Launch a probe
Pick a launch date, a speed and an aim, and see if you can reach the outer planets. Rockets alone barely get you to Saturn. The trick that took Voyager to the edge of the Solar System is to steal speed from Jupiter.
Ready on the launch pad. The gold arrow is your aim; the dashed circles are where Jupiter and Saturn will be when you could arrive.
- Pick a launch date. Watch the dashed circles: that’s where Jupiter and Saturn will be when you could get there.
- Set your rocket’s speed and aim. The gold arrow on the map shows the direction.
- Press Launch and watch the white probe. It slows down at each planet so you can see the slingshot.
Challenges
- Reach Jupiter
Jupiter’s distance needs about 8.8 km/s, but Jupiter has to be there when you arrive. Try launch dates a year apart.
- Saturn on a budget
Saturn’s distance alone needs about 10.3 km/s. Reach Saturn with less than 10.0 km/s by borrowing speed from Jupiter.
- Leave the Solar System
From Earth’s orbit you need about 12.3 km/s to escape the Sun, or less with a slingshot.
- Grand tour
Visit Jupiter, Saturn, Uranus and Neptune in one flight. Voyager 2 did it. (Tiny changes matter here.)
How can a planet speed you up for free?
Throw a tennis ball at the front of an oncoming train. It bounces off far faster than you threw it, because the train was moving. A spacecraft swinging around Jupiter is the same: seen from Jupiter, it leaves exactly as fast as it arrived, only turned. But Jupiter itself is racing around the Sun at 13 km/s, so seen from the Sun the spacecraft can leave much faster. Jupiter slows down by an amount far too small to measure.
The line-up of 1977
In 1965 Gary Flandro, a summer student at NASA’s Jet Propulsion Laboratory, noticed that in the late 1970s Jupiter, Saturn, Uranus and Neptune would be lined up so that one spacecraft could swing from each to the next, an arrangement that comes round about once every 175 years. Earlier, Michael Minovitch had worked out how gravity assists could be calculated. Voyager 2 used the line-up to visit all four giant planets.
A toy model: planets move on circles, the probe starts from Earth with no Earth gravity, and there are no course-correction burns. Real missions fine-tune their path many times on the way. Dates in the presets differ from the real Voyagers for those reasons.