How the PlayStation-powered New Horizons probe flew to Pluto

Sense and Sensitivity

And there’s more. “Pluto is over 30 times as far from the sun as the earth”, Jerram said, “so the sun will appear 30 times smaller. This means there is 1,000 times less light. Also, New Horizons will go past Pluto at 30,000 miles per hour, so very high-sensitivity imagers are required.”

This is evident if we look at the size of the sensor. Commonly, the CCD in a cheap consumer camera measures 6.17×4.55mm, which works out at 1.75 square microns per pixel for a 16-megapixel camera. By way of contrast, the pixels in Ralph’s CCD are almost a hundred times larger.

Moving on from the CCD, the other element of a camera that has a major impact on image quality is the lens, and here the figures for the New Horizons’ instruments, and LORRI in particular, make interesting reading. LORRI has a field of view of 0.29 degrees, compared to around 40 degrees horizontally for a camera’s lens zoomed in to the point at which the scene appears normal. Dividing one by the other gives the amount by which the lens magnifies the image, so this would be a massive 138 times. This is far more than any regular camera zoom lenses, but that’s just a start.

^ After spending most of its voyage in hibernation, New Horizons is woken up for its Pluto encounter in December 2014. Radio signals take four and a half hours to reach Earth

A large CCD needs a large focal length lens to achieve the same magnification factor of a camera with a smaller CCD. Because of the need to work in low-light conditions, LORRI’s CCD is much larger than in most digital cameras, again pushing up the focal length. What’s more, as the focal length increases, so must its diameter if its light-gathering capacity is not going to be jeopardised.

The bottom line is that LORRI has a huge and vastly expensive 2,630mm focal length lens with an aperture of 208mm. While for simple lenses we might think of the aperture as one and the same as the diameter, LORRI’s lens requires the largest optical element to be 758mm in diameter and the whole assembly weighs 5.64kg. When we add in the rest of LORRI, we end up with something that weighs about as much as 60 compact digital cameras and we can’t begin to imagine how many times more expensive.

Communication System

When design work on New Horizons started in 2001, the fastest Wi-Fi available, 802.11a, topped out at a theoretical 54Mbit/s. However, because space probes and their ground-based Earth stations don’t have to adhere to standards-based communication protocols, it might seem reasonable to believe that much higher rates of data throughput would have been possible.

The International Space Station (ISS) has several communication links each offering 300Mbit/s. There’s an important difference between the ISS and New Horizons, though, that will be all too familiar to users of home Wi-Fi networks. As you get further from your access point, the speed decreases dramatically with the decreasing signal strength, which is why it doesn’t reach down the bottom of your garden. Even today’s top-end 802.11ac routers will see a halving of their throughput by increasing the range to just 10 metres.

New Horizons, on the other hand, is 480 billion times further away than your the bottom of your garden. It would therefore either need to have been fitted with some very powerful kit or data throughput will be seriously slow. It’s not surprising to learn that both are true.

^ New Horizons sees Pluto for the first time in January 2015. Still 200m km away, images aren’t yet as good as those from the Hubble space telescope

X marks the spot

In common with many deep space missions, New Horizons uses the X band for radio communication as it offers a good compromise between atmospheric absorption and low noise. The frequency is about 8GHz, which isn’t too different from the 2.4GHz and 5GHz used in Wi-Fi equipment. There the similarity ends, though. The maximum permissible transmit power of Wi-Fi equipment in the UK is a tenth of a watt on 2.4GHz or one watt on 5GHz. In fact it’s often a bit less, because the regulations refer to the effective radiated power, which also takes the performance of the antenna into consideration. New Horizon’s transmitter, on the other hand, outputs 12 watts.

While a 12- or 120-fold improvement is not inconsiderable, it pales into insignificance when we compare the antennas. Typically, Wi-Fi equipment has small antennas that are more or less omni-directional so have a gain – that is, the amount by which the antenna magnifies the signal – of perhaps two decibels (dB). New Horizon’s antenna is a 2.1m dish with a gain of 40dB, and the antennas in NASA’s deep space network that are used to communicate with distant space probes are massive 70m dishes with a 74dB gain. Since both the transmit and the receive antenna have an impact on the strength of the received signal, this setup provides approximately 108dB more gain than the link between a Wi-Fi access point and a laptop.

^ Even with NASA’s huge 70m antennas, the signal received from New Horizons was tiny

When you consider that the decibel is a logarithmic method of measurement, and each additional 3dB corresponds to a doubling in the power, you get some idea of how huge this difference really is. Taking the increased transmit power and the higher gain antennas into consideration, the link from Pluto to Earth uses the equivalent of a thousand billion times more power than a typical Wi-Fi link.

