“The New Horizons Spacecraft” is the third article of our New Horizons series: “New Horizons Mission – Its History, Route, Success and Future”.
On previous articles, we mentioned about historical background and scientific goals of New Horizons Mission.
This article will include details about New Horizons Spacecraft, its operational systems and the scientific payload.
Below you may find a list of links for all article in series. The link for the last article will be here as well once it is published.
- New Horizons Mission – Historical Background
- New Horizons Mission Goals
- New Horizons Spacecraft
- New Horizons Mission From Launching to Pluto Encounter
- New Horizons Pluto Flyby
- New Horizons Mission Ultima Thule Flyby
A pioneer mission to explore far worlds and reveal many facts about Pluto, Charon and possible other candidates from Kuiper Belt was on the way. As article #2 demonstrates, the mission was planned while the design and construction of the New Horizons Probe was ongoing.
On this article we do not intend to mention about design and construction process. We will directly present New Horizons Spacecraft’s structure and devices, and simply reveal how it functions.
New Horizons Spacecraft
The New Horizons Spacecraft was designed and built in John Hopkins University Applied Physics Laboratory (JHUAPL). The scientific instruments of the project were provided by Southwest Research Institute, JHUAPL, NASA – Goddard Space Flight Center, University of Colorado, Stanford University and Ball Aerospace Corporation.
In terms of its size, New Horizons Space Probe is mostly defined as “piano-sized” since it also looks pretty much like a classical piano. It is 0.7 x 2.1 x 2.7 meters wide where a 2.1 meters wide antenna is attached to the body. It totally weighs 478 kg including the fuel, scientific instruments and other weights onboard.
Major cargoes to be carried by the spacecraft can be listed as follows:
- Propellant (hydrazine) – 78 kg
- Science payload – 30kg
- RTG and balancing tools – 50kg
- Minor memorial items including the ashes of Clyde Tombaugh who discovered Pluto in 1930.
New Horizons Probe – An Overview
We can classify the devices and systems of New Horizons Spacecraft in two groups: Scientific and Operational equipments.
The Scientific Equipments of the probe are:
- An image spectrometer for analyzing atmospheric and surface compositions
- A camera for analyzing visible and infrared wavelengths for mapping surface features in best resolutions
- A telescopic camera for more enhanced resolution
- A particle spectrometer for atmospheric particle detection and analysis
- A space-dust particle measurement detector
- 2 pcs radio science devices for performing basic examination of atmosphere, size and temperature of Pluto and Charon together with possible future targets
While the Operational Equipments are:
- Spacecraft body and tanks
- Thermal protection equipment including heaters, insulation sheets and heat vents
- 16pcs thrusters for trajectorial corrections
- Antennas for communication
- Star and Sun tracking cameras&sensors for navigational purposes
- Radioisotope thermoelectric generator (RTG) for producing power onboard
- Propulsion units
- Digital communication & power equipments
- Data handling and storage units
And here comes the basic information about the above list.
New Horizons Spacecraft – Scientific Payload
On previous article of the series: New Horizons Mission Goals, we mentioned about the objectives with different priorities. On below list you can track the distribution of tasks for the specific science instruments and eventually find more details.
Alice is a UV spectrometer developed by SWRI for analyzing atmospheric composition, temperature on various altitudes and surface features of planetary targets. With Alice, it was also planned to observe atmosphere of Charon (if Charon had one).
Alice was designed to work on two different modes: Air glow mode to detect direct UV emmissions of atmospheres and occultation mode to use refracted star lights from target atmospheres. In this case, the major star occultation plan was with the Sun.
This equipment is the basic visual imaging unit however it has many additional duties too.
Ralph was built to perform detailed surface mapping and surface composition & temperature calculation of all visited objects. It was also designed to process best real images of Pluto, Pluto moons and other visited objects. In addition, it was assigned to calculate fine-tuned proper diameter of objects and orbit calculations on Pluto System. What is more, there is another duty of this device: Ralph was going to capture the atmospheric haze of Pluto and observe Pluto rings if existed.
Ralph consists of Multispectral Visible Imaging Camera (MVIC), where name implies what it means; and Linear Etalon Imaging Spectral Array (LEISA) which is an infrared compositional spectrometer.
It is developed by Ball Aerospace Corporation, NASA Goddard Space Flight Center and SWRI.
