated low in the atmosphere. In practice, there was no pause, suggesting detonation occurred higher up than expected.
The nice thing about the photopolarimeter was that it was relatively light on data usage - a 41 minute observing sequence could be stored in the flight computer's memory and sent back to Earth in only 12 hours.
This mode was used to observe the B, C, G, H, Q1 (fragment Q disintegrated further after the comet's discovery), R, and S fragments. B ended up being too small for the photopolarimeter to detect, and it was discovered after the fact that the R observation was mistimed.
Finally, the near-infrared spectrometer would measure how much heat was emitted by the impact. This instrument was very slow, but could track the initial release of energy and how quickly the impact site cooled down.
The data collected by this instrument during the G and R impacts is really incredible. You can see the initial growth and dissipation of the impact fireball, then a few minutes later a gradual increase in energy as material blasted upwards by the impact made its way back down.
You know the spectacular Hubble image of the G impact site? The infrared spectrometer explains the structure. The intense black central region is the site of the fireball, while the diffuse halo surrounding it is the material "splashed" by the impact.
Terrestrial instruments were much more capable of investigating the after-effects of the Shoemaker-Levy 9 impact (how it changed atmospheric composition and weather patterns).
But Galileo was probably most responsible for us learning about what goes on in the moments immediately after an impact. It got data no other instrument was capable of collecting. Not bad for a mission that only four years earlier was almost considered unsalvageable.
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