The reason such a huge power level is needed is because power decreases with the square of the distance. So, double the distance and the power drops to a quarter, at 10 times the distance it’s a hundredth, and so on. Going back to that Wi-Fi link at a range of 10m, and comparing it with the 4.8 billion kilometres to Pluto, we find that the signal strength would be 230,000 billion billion times less. Now that thousand billion times increase in effective power doesn’t look too excessive after all.

Bit part

The received power is so low that, during the Pluto encounter, New Horizons was designed to return data at a minimum speed of just 600 bits per second. Note that we’re talking of bits per second, not kilobits or megabits, so this is at least a million times slower than an 802.11ac Wi-Fi link. Returning a single image of Pluto to Earth takes several hours, and NASA is having to prioritise the remaining data for transmission, a process that will take months.

As well as the data rate being miniscule, the huge distance to Pluto affects another important characteristic of a communication channel: its latency. While the data transmission rate is sometimes expressed as speed, latency genuinely is the speed, in this case the time it takes for a radio signal to travel from Earth to Pluto. Even though radio travels at the speed of light – around 300,000 kilometres per second – signals from New Horizons are currently taking four-and-a-half hours to reach Earth.

Chris DeBoy, the New Horizons communications systems engineering lead at Johns Hopkins University, told us how such signals were still able to return useful information to mission controllers on Earth. For a start, more advanced coding schemes were used than those in, for example, the familiar Wi-Fi networks.

^ After almost 14 years in planning and in transit, New Horizons has reached its target and scientific exploration starts in earnest

“New Horizons and NASA’s Deep Space Network have taken advantage of developments in advanced coding techniques to maximize the downlink data rate,” he said. “New Horizons uses a rate 1/6 Turbo code for forward-error correction. Turbo codes can perfectly reconstruct the transmitted message at the receiver when the signal is seemingly buried in noise, and they approach the best theoretical code performance achievable.” That ‘best’ is defined by a complex piece of maths known as the Shannon Limit, and it’s not going to get any better any time soon.

DeBoy explained how, despite the sophistication of the coding scheme, the data throughput is much lower than most terrestrial systems, but there are ways of getting round this. In keeping with the philosophy of providing redundant circuits, the communications system has two transmitters and two Travelling Wave Tube Amplifiers (TWTAs) but, if all’s well, it’s possible to use them together.

“It’s possible to nearly double the downlink rate by powering both TWTAs, and we do it often,” he said. “A special signal splitter connects each radio transmitter to each TWTA, so if a TWTA were to have a problem, we could still use either radio. Each TWTA is connected to a specific input on the high-gain antenna. One TWTA feeds the port that transmits right-hand circular polarisation from the high-gain antenna, the other feeds the port that transmits left-hand circular polarisation. The polarisation of a radio frequency signal describes how the electromagnetic wave (specifically the electric field) behaves as it moves through space. Right- and left-hand circularly polarised signals are independent of each other and so it’s as if we have two separate channels for transmission.”

However, this isn’t always possible. “We can’t use this technique all the time, however, because the TWTAs are two of the more power-hungry components on the spacecraft. Sometimes there is not sufficient power to turn both TWTAs on during a downlink. But during the playback time after Pluto encounter, the dual-TWTA downlink will be routinely used to get the science data and pictures behind exciting new discoveries back to Earth as quickly and reliably as possible.”

^ In the future, New Horizons will head further into the Kuiper Belt, a ring of small objects the team hope to explore, subject to approval and funding

Sophisticated as New Horizon’s communication system may be, like the computer system, it relies on technology that may seem outdated. However, in this case, that isn’t because the probe has taken so long to reach its target. Instead, it’s testimony to the fact that the demands of space travel place different demands on equipment and, in some cases, that means that the old ways are sometimes still the best.

The technology in question is the Travelling Wave Tube Amplifier, or TWTA, we’ve already encountered. It serves the same purpose as the power transistors in most data communications equipment, namely to amplify a tiny signal to an adequate power level to be fed to the antenna for transmission. While the term TWTA might sound high-tech, it’s actually a rather specialised type of valve: the red-glowing glass tubes that adorned radios before the invention of the transistor in 1947 and the development of transistor radios in the 1950s and 1960s.

Onwards and Upwards

Its encounter with Pluto and Charon is New Horizons’ pinnacle of achievement, but it doesn’t represent the end of the road for the space probe. It now heads into the Kuiper Belt, a huge area of rocky material extending well beyond the orbit of Pluto. If funding allows, it will be tasked with searching for other dwarf planets, many of them as yet unknown.

This brings us to Dawn. This may not have captured the public imagination in the same way as New Horizons, primarily because few people other than astronomers have heard of its destination, but this spacecraft also visited a dwarf planet, called Ceres, this year. NASA’s Dawn probe went into orbit round the dwarf planet – the largest object in the asteroid belt between Mars and Jupiter – in March, returning images of its crater-potted surface and mysterious bright objects. It appears that 2015 truly is the year of the dwarf planet.