Radio Science Experiment (REX)
Built by JHUAPL and Stanford University, REX had a special duty: To capture the powerful radio signals released from Earth towards Pluto’s atmosphere. We might call this a “radio signal occultation”. Hence REX was going to collect the signals right after the closest Pluto flyby when New Horizons Probe is positioned “behind” Pluto and perform a seperate atmospheric analysis.
This was a unique task and was to be performed for the first time. Normally the spacecrafts use the opposite method: Create the signal from spacecraft and send it to Earth through planet atmosphere.
Side tasks of REX were to perform basic day&night temperature measurements of encountered bodies, their mass&diameter measurements and to detect trajectorial changes of New Horizons Space Probe.
Long Range Reconnaissence Imager (LORRI)
LORRI of JHUAPL is basically an imager consisting of a digital camera with a big telescope used for detecting visible light. It was intended to be used for both obtaining clearest surface photos and for navigational purposes in case a minor trajectorial correction needed.
LORRI was planned to capture the closeby images from Pluto surface, Charon surface and others. On Pluto, pre-calculated surface resolution of images obtained from LORRI were about 50 meters. The world had to be ready for UFO imitations of conspiract theorists!
Solar Wind At Pluto (SWAP)
SWAP was designed by SWRI for measuring atmospheric escape rate and the solar wind effects on Pluto’s atmosphere. This is another important task for understanding Pluto’s atmosphere, its phases and possible disappearance in different seasons.
Pluto Energetic Particle Spectrometer Science Investigation (PEPSSI)
PEPSSI is another device developed by JHUAPL for atmospheric measurements. Its duty is to measure atmospheric composition, but in a different way: It detects the materials reaching to its detector which escape from the atmosphere and get ionized via solar winds. Those calculations also helps calculating the atmospheric escape rates.
Student Dust Counter (SDC)
This was the first scientific instrument ever on such a mission and it was developed by the students of University of Colorado.
SDC’s duty is to calculate the number and sizes of tiny dust particles during complete journey and perform the best estimations of the collision rates; especially on far corners of Solar System.
New Horizons Spacecraft – Operational and Navigational Systems and Instruments
Now let’s see details about New Horizons Probe’s operational equipments.
The carcass of New Horizons Probe is a cylindirical aluminum where the RTG, tanks; side, top and bottom panels together with the launching interface are mounted. The scientific and operational equipments are mostly mounted to above mentioned panels.
In addition, extra weights are fixed on specific positions in order to balance the center of mass based on pre-launching spin tests.
New Horizons Spacecraft had to be designed to work in coldest spots of the Solar System. Hence the thermal condition of spacecraft equipments needed an extra care.
The body of the probe is in a “thermos bottle” form and completely covered with an insulation material (the golden color) which efficiently traps the excessive heat from the radioactive decay.
Temperature sensors placed on different locations of the spacecraft provides the ambient temperature data. With that knowledge, additional processing units run to automatically adjust the operating temperature within 10 and 30 ºC.
If the temperature is below the limits, independent heaters dedicated to specific instruments or internal ones start working for increasing the temperature above minimum limit. This is a possible case for most of the journey.
If the temperature is above the limits, dedicated louvers are opened for reducing it. This can only happen while the probe is close to Earth.
The power system on New Horizons Spacecraft Consists of below units:
- Radioisotope Thermoelectric Generator (RTG)
- Shunt Regulator Unit (SRU)
- Power Distribution Unit (PDU)
Radioisotope Thermoelectric Generator (RTG)
Since the spacecraft was designed to operate far from Sun for a remarkably long time, RTG power was a wiser choice instead of solar power.
On the other hand, decay of radioactive element means decay of power. That’s why the power usage is one of the essential issues on RTG-powered spacecrafts.
New Horizons Probe began its journey with 247 Watts and the output power of RTG decreased constantly while the minimum power needed for the spacecraft to operate is 160 Watts. According to current calculations in 2019, the spacecraft still has enough fuel to subsist more than 10 years.
RTG creates the power needed for the probes’ scientific, operational and navigational equipment and less than 10kg of Plutonium dioxide is used as the fuel for this process. The spacecraft does not include any kind of batteries for power storage.
