The sun, the most massive object in the solar system, the source of the biggest wrinkle in the fabric of space, you’d think it would be easy to approach. Not so, as NewScientist (31 March 2018, paywall) makes clear in this interview with the planners of the Parker Solar Probe:
“When you launch a spacecraft from Earth, it possesses Earth’s orbital velocity, about 30 kilometres a second. To get to the sun, you have to cancel out most of that, slow it down so it can fall in under gravity. That takes a lot of energy. If you want to launch directly from Earth to the sun, you need 55 times more energy than to get to Mars. It’s more than twice even what you need to get to Pluto.
For five decades, we had been studying this problem on and off, and had come to the same conclusion: to get to the sun you need a Jupiter gravity assist. Instead of going directly to the sun, you launch out to Jupiter, and use its gravity to reduce the spacecraft’s speed so it falls inwards.
But at Jupiter’s distance, solar power won’t work: you need nuclear. Everyone said the problem was impossible, but I started looking at whether you might use the gravity of the inner planets instead. Venus is much smaller than Jupiter, so its gravity assist is much less. You can flyby multiple times, each time losing some velocity and falling in closer to the sun, but that means manoeuvring to pass Venus in the right orbit each time, which is tricky and uses up fuel.
Eventually, I found a trajectory with seven Venus assists that passes the sun 26 times, each time closer. The closer the probe falls, the faster it gets. At its fastest, it will be travelling at 200 kilometres a second – the fastest spacecraft ever.”
The spaceflight folks do get to work on some fascinating problems, don’t they? Sure wish I had been smart enough to give it a swing when I was younger. From the NASA introductory page:
Credit: NASA
Parker Solar Probe will use seven Venus flybys over nearly seven years to gradually shrink its orbit around the sun, coming as close as 3.7 million miles (5.9 million kilometers) to the sun, well within the orbit of Mercury and about eight times closer than any spacecraft has come before.
Parker Solar Probe is a true mission of exploration; for example, the spacecraft will go close enough to the sun to watch the solar wind speed up from subsonic to supersonic, and it will fly though the birthplace of the highest-energy solar particles. Still, as with any great mission of discovery, Parker Solar Probe is likely to generate more questions than it answers.
