Welcome to our New Horizons Mission series and its fifth article: New Horizons Pluto Flyby!
On previous articles, we came after the process of realization and planning of the New Horizons Project (1-2), technical and scientific details of New Horizons Spacecraft (3). And finally we got onboard the probe, travelled all the way from Earth to Pluto (4) while visiting an asteroid and Jupiter System on the road.
- New Horizons Mission – Historical Background
- New Horizons Mission GoalsNew Horizons Spacecraft
- New Horizons Spacecraft
- New Horizons Mission From Launching to Pluto Encounter
If you like to remember where we were, feel free to check out the forth article in series.
In short, this article begins from New Horizons Team’s final “Wake Up Call” to the spacecraft for terminating the hibernation mode in the end of 2014. Later on this text, we will cruise with New Horizons Spacecraft to Pluto, experience the New Horizons Pluto flyby days (14 July 2015) and seize its scientific outcomes.
New Horizons Pluto Journey
6 December 2014: Knock knock, duty calls!
It had been almost 9 years till the spacecraft was launched, and it was time for reaping the fruits of victory. Time to direct spacecraft instruments to Pluto area.
After few weeks of tests following the wake-up, pre-flyby operations began on 4 January 2015. Later on 15 January 2015, New Horizons’ First Approach Phase and Pluto observations were already initiated. Within this period, a Kuiper Belt Object 2011 KW was also observed from 75 million kilometers away.
By virtue of above, on 10 March 2015 a trajectory correction was performed . Those were the final chances to catch Pluto in place and on time!
Approaching to Pluto – Importance of a Safe Flyby
Consider you’ve performed plantation to your huge field 9.5 years ago. You can only remotely command your harvesting machines 4.5 hours before the operation and you have few hours to harvest the best products where you can not assure 100% safety of your machines. That’s how lunatic a distant flyby to an unexplored planet can be!
Apart from being in the right place at the right time, there was another issue of utmost importance: Safety of spacecraft instruments during the closest flyby.
The probe should both be avoiding any kind of object (e.g. rings or even possible undiscovered moons) and should be located in an area 100 x 150 kilometers wide within a time period of 100 seconds. And during this period, all science instruments should be pointing to the right place at the right second where the instruments can not be rotated seperately! Spacecraft itself should point the dedicated instrument to the target.
We can easily quote now: “It Is Rocket Science!”
In that regard, there are some handicaps. Spacecraft was going to be very fast (about 50.000km/h) during the flyby and scientists had to specify a set of coordinates for the safest Pluto flyby with best images and scientific data. But specifying such coordinates needs more precise information of planetary coordinates, and this could only be more and more clear when spacecraft is closer to Pluto area.
A similar mechanism had to be ran for the possibly dangerous objects in the trajectory. Spacecrafts’ advanced cameras were scanning the route and looking for any kind of obstacles on the road. There were backup plans for altering the trajectory, but mostly the fingers were crossed for a smooth flyby.
The team and the spacecraft were extremely busy with above calculations. In the meanwhile, better images of Pluto and Charon were also collected and downlinked to Earth.
On 9 April 2015, first colored image of Charon and Pluto was captured by Ralph from 115 million kilometers away.
With the exposures of complete Pluto System on 11-12 May 2015 and analysis performed afterwards, New Horizons Team finalized the primary calculations of orbits and the simulations of possible dangers. The team searched for additional objects, even 4 times less dim than moon Styx. However no rings were detected and yet all was clear ahead for New Horizons’ Pluto encounter!
On 13 June 2015 and 30 June 2015, two more final and minor trajectory corrections were applied.
All was set and everything was in order for the flyby! What could go wrong after that point?
The Software Glitch Before The Encounter!
4 July 2015 was an unfortunate day. Spacecraft computer went into safe mode, scientific operations were interrupted and communication with the spacecraft was lost for 1 hour and 21 minutes!
That was the second software problem faced after the bit flipping issue 8.5 years ago in 2007. But this time, the timing was quite challenging.
What to do? The team members had to stop watching fireworks and get back to work..
The team worked on the problem and shortly maintained the data link. And on 5 July 2015 they realized that the issue was a computer malfunction. On 6 July 2015 project manager Glen Fountain said “While one part of the computer was compressing data it had already gathered, another part was burning a future command sequence into flash memory. The two tasks were more than the processor could handle at one time”.
This was one of the worst case scenarios during Approach Phase 3, last phase of scientific course and there was no room for a mistake. The problem on CDH had to be fixed immediately.
The team was running against time, because they had to fix everything latest on 7th July for handling the pre-planned Pluto flyby and scientific operations properly.
Since the time was limited and two-way data transmission speed was about 9 hours, corrective actions were prepared, partially tested in laboratory, got approved and immediately uplinked to the spacecraft.
Several fixes including switching from main to redundant CDH were applied in 3 days and finally on 7 July 2015 morning, software team managed to fix all with a tremendous amount of effort. Spacecraft was in normal operation in 3-axis mode.