Shunt Regulator Unit (SRU)
Total output thermal power of RTG system on New Horizons Probe is 3.948 Watts; however the power for usage is limited with 250 Watts. That’s why the accessive heat is either used internally or discharged to the outer space. That bulge is filtered out through the cooling tubes for preventing overheating within the spacecraft.
So that’s what SRU does; receives the power from RTG, regulates it for onboard usage. And that provides the discharging of surplus out. This brings an additional advantage for the Thermal System: Some part of this accessive heat is used for onboard heaters if necessary.
Power Distribution Unit (PDU)
PDU, as its name implies, receives the regulated power and distributes it to spacecrafts subsystems, sensors and devices.
Fellows with electrical knowledge should be familiar with the names. 🙂
Propulsion System and Thrusters
New Horizons Probe has no reaction wheels. Instead of them, tiny thrusters handle the propelling job. Those monopropellant hydrazine thrusters basically use the power created by the reaction of hydrazine molecules with aluminium oxide.
Four of the thrusters are capable of 4.4 Newtons of buoyancy for trajectorial corrections. Remaining 12 pcs have the capacity of 0.8 Newtons and used for similar operations in groups. The combination of thruster usage varies in time according to assigned maneuvering job.
Trackers and Sensors
On New Horizons Spacecraft, there are two pieces Star Trackers, two Fine Sun Sensors and Sun Pulse Sensors for accurate determination of coordinates and directions. Those sensors are also connected to RIO‘s for transmitting instant position of the probe to CDH.
In every second, ten different images of space is forwarded to CDH. Those images are automatically checked against the pre-installed star maps consisting of 3000 stars for calculating momentary coordinates.
For proper calculation of spin rates, Sun Pulse Sensor sends a pulse to CDH every time the Sun is on the frame. On the other hand, Fine Sun Sensors are used to locate the Sun with high accuracy in case the spacecraft’s antennas need to be directed towards Earth.
In addition to above, there are two gyros onboard for detecting the amount of acceleration on 3-dimensional axis.
Command and Data Handling System (CDH)
Command and Data Handling System is the main unit on New Horizons Probe for data handling and execution of almost all scientific and operational processes. CDH includes two seperate and completely redundant Integrated Electronics Module(IEM)‘s and sends processed commands&receives the data via Remote Input Output (RIO) Units.
The system includes modules for data processing, storage and transmission together with timekeeping and interpretation of overall condition of all units as well as internal power regulation.
Pulse Oscillators and Timing
Another important thing to mention for a travelling spacecraft is how you count the time.
This is very similar to GPS Systems: the satellites orbit the Earth from a distance, and the elapsed time on the satellite is not the same with the time on Earth. Without calculating the gravitational bending effect of Earth over space time, we wouldn’t have GPS Systems at all!
And when it comes to New Horizons Spacecraft, it gets more complicated. Because the relative coordinate of the spacecraft changes all the time: From launching till the end of the mission.
For that reason, all time-related coding needs a time signature (elapsed Earth-Time for the mission). Thus the proper time is calculated via ultra sensitive “Pulse Per Second” oscillators onboard.
Communication System and The Antennas
The antennas of the spacecraft are used to send the signals to and receive the signals from bases on Earth. Those signals are processed via complex switches and communicate with IEM‘s.
The spacecraft has four antennas onboard: 2 Low gain, 1 mid gain and 1 high gain. While the big and high gain antenna is used for all communications with land, lower gain ones are there as backup antennas and mostly for near-Earth communication.
How Is New Horizons Spacecraft Operated? How Does It Work?
New Horizons Space Probe is strictly pre-programmed for autonomous operation. However, for such a long and distant journey, minor deviations and additional thrusts are inevitable.
So let’s continue with the operational details and see in which conditions the spacecraft might need an intervention.
During the normal autonomous operation of the spacecraft, CDH decides what to do in general. And in terms of navigation, that means as follows:
- The spacecraft uses its thrusters on pre-decided moments for maintaining a stable route.
- New Horizons Spacecraft is designed to travel with a stable spin rate unless there is a scientific detection situation (like Pluto encounter, Ultima Thule encounter and close Jupiter flyby)
- The information about coordinates and speed of the spacecraft are continuously gathered by CDH from star sensors, gyros and REX (Smart move to use science payload for navigation purposes too!). And based on those data, trajectorial declinings calculated accordingly
- Automatic corrections via running relevant thrusters performed for correction of the course.
