9½ Years to Pluto, No Go-Arounds

Here's the scene. You are Alice Bowman, who in 2018 will give a keynote at DesignCon. But that is years in the future. Today, it is 2pm on July 4, 2015. However, you don't get to eat grilled meat and drink beer since you are in the mission operations center for NASA's New Horizons space probe, along with a handful of others. New Horizons is a mission to take the first close photographs of Pluto. It was launched way back in 2006, nine and a half years earlier. Before Steve Jobs stood up in YBCA and announced the iPhone. Yes, Pluto is a long way away. in fact, it is so far away that it takes four and a half hours to send a message to the craft, and another four and a half hours to receive any type of acknowledgment. It has taken so long to get there that Pluto has been re-classified as a dwarf planet in the years since the launch.

The approach to Pluto is imminent, starting on July 7 in three days time. The closest approach will be on the 14th. For reliability reasons on such a long flight, the spacecraft is not gimballed, meaning that its antenna and its camera point where the spacecraft points. In particular, it can only take pictures or communicate, not both at the same time. July 4 is the day that the final instructions for the photographic sequence are to be uploaded to the probe. After that, it will run the program incommunicado, storing all the pictures. Later it will download them. The upload will take five hours from a 30m dish in Australia, one of the fastest, capable of 1K bits per second (the smaller dishes can only manage 50 bits per second).

You listen for the response from the upload but there is nothing. You have lost contact with the probe. The probe is moving at 31,000mph. That is actually slower than it left Earth, at 36,000 mph (still the fastest ever departure from Earth) since over the years the gravitational pull of the sun has pulled its speed down. At 31,000 mph, that means that in the 4.5 hours since the expected transmission, the craft has moved nearly 150,000 miles. In a couple of weeks, it will have blasted past Pluto, and if the data cannot be uploaded, it probably won't even have bothered to take any pictures. You had flown for nearly ten years with one chance to get it right, and you were about to get it wrong.

Failure Is Always an Option

For about ten seconds, Alice did what anyone would do and felt totally defeated. Then her training kicked in. The first thing is to check that the communication links to Australia and the dish there are all working normally. They are, which means it is a problem with the craft. One advantage of such a long mission is that the team has all worked together for years. They get the guy who wrote programmed the flight computer on the phone.

“What could be going on? Don’t tell me everything, there are obviously lots of things. Tell me the most likely."

"There are two things that come to mind. One that we've seen before and one we've not."

"How long will it take for us to be able to tell?"

"108 minutes for the situation we've seen before, and 55 minutes for the one we've never seen."

They were already 60 minutes at that point, so they reconfigure the communication to see whether one of these two anomalous behaviors is happening. They connect. Relief. But the probe is transmitting at just 10 bits per second which means that for some reason it has fallen back into "safe mode". That also means it is spinning (to keep its antenna pointed back at earth) meaning photos are impossible. However, it also means that they can recover the spacecraft.

But it is a race against time. They figure that they have four roundtrip message times to the craft before the flyby sequence has to start. At 3am on July 5, they send the first set of commands to provide all the detailed instructions for the flyby. On July 7, they see it was successful and the craft has transitioned from spinning into 3-axis mode (of course, with the delays, that means it actually transitioned hours earlier). They wait. It is just four hours later that the first set of commands to take photos should engage. And it did, although they couldn't know that yet. Remember, the probe can either align itself to take photos, or align itself to communicate. It can't walk and chew gum at the same time.

Twenty-two hours later, the spacecraft has a small communication window, and they discover that the photograph sequence engaged correctly.

Then, on July 14, they get a 15-minute communication window to check that the data from the photography was all in memory correctly. Alice tells everyone the good news:

“We have a healthy spacecraft, we have recorded data, we are outbound from Pluto”

After Pluto

Pluto is three billion miles from earth, nine and a half years of flight. They hit the aim point 88 seconds early and 26 miles off. Alice thinks they could have done better, but after the July 4 anomaly, they were happy.

New Horizons is going on to the Kuiper Belt. In fact, there are thousands of objects out there, but it was only in the 1990s that it was determined how to detect them. Pluto was just the first one discovered (in 1930). The craft is now 44AU from Earth, and the roundtrip speed of light time for radio communication is up to six hours each way.

In 2019, it is scheduled to take pictures of the inventively-named object MU₆₉. We have no idea what it looks like. The Hubble telescope found it. There have been three occultations (where it passes in front of a distant star) but they don't even know if it is a single object or a lumpy binary object, or maybe has a moon. The plan is to fly just 2200 miles from it.

The craft is currently in hibernation and the plan is to wake it up on July 4 (isn't anyone superstitious about that date?) and start doing imaging in August, although probably it will still be too far away to see. They have seven chances to do a trajectory change, although hopefully they will not require that many. The encounter period starts on December 25 with the closest encounter on January 1 (this spacecraft loves holiday dates). It is actually 33 minutes after midnight and they decided not to adjust it to be precisely at New Year because they didn't want to share the glory with everyone in Times Square.

They will get a first 15-minute look on January 1, but they won't be able to bring the data down until it goes into spinning mode on January 9. After that, New Horizons will become the third craft to leave the solar system completely.

Some More About the Spacecraft

The craft is about the size of a baby grand piano. It weighed about 1000 lbs at launch, including the hydrazine that is used to maneuver it, which gradually gets used up, of course. It is too far from the sun for solar power to work, so its electrical systems use nuclear power. Since it would be nine and a half years from launch to arrival at Pluto, the craft had to be designed for extreme reliability. With the long communication times to earth, it would have to take care of itself. 

For the DesignCon audience, it would have been nice to get a bit more detail one the electronics. But here is what we were told. There is 512KB of memory for events. They normally use one half for the currently executing program, and the other half for the next mission being uploaded. However, for the flybys, they use 80% and just keep 20% for communication. The craft can be in two main modes of flight, three-axis and spinning. In three-axis, the craft can point itself in any direction to take pictures or to communicate. However, for longer communications, it goes into spinning mode, when it gyroscopically stabilizes itself to point at Earth and can transmit at higher data rates. The speed of the spacecraft and the communication delay mean that talking to it is a bit like skeet shooting, where you have to aim ahead to intercept the flight (we call them "clay pigeons" in Britain, by the way, isn't that a nicer name?).

Watch New Horizons Flyby Past Pluto

This video is actually a montage of photos since New Horizons didn't directly take video (remember, the spacecraft launched in 2006, and was designed years earlier. Google hadn't bought YouTube yet, video just wasn't a thing).

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