What was the cost of this malfunction? About 30 pcs science jobs were wasted, but considering the overall success of New Horizons Pluto flyby and it did not happen during the closest approach; that was eluded without a big headache.
New Horizons Pluto Encounter
The Pluto mission was now safely back on track and a dense series of scientific observations were going on.
10 July 2015 was the date when the diameter of Pluto and Charon were determined with less than 0.1% error.
On 12 and 13 July 2015, backup images of the closest flyby activities were transmitted to Mission Operation Center. That was a plan to cover images of Pluto and Charon with best possible resolution in case of a problem during the encounter. That could possibly be a damage on instruments or spacecraft, where the calculated risk was about 0.01%.
Most important part of more than two-decades long mission was ±12 hours of the encounter. Almost 400 pcs science operations and 51 different data types using all 7 scientific payload was planned to be executed during that period.
New Horizons Pluto Flyby on 14 July 2015
And it is 14 July 2015!
At 11:49:57 UTC, New Horizons Spacecraft successfully flew 12.472km over Pluto surface with a speed of 49.608km/h and made history.
Minutes after at 12:03:50 UTC, the probe performed the closest Charon flyby from 28,858km away with a relative speed of 49.932km/h.
Since the spacecraft was extremely busy with science operations and related maneuvers, there was no room to direct the antennas to Earth for sending processed data packages. After 13 hours of waiting on 15 July 2015 at 00:52:37 UTC (it was still 14 July evening in Mission Operation Center), NASA’s Deep Space Network reported that the flyby mission was successfully performed, all subsystems were functional and spacecraft was running on its course.
Finally the dreams came true! Words spelled in 1989 in an Italian Restaurant in Baltimore. An idea brought to maturity in decades. And 26 years of many lifetimes dedicated. There it was!
That was the beginning of a 15 months of data transmission period. 6.25Gb of science data was stored during the Pluto flyby and it could only be transmitted with 1-2 kb/sec speed from that distance.
Post-Pluto Flyby and Collected Data
A bunch of major scientific operations were handled more than 1 hour after the encounter: Examination of atmospheres of Charon and Pluto when both rocks are between Earth and the spacecraft. Powerful radio signals were spread towards Pluto System and captured by REX and Alice. Those occultations were performed on 14 July 2015 at 12:51 UTC for Pluto and 14:17 UTC for Charon.
The data collected by CDH was prioritized and sent to Earth in respective sequences. Between 15-20 July 2015, complete data packages of prioritized data including overall colored and non-colored views of Pluto and Pluto moons, together with initial atmospheric data and some of the enhanced high resolution views of specific areas on Pluto and Charon surfaces were transmitted to Deep Space Network.
The cameras onboard ably captured overall images of Pluto and Charon with couple of kilometers resolution. When it comes to pre-selected areas, focused regions were pictured with an incredible resolution few dozens of meters!
Best resolutions for the smaller moons’ images were: Nix: 330m/px, Hydra: 780m/px, Styx: 1.8km/px, Kerberos: 1.8km/px.
New Horizons Pluto Flyby and Its Scientific Outcomes
Marking a time stamp in the history, New Horizons Spacecraft visited Pluto System. Beyond being only a “visit“, this Pluto mission returned valuable data about unknown worlds: New facts about Pluto and Kuiper Belt which led to a broad increment on our Solar System knowledge only in few years.
Was it a successful mission? Did New Horizons ably perform all tasks?
Only problem faced was the computer malfunctioning on 4th July, but luckily this did not effect any part of the mission objectives! All data aimed to be collected was fully and successfully gathered.
New Horizons sent the last part of scientific data package on 25 October 2016 21:48 UTC, which officially meant the end of the Pluto Flyby task for the mission.
In terms of science, 14 July 2015 and 25 October 2016 are not dates of an end. Those gigabytes of data were to be analyzed and interpreted. As of today, scientists are still working on those data and still come up with new researches and findings.
In this section, we will briefly explain the scientific outcomes of the mission following the scientific purposes stated on Goals of New Horizons Mission article. We placed additional links to our related articles where we explain the studies in details.
First of all, whatever the scientists would face after a closer look to Pluto System, it would be surprising. Because we have never looked at such a planetary system before.
However, the panorama of such a small planetary system was beyond expectations! It seemed Pluto was a living, reshaping and a unique in terms of its geology, surface variations and atmosphere. And comparing to Charon, they were totally different from each other.
As a consequence of above, the colors and global views of Pluto and Charon are completely different. While Pluto seems to be dyed with red, white and yellow; Charon looks more like a grey world with a brown/red cap. The red colors here are mostly deceiving since the true color of Pluto is brownish. And that applies to Charon’s north pole as well.
When it comes to smaller Pluto moons, their characteristics were very similar to what was predicted before the mission but researchers realized that their sizes were slightly different than previously calculated.
Mission Objectives and Related Scientific Results
Surface Findings and New Horizons Maps
Detailed surface images of Pluto and Charon with less than 1 kilometer resolution were captured as planned. In addition, surface compositions of both rocks were identified. Detailed maps of Pluto and Charon’s surface features were created. For details of those New Horizons maps, see related articles: Pluto Map and Charon Map.
