The exploration of space has been a two-way battle. Not between the Russians and Americans, but between humans and robots. The race to the Moon was a victory for the humans. But it is notable that the humans have been in retreat since. We no longer go beyond low-Earth orbit: we could not go back to the Moon even if we wanted to. In fact, neither the US nor Europe currently have the capability to put people in space: only Russia and China do. Robots have gone from strength to strength, and rule the Solar System. Robots have landed on Mars, Venus, Titan, the asteroid Itokawa (a crash landing), and the comet Churyumov-Gerasimenko (two bounces and a crash), and will land on Churyumov-Gerasimenko (again) at the end of September. The battle for space between humanity and robottica has been fought – and lost.
The advantages of robots are clear. They are much lighter than people (a very important fact for space travel) and have no need for food, drink or air. They can last much longer (the two Voyagers, launched in the 1970’s, are still going strong). There are disadvantages. Robots are not as versatile as people and can’t fix hardware issues. They can be excruciatingly slow (for instance the rovers on Mars): it is like sending snails on exploration. They are functionally blind: robots cannot see the unexpected. Like any computer, they do what you tell them, not what you want. If we could have had a human on Mars, we would have learned much quicker and much more. Hopefully one day we will. But first we need to re-learn how to get people in space.
New Horizons was a triumph of the robots. How did it work? And how did it manage to get to Pluto so fast?
New Horizons was launched on Jan 19, 2006, on a Lockheed Atlas V-551 rocket (with 5 attached boosters), at third attempt. The spacecraft itself weighed just under 500 kg; the rocket was almost 600,000 kg. It takes an awful lot of fuel to travel in the solar system! Still, there are ways to make it sound more respectable. Over the distance to Pluto, New Horizons managed a respectable 25,000 mpg. This level of fuel efficiency is apparently beaten only by racing pigeons (which manage 60,000 mpg) and by Volkswagen diesels when being tested in the lab. The 500-kg weight was low for such a mission, but this allowed it to get to Pluto on a fast, almost direct track. Heavier satellites can’t be launched as fast and need to do a number of fly-by’s of Venus and Earth to pick up speed.
The launch window lasted for two months, but these were for different trajectories. The earlier, January launches aimed at Jupiter. February launches would instead aim for a direct trajectory to Pluto. These were actually slower. The eventual arrival date, July 4, 2015, was the earliest one possible. February launches would have gotten to Pluto as late as July 2020. That delay would in itself not have been too bad, after all we are used to such exploration missions lasting decades. But the risk of spacecraft failure increases with time, due to ageing of components and the harsh radiation environment. Faster is safer.
New Horizons carried 7 instruments, including one build by students, with names such as as Alice and PEPSSI. The instruments weighed 30 kg all together. (A lot of the rest of the weight is the communication dish.) Everything ran from a 200 W power supply. New Horizons lacks a battery and needs a continuous power supply, although there is a capacitor bank to damp out load variations. The system is run at 30 V. The power supply uses the heat from the decay of 11 kg of plutonium; NASA missed a green trick by not stating that it was committed to taking plutonium back to Pluto. The nuclear fuel was stored in the form of plutonium dioxide, and incorporated in an inert ceramic material. Nuclear power is really the only solution for long-distance space travel. Solar panels do not work well far from the Sun: to get 200 W from a solar panel at Pluto, you would need a panel of 50 by 50 meter. (Maybe one day we can create a solar sail with solar cells.) The main risk of using nuclear material is in launch incidents. The ceramic material is designed to be fire resistant, to break into large particles (better than microscopic ones), and to not dissolve into water.
The radiation-hardened CPU runs at 12MHz, with on-board solid-sate storage capacity of 64 Gb. All electronics, including the CPU and storage, is duplicated, for reasons of redundancy: if one fails, there is still a fully working system. The spacecraft can autonomously switch between the redundant systems, based on internal fault checking. There are 18 thrusters for course corrections and pointing, also twice as many as needed.
