How Dawn, New Horizons cross huge distances: Bob McDonald

The New Horizons mission to Pluto and the Dawn mission to Ceres demonstrate two approaches to space travel - the brute power of rockets, versus advanced solar-powered ion propulsion.

Two different space probes using two very different rocket systems are approaching Ceres and Pluto

An artist's impression of the Dawn probe at Vesta in the asteroid belt. (NASA/JPL-Caltech/UCLA/McREL)

The exploration of dwarf planets begins this week as the Dawn spacecraft begins orbiting Ceres, in the asteroid belt between Mars and Jupiter. Later, this July, the New Horizons probe will whiz by Pluto. One is an interplanetary cruiser,  the other an interstellar bullet.

While we wait for close-up images of these icy dwarf planets to be beamed back to Earth from space, it’s interesting to note how differently the two probes reached their destinations.

The Tortoise: Dawn's ion drive

The Dawn spacecraft has been cruising around the inner planets since it left Earth in 2007 on the gentle power of an ion engine, which is very different from what we normally think of as a rocket.

Rather than belching flame and smoke, producing tremendous power, and accelerating like a rabbit, Dawn's ion engine takes a more leisurely approach, with a quiet blue glow of electrically charged xenon ions that push with a force of only 91 millinewtons.

An image of Vesta taken by the Dawn probe. (NASA)
To put that into perspective, that’s about the force exerted by a piece of paper lying on your hand. At that rate of acceleration, it would take Dawn four days to go from zero to 100 km/hr. At the drag strip, you wouldn’t use a stopwatch to time it, you’d need a calendar!

But while this kind of feeble power might seem ridiculous for a vehicle that must cover millions of kilometres through space, it works.

Chemical rockets generate higher acceleration, but they burn fuel at tremendous rates - usually exhausting their gas tanks in a matter of minutes. Ion engines, on the other hand, can run at low thrust for years without stopping. After a slow start, speed adds up over time and keeps adding up because the spacecraft is always accelerating.

An ion engine is seven to 10 times more efficient than a regular rocket, partly because it’s solar powered. Run the ion engine long enough and you can reach interplanetary speeds — you just have to be patient.

Dawn’s more controlled approach to spaceflight also enables it to slow down and take in some sights along the way to Ceres, which it did in 2011 when it passed close to the largest asteroid Vesta. Dawn put itself into orbit and mapped the entire surface of that world for a year.

Then it fired up its slow burning ion engine again and cruised out to Ceres, where it will park itself for another year or so. 

The Hare - New Horizons' big blast

New Horizons, on the other hand, was sent off to Pluto in 2006 the old fashioned way.

It was placed on top of a mighty Atlas 5 three-stage rocket. With five boosters strapped to its base, it was the fastest rocket ever launched. After leaving Earth, it took only nine hours to cross the orbit of the moon. In comparison, Apollo astronauts took three days to get that far.

An artist's concepttion of the New Horizons spacecraft passing Pluto. (Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute)
A year later, this dragster of a spaceship reached Jupiter, where it was able to test its cameras with a very cool portrait of our largest planet and its volcanic moon, Io. 

More importantly, it took advantage of Jupiter’s huge mass to get a gravitational boost. Sometimes called a gravity slingshot, this accelerated the spacecraft even more, sending it on a direct path to Pluto and shaving several years off its flight time.

By the time it reaches Pluto in July, New Horizons will be travelling at 14 km/second (50,200 km/hr) and will whiz through the Pluto system in one day. Its closest approach to the planet will only last half an hour. 

Unfortunately, the drawback of getting yourself going that fast is that it’s hard to slow down unless you carry a big load of fuel to use for braking, and New Horizons doesn’t have any brakes. It used all of its energy at launch, like the gunpowder in a rifle shot.

Once blasted off the Earth, New Horizons coasted the rest of the way like a bullet (with a little help from that gravity assist I mentioned). So after snapping as many pictures as possible and scanning the Pluto system with many other instruments, the spacecraft will just keep on going.

There are plans to fly by another, yet to be determined, icy body beyond Pluto, but after that, New Horizons will continue to fly right out of our solar system altogether. It will follow the two pairs of long-distance probes, the Voyager and Pioneer spacecraft, which are the only other human-made objects so far destined to drift among the stars of the Milky Way for hundreds of millions of years.

This tortoise and hare approach was not a race. They are both effective ways of getting around the solar system.

Pluto is so much farther away that New Horizons needed a big push to get there, and even then it took almost a decade. In theory you could get there with an ion engine in a slow and steady way, with constant low acceleration. But the problem would be power. The ion engine on the Dawn spacecraft is solar-powered, and Pluto is so far from the Sun that there just isn’t enough energy available out there to run an ion drive.

Both of these missions also show how far away the planets are. Both probes took about a decade to get to their destinations.

We really need newer and faster ways of getting around, especially if humans are planning to explore beyond Earth.


Bob McDonald is the host of CBC Radio's award-winning weekly science program, Quirks & Quarks. He is also a science commentator for CBC News Network and CBC-TV's The National. He has received 12 honorary degrees and is an Officer of the Order of Canada.