Detailed analysis of Pluto surface showed that Pluto had been continuously regenerating her surface in last 4 billion years. The icy surface was found to be dominated by nitrogen together with methane, carbon monoxide, water and ethane. Besides above and a great variety of surface features, signs of regeneration, glaciation and volcanism were concluded.
On the other hand, the studies on Charon surface revealed that Charon had a much more stable and old surface structure comparing to Pluto. Charon surface was made of water ice and ammonia compunds, and reflected similar geological properties like Pluto with one difference: Very few surface activities compared to Pluto.
Researchers identified complex organic molecules called tholins on both icy rocks, which are responsible for the red/brown color pallette of both surfaces. Those tholin layers are formed via the interaction of methane and solar radiation. But that made sense only for Pluto, not for Charon as Charon is in lack of methane. Further studies showed that the brown mark on Charon’s north pole was a consequence of methane molecules stole by Charon from Pluto.
Major Surface Features
On initial high resolution global images, Pluto welcomed New Horizons Team with a huge white heart. North part of this area is called Sputnik Planitia, the biggest glacier detected in Solar System. The area is found to be crater free, smoothened by winds and glacialism and geologically young.
Besides big mountain ranges, pits and canyons; possible cryovolcanic areas were detected and later named as Wright Mons and Virgil Fossae.
Detailed Charon images highlighted a huge tectonic belt covering the equatorial area which is anticipated to be a result of a frozen subsurface ocean. In addition to this, signs of ancient cryovolcanism is detected on Kubrick and Clark Mons which also relates to the same ocean.
Further researches of craters on Pluto and Charon performed and signs of minor impacts during bombardment of collisions from early Kuiper Belt periods were concluded. That was a crucial data for Kuiper Belt and Solar System studies.
Inner Layers and Oceans
Researches on both rocks also resulted with calculation of inner layers. 70% of Pluto is made of a rocky core and the rest of the 30% is the icy crust. On moon Charon this ratio is 55% to %45, and the ice crusts are dominated by water.
Speaking of water on Pluto and Charon, further studies revealed that Charon once had a subsurface ocean, but they are completely frozen for billions of years. However there’s a great chance that Pluto has a hidden ocean made of water beneath its icy crust at the moment.
One of the most important objectives of this mission was to identify Pluto’s atmosphere, its composition, formation and escape rate. Experiments and further studies revealed all clearly.
On the other hand, the haze layers, atmospheric pressure and temperature, interaction with solar winds and the escape rate were totally different than predicted. And the mission lead to extensive revisions on atmospheric models.
Detailed atmospheric calculations also revealed that Pluto had rivers or lakes on its surface in the past.
As a part of mission plan, night side of Pluto was also captured. Besides the unpredicted results of Pluto’s atmospheric analysis, Pluto atmosphere also amazed scientists visually. Would anyone predict that Pluto had a blue atmosphere before the mission? It does!
Before the mission, there were no signs of a stable atmosphere on Charon. New Horizons Probe’s measurements confirmed that Charon has almost no atmosphere at all.
Pluto Moons and Pluto Rings
Particle analysis and extensive researches showed that there are no rings and no additional moons around Pluto. In addition to this, detailed analysis of dust counting results showed that the area does not include more debris than the amount in empty space.
As well as Pluto and Charon, sizes of tiny Pluto moons were sensitively calculated and specified. All smaller moons are found to be in amorphous shapes and have chaotic axial rotations.
Those moons were additional targets of the mission since some were discovered months before the spacecrafts launching and some were located during the voyage.
Bigger moons Nix and Hydra‘s surfaces analyzed and ammonia compunds found to be accompanying water ice. Styx and Kerberos were not planned to be analyzed in details, that’s why there are no direct findings about surface materials. However, higher albedo of those suggest existence of clean water ice on both.
Comparing the albedos of those moons to similar Kuiper Belt Objects suggest that Pluto System was formed after the initial bigger collisions and the residents created their own habitat about 4 billion years ago. And that theory is supported by the calculation of crater ages.
The success and the scientific results of New Horizons Pluto Flyby dramatically changed what we knew about Pluto and Kuiper Belt. And it seems subsidiary consequences of exploring Pluto will keep on transforming scientists knowledge.
While New Horizons Spacecraft was heading towards its new target(s) in second half of 2015, researchers were already excited to learn more about other worlds in Kuiper Belt. Our next article will be there for New Horizons Probe’s journey to deep Kuiper Belt, Ultima Thule and its current course.
03-2019, “Fifty Years of Exploring Pluto: From Telescopes to the New Horizons Mission”
D. P. Cruikshank, S. A. Stern, H. A. Weaver, C. B. Olkin, L. A. Young, K. Ennico, F. Bagenal, New Horizons Science Team
Link: ( https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20190002167.pdf )
07-2015, “Pluto-bound Probe Faces Its Toughest Task: Finding Pluto“
07-2015, “Pluto spacecraft temporarily loses contact with Earth“
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