Data is send to Earth through the high-gain antenna, which has a beam of only 0.3 degrees. The whole spacecraft is turned to point the antenna accurately at Earth, something that obviously was not possible during the Pluto encounter! There are two transmitters, using separate polarizations, where each one uses 30 W and transmits 12 W. Originally, only one transmitter was planned to be used at a time. However, during the mission it was found that both could be used simultaneously which doubled the data rate. There is also a medium gain antenna, which does not need to be pointed at Earth as accurately. This can send but is mainly used as the receiver for commands from Earth. If the spacecraft loses contact with Earth, it goes into a safe mode, points at Earth, and waits for instructions. If that doesn’t work, it points at the Sun: at that distance, the Earth will be within 2 degrees of the Sun.
The data rate from Pluto per transmitter is less than 1 kb/s. Without compression, it would take 100 million seconds to dump all storage to Earth, or about 4 years! In practice, it took about a year to retrieve all data taken during the passage of Pluto. At first, highly compressed images were send to ensure we got something in case of failure of the craft. Full resolution images were transmitted later.
And here is another bit of curiosity. All data was transmitted at the speed of light. It takes light just over 1 second to get to the Moon, a distance which took the Apollo astronauts (back in the days we could still get there) 3 days. The same light takes 4.4 hours to travel from Pluto to Earth! The Solar System is that big. The Apollo astronauts would by now only have been half way to Pluto, and perhaps getting slightly bored with their 1960’s entertainment system and reruns of I love Lucy. Let’s not mention going to the stars: the nearest one is 5000 times further than Pluto. Pluto is 15,000 times further than the Moon, which is a similar ratio. New Horizons would take 45,000 year to get there. Neil Armstrong would follow 570,000 year later. A big step for a robot, an impossible one for humanity. The robots win.
New Horizons was launched with a speed of 16 km/s. (This is the velocity after it has escaped from Earth’s gravity which required 11 km/s.) For comparison, missions to Mars are launched at 2.5-3 km/s, although it should be noted that they need an additional velocity change on arrival, which New Horizons did not do. As it moved away from the Sun, the velocity slowly decreased. It arrived at Jupiter on Feb 28, 2007, just over 1 year after launch. Juno, the satellite currently in orbit around Jupiter, took 4 years to get there.
Rockets are actually a poor way to travel. The problem is that you need to carry your fuel with you, which means accelerating not just the payload but the fuel as well. The push comes from the speed of the exhaust out the back end of the rocket, which for chemical rockets happens at 3-4 km/s. If you need to go faster than this, rockets become very inefficient. It is like propelling your car by throwing things out of the back window. Nuclear rockets would have much higher exhaust speed and be much better, but environmental concerns are prohibitive and the test ban treaties rightly forbid them in our atmosphere. Ion propulsion drives are also faster but can only move small loads. For now, we are stuck with chemical rockets and this is the main reason why exploration of the Solar System is tortuously slow. Something being ‘rocket science’ is not a compliment.
Gravity assists are one way to improve on rockets. For New Horizons, the Jupiter fly-by increased the speed by 4 km/s. If you are wondering how this can work, imagine that the spacecraft is in orbit around Jupiter. Jupiter is orbiting the Sun at 13 km/s. If the spacecraft orbits Jupiter at 5 km/s, than at the point in its orbit where it moves against the direction of Jupiter, the total speed is (13 – 5) = 8 km/s. Half an orbit later the spacecraft moves in the same direction as Jupiter, and now its total speed is (13 + 5) = 18 km/s. The trick is to arrive at Jupiter at the first point, and leave it at the second, effectively a slingshot where your direction changes by 180 degrees (this would indeed be possible but in practice the change in direction is rather less). You will have gained 10 km/s speed for free. It is not entirely free: Jupiter pays the bill and slows down a bit (the laws of physics are unlike Irish tax laws: they apply to everyone and insist on balancing the books), but because Jupiter is so much heavier its change in speed is immeasurably small. It is like Bill ‘Jupiter’ Gates paying for your coffee.
For New Horizons, the Jupiter fly-by shaved several years of the journey time. Normally, gravitational slingshots are used to makes higher payloads possible: you can get away with lower launch speeds because you pick up the missing speed later. For instance, Cassini was a massive 5700 kg and could only get to Saturn using two fly-bys of Venus, one of Earth, and one of Jupiter. It started its journey to Saturn by heading the other way.