- Communication with Earth is done, system status and relevant data is sent to Earth via desired antenna.
- No additional operations performed if there are no emergency situations or no corrective commands from Earth.
In case of an emergency or fault, 126 pcs (the initial number of codes where new codes can be added) pre-programmed codes are scanned and the automatic correction is executed for that specific situation.
If the problem was not solved, the spacecraft goes into a safe mode for safely sending an alarm signal to the operators on Earth.
For a very long time, the spacecraft is planned to work in hibernation mode for power saving. During that mode, only the system status information is transmitted to Earth via the low-gain antenna.
As a part of above navigation plan, New Horizons Probe has three different and “major” attitude modes:
- 3-axis mode
- Active spin mode
- Passive spin mode
The spacecraft is in Passive Spin Mode during most of its flight. For fuel-saving concerns, this mode will be applied for about 8 years of flight between Jupiter and Pluto. Together with avoiding minor trajectory / spin corrections during this hibernation mode, monitoring and fuel consumption will be limited to minimum.
The Active Spin Mode is activated if the spin-rate and orientation of the spacecraft needs to be varied significantly.
The most important but least used mode is the Three Axis Mode. This mode will only be active when there is a scientific incident. This is when the detectors and spectrometers work and when the imagers capture beautiful pictures we know!
During this mode, the probe can either be balanced at zero-spin for focusing a target or maintain a fixed single-axis spin for the desired scanning operation.
Handling the Scientific Payload Data
All scientific instruments of New Horizons Space Probe are directly linked with the Instrument Interface Units of IEM‘s.
Since all the science instruments are non-rotative, whenever an experiment or measurement is carried out; “spacecraft itself” needs to be precisely rotated and a directional balance needs to be provided. And that explains the importance and the required sensitivy of 3-axis mode.
OK. The spacecraft is oriented properly, the detectors got the data and sent them to CDH. How to store this data and send it to Earth?
The Solid State Recorders (SSR) of CDH are responsible for this job. Acquired data is sent to SSR’s, zipped and stored until the relevant data package is successfully received by NASA’s Deep Space Network. The high gain antenna is used for such prioritized scientific data delivery as the distance between Earth and New Horizons Space Probe becomes enormous during the major measurements.
New Horizons – Launching Device
It was decided to use Lockheed Martin Atlas V-551 and a Boeing STAR-48B solid-propellant rocket for the launching. It was a 59.7 meters high and 575 tons weight mechanism with full fuel. The duty was to carry the spacecraft, release it from Earth till sufficient altitude and maintain its preliminary speed for Jupiter bouncing. The job was going to take about 50 minutes and the launching equipments was going to detatch and fulfill its operation at that point.
This rapidity of launching together with the designed overall speed was intended to name New Horizons Space Probe as “Fastest spacecraft of its era” in 2005.
The Article Is Over
But the journey is just beginning! We came all the way to launching moment through our article series.
The epic fly was on the way to go! Did it go well? Did the team and the spacecraft successfully perform all tasks?
Our next articles will serve you the answers: Next article(#4): New Horizons Mission From Launching to Pluto Encounter will cover the journey till Pluto. Next article of the series includes the Pluto flyby and its scientific outcomes. Last part of the journey will follow soon, tracking the days of Ultima Thule and Kuiper Belt.
Final words as usual: Coming Soon & Stay Tuned!
- John Hopkins University Applied Physics Laboratory (JHUAPL)
12-2005, “New Horizons Press Kit“
Link: ( https://www.nasa.gov/pdf/139889main_PressKit12_05.pdf )
- 2007, “The New Horizons Spacecraft“
Glen H. Fountain, David Y. Kusnierkiewicz, Christopher B. Hersman, Timothy S. Herder, Thomas B Coughlin, William T. Gibson, Deborah A. Clancy, Christopher C. DeBoy, T. Adrian Hill, James D. Kinnison, Douglas S. Mehoke, Geffrey K. Ottman, Gabe D. Rogers, S. Alan Stern, James M. Stratton, Steven R. Vernon, Stephen P. Williams
Link: ( https://www.boulder.swri.edu/pkb/ssr/ssr-fountain.pdf )
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