Fly-by missions—what is the point when we have the technology to go into orbit?

Fly-by missions—what is the point when we have the technology to go into orbit?
Artist’s impression of Giotto approaching a comet. Credit: Mirecki/wikimedia, CC BY-SA

New Horizons' fly-by of Pluto and its moons is the latest in a historic string of missions to objects in the solar system. But given that a fly-by lasts for just a short time, how much can we really get out of it? There's no doubt that the mission will yield a great deal of interesting data, but surely more would be gained if the spacecraft could go into orbit for a number of days or actually land on the surface and take physical samples.

New Horizons is hugely important because it is giving us a first glimpse into the unseen world of a third class of objects in the Kuiper belt – the building blocks of the outer solar system, located beyond the terrestrial and gas-giant planets. Fly-bys such as this are very exciting as they provide just one chance for unique measurements at the target.

While we are only at the very first stage of exploring Pluto and its moons, the fly-by will provide the foundations for future missions. Indeed, a fly-by is the first in the classical four stages of solar-system exploration and is followed – in this order – by an orbiter, a lander and the return of a sample from a body (marked 1-4 in the table below).

Lessons from the past

The first fly-by was of our Moon, made in 1959 by the Russian Luna-1 spacecraft. And 50 years ago, nearly to the day (July 15), the US Mariner 4 made the first fly-by of Mars.

My generation was captivated by the historic fly-bys of the outer planets and some of their moons, and I've been lucky enough in my own career to have been involved in instrument teams for several historic fly-bys. These were the Giotto mission to comets Halley (1986) and Grigg-Skjellerup (1992), as well as several close "fly-by firsts" in the Saturn system with the Cassini mission (such as moons Titan, Enceladus, Rhea, Dione, Hyperion).

Fly-by missions—what is the point when we have the technology to go into orbit?
Flyby firsts. Author provided

The Giotto fly-by of comet Halley only lasted a few days, but our knowledge of comets was revolutionised by this encounter. One of several probes to explore Halley in the mid-1980s, Giotto had the widest and most capable set of instruments and passed closer to its target than any of its companions.

It found cometary jet activity, a surprisingly dark surface, hydrocarbons in a crust and a complex bow shock and tail formation mechanism. These discoveries are now being followed up by the Rosetta mission and Philae lander at comet 67P.

Fly-by missions—what is the point when we have the technology to go into orbit?
Comet Wild 2 as seen from Stardust on January 2, 2004. Credit: NASA/wikimedia

But the fact that fly-bys happen so quickly can also make them very stressful and difficult to manage. When we were monitoring the Giotto spacecraft, flying past Halley at 68.4 km/s, it suddenly started spinning off its axis after encountering a dust particle near its closest approach. Fortunately it was possible to stop the wobble.

There are many other examples where data have been rescued – including with New Horizons during its worrying glitch (now fixed) on July 4.

New Horizons and beyond

After launch on an Atlas V in 2006, the 478kg spacecraft passed Jupiter only 13 months later, which was an express route. The main reason for the hurry was to reach Pluto before its tenuous atmosphere collapses by freezing as the planet moves further away from the Sun. The mission design of New Horizons gives a very fast fly-by at over 14 km/s (50,000 km/hour), with only a few hours and days for the highest resolution measurements.

Measured in "astronomical units" (one AU is about 149.6m kilometres), Pluto's orbit takes it from its closest point to the Sun (29.7 AU, 1989), inside Neptune's orbit (30.1 AU), through its current distance (nearly 33 AU) out to its furthest distance from the Sun (48.9 AU, 2113). Receding from the Sun, the surface temperature reduces from its current 40 Kelvin, leading to freezing of the atmosphere.

Fly-by missions—what is the point when we have the technology to go into orbit?
Image of Pluto and Charon, taken with New Horizons. Credit: NASA

But why fly past, rather than going into orbit? The most simple answer is that a lot of energy, meaning a lot of fuel, would be needed to slow New Horizons enough to capture it into orbit. Instead, NASA opted to get to the Pluto-Charon system quickly with a relatively capable 30 kg payload, rather than taking a large amount of extra fuel using a different fly-by scheme, to get there before the atmosphere collapses.

New Horizons is already expanding the thin textbooks on the Pluto-Charon system with early images, and the data to be returned over the next 16 months from visible, infrared and ultraviolet spectrometers as well as plasma, dust and radio science instruments will broaden and rewrite them again.

But new questions will almost certainly arise, which can only be answered by a more detailed orbiter mission, following the usual exploration sequence. When that would happen is hard to say. The relative priority will need to be compared with missions to other objects, particularly those where the exploration stage is low, before possible implementation.

In the future, we can look forward to more detailed fly-by missions of objects where our knowledge is limited and later-stage missions such as Rosetta. Also we will visit new dimensions, such as ExoMars drilling underneath the Mars surface by up to 2 metres for the first time. The JUICE fly-bys of Jupiter's moons Europa, Ganymede, and Callisto, before entering Ganymede orbit, will allow comparison of subsurface oceans, and Europa Clipper will fly past Europa 45 times to complete a detailed reconnaissance there.