After the Jupiter encounter New Horizons was put into hibernation. The instruments and electronics were mostly turned off, although one receiver was left on. Once a year the spacecraft was woken up for a 50-day systems check. The hibernation reduced wear and tear on the internal systems, but also saved money back on Earth! This was the first time electronic hibernation was used in deep-space missions. The European Space Agency used it with Rosetta, but although Rosetta was launched earlier, it started hibernation later than New Horizons.
New Horizons approached Pluto from the south, which was the illuminated hemisphere (naturally, as New Horizons came from the direction of the Sun). The northern cap was at night during the approach, and we only got lower resolution images of it taken a few days earlier, at considerable distance. Remember that Pluto’s day last 6.4 Earth days. The full sequence of actions, from 7 days before the Pluto encounter to 2 days after, was programmed into the satellite, together with as many as 250 contingencies. The time delay of 9 hours in communication with Earth was too much to allow for any interaction. The sequence was finalized a long time before the encounter. This caused problems when during the New Horizons cruise, more moons were discovered around Pluto. There was little flexibility left in the sequence, and it was not possible to change the sequence to allow for good images of these moons.
New Horizons at Jupiter: volcanoes and storms
The fly-by of Jupiter was a nice bonus: why waste a good opportunity to do accidental science? The instruments were turned on and aimed at Jupiter and three of its major moons. What did we learn?
One of the most exciting findings was that of a major eruption of Io’s Tvashtar volcano. The plume was a staggering 350 km high. It was in the same region where the Galileo satellite saw an incandescent fire fountain, in 1999. A good place to stay well away from. The trajectories of the ejecta showed that the eruption plume started out gaseous, and ash condensed higher up in the plume. The Tvashtar plume was very sulfur-rich. Some of Io’s sulfur escapes Io altogether and reaches the surface of Europa. In the league of volcanic sulfur polluters, Io puts Iceland to shame.
Changes on the surface indicate that there were at least 19 eruptions on Io between 2007 and the last images taken around 2000. In some places the structure of the old surfaces could still be seen through the new ones, suggesting that the deposits are not very thick. One new lava flow is 130 km long! Concentric rings indicate explosive, ashy ejecta. Hot spots on Io’s surface show several active regions: the high temperatures show that the lavas are basaltic. One hotspot called East Girru is located on a linear feature and is offset from the Girru volcano: this is probably a fissure eruption. It is interesting how many of the characteristic volcanic features on Earth also occur on this alien world.
On the non-volcanic front, New Horizons discovered polar lightning on Jupiter. Lightning had been seen elsewhere on the planet, but this was the time lightning close to the poles had been seen outside of Earth. The lightning was located in or just above the water cloud layer, within strong convection. The energy of the polar lightnings, typically 2-3 GJ each, is very similar to that near the equator. This shows that heating by the Sun is not important in driving lightning on Jupiter. The energy that drives convection in its atmosphere instead must come from within the planet. This is not a surprise, but a nice confirmation. New Horizons also found that the cloud thickness near the equator was much less than it had been around 2000. This may be related to the current fading of the Red Spot which is probably due to changes in the visible clouds rather than a change in the storm itself.
The rise of the robots
Robotic spacecraft have been spectacularly successful. We would know next to nothing about the outer Solar System without them. New Horizons shows what a well-designed, autonomous, robotic spacecraft can do. But it is also limited, by the available instruments, the lack of something like the human eye which can see the unexpected, and in-build inflexibility.
For Mars, our goal should be to put humans on it, and for more than just a ‘wave the flag’ mission. A two-year stay on Mars, with ample local transport to go off and explore, would be a science power house, putting any robot to shame. It would also be extremely motivational back on Earth, an inspiration for a generation. The cost? At measly 100 billion dollar – a lot, but no more than the cost of one irresponsible bank, four BPs, or a few weeks of tax avoidance by multinationals. The world can do it. The two richest people on Earth could do it. What on Earth are we waiting for?
Beyond Mars, the future is with the robots. We either live with their limitations, or we develop better artificial intelligence to aid them.
This was part 2 of the Pluto trilogy (see here for part 1). To be continued