These missions, and their follow-ons, will help us discover more about the humankind's place in the universe – and whether we are alone. But it is clear that, while we have achieved a lot in exploration so far, there is still a large amount left to do.

Explore further

New Horizons finally gets up close with Pluto – for 15 minutes

This story is published courtesy of The Conversation (under Creative Commons-Attribution/No derivatives).
The Conversation

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Jul 15, 2015
Probably need something more like Dawn to do an orbiter mission for Pluto.

Perhaps you could launch several orbiters at once, and have them ride on a Solar Sail out into the solar system. Each orbiter would also have a Dawn-like ion engine, and would disengage from the Sail as it reached various points along hte journey, to stop and orbit it's specific target object.

Charon is small enough that I think an Ion Engine could be enough to power a lander mission there, and if you land on the inside face you'll be able to turn your instruments to Pluto and study it more too. So you could take samples of Charon, and take any type of photos you want of that face of Pluto.

What is interesting to me is the fact Pluto and Charon lose mass in the modern solar system, therefore they must have been either farther away at some time in the past, or the Sun was much cooler in the past, in order to explain how they accumulated so much of their icy mass in the past.

Jul 15, 2015
Orbiting Pluto is tricky:
- you can opt for a slowdown (needs lots of fuel as the article mentions)
- or you can opt for a gentle insertion (creeping up from behind) which would mean it would have taken MUCH more time to get there. Space probes are hardy, but every year you add to a mission increases the risk that something crucial will fail en route (energy source, cameras, memory/computer, antenna/communications, attitude thrusters, ... ) There are just so many things that can go wrong before all you have is a piece of junk hurtling into the cosmos.
- then there is Charon. It's relatively close and of comparable mass. An orbit may bring you close to L1 (which is good because you can get a free ride around Charon and which is bad because you'd need to continually correct to stay in orbit around Pluto).

Yes, we have the technology. It just depends how much money/time we're willing to spend on such a mission.

Not least because of Charon. The L1

Jul 15, 2015
With today's technology of very high specific impulse ultra efficient rocketry for space propulsion, and the Chinese developed EmDrive on the horizon with no material propellant at all, just photons, it is the ultimate waste to design much less build and use chemical rocketry in space for primary mission propulsion.

New Horizons is powered by plutonium, so it has power for decades. If it had been equipped with a system like DAWN, now at Ceres, it would have been able to orbit BOTH Pluto AND Charon and its other moons indefinitely and gather much MORE data and science; and with enough fuel or with a Bussard Collector of an M2P2 design could stay exploring literally forever. Well maybe a little more Pu139 power fuel. Not only that, almost all the Kuiper Belt could have been its oyster. Places like Quaoar and the other large trans-main stream planetary objects could be 'explored'. It could truly go where man has never gone before! Quantum commo and it does it in real time.

Jul 16, 2015
Such nonsense.

Unless the flyby provides solid evidence of present life on the surface, There will be no follow up mission to Pluto, not in our lifetime.

Once the staged hoopla dies down, people will realize that the only thing this mission accomplished was proving Triton was a perfect stand in for Pluto.

According to this article, long before a Pluto flyby we'd have been treated to a Neptune orbiter to follow up on the truly spectacular Voyager 2 findings re Triton.

Yet we opted for Pluto, even though Neptune's immense gravity well means establishing an orbit around it would have been much easier.

The Pluto flyby was nothing more than a well-connected astronomer's last hurrah.

Since the public has lost interest in funding such ego-driven curiosity, confirming what we already suspected that Triton and Pluto are practically twins is never going to justify a follow up mission, unless that confirmation includes Pluto is like Triton with life.

Jul 16, 2015
What's needed is better management of human life on Earth and its stupid addiction to wars and military expenditure ($1,700 billion)--money that should instead fund space exploration ($72.1 billion).

Jul 16, 2015
What's needed is better management of human life on Earth

That's a tired old argument that somehow never seems to die.
It's NEVER a good idea to put all eggs (money) in one basket. It's always far better to do a number of things - even if that means that no one issue will get full attention.

Reason being: You can always argue that "issue X is more important than issue Y therefore issues X should not be funded at all". If we do that then we can argue to stop all food production, All infrastructure and all commerce because it's far more important to supply water to those who don't have it (most immediate threat).

Such an approach isn't sensible.

You can always argue that X is getting too much or too little money but we should always invest some money in knowledge gain - even if we don't know what good that knowledge will be, yet. Turns out that most all knowledge ever gained has been of some used (and paid for itself many times over).

Jul 18, 2015
Glad someone is questioning this. I question the whole enterprise of of going out on billion dollar missions to photograph dead rocks, frozen or molten. We could have just ignored it like every other item in the belt.

Jul 19, 2015
Pu139 [sic] ->Pu 239

Jul 19, 2015
Space exploration, like all exploration of unknown places, is an educated gamble. There will always be some risk of failure, but there will always be the probability of great rewards for the human population on Earth. We must balance our needs at home with our curiosity about the universe; without curiosity, we are just dumb animals in the forest. We can easily put too many demands on one spacecraft to the point of almost guaranteed failure; simple is always better with technology.

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