Last week, I was having lunch with some friends, which included a number of programmers. One of them mentioned an old urban myth, that I hadn’t heard in several years, which claims that, due to a programming bug (involving a misplaced semicolon), NASA once accidentally sent a probe into the Sun. I pointed out to my friend how implausible this was. He didn’t believe me, and we ended up having a conversation about the logistics of solar system navigation, some of which I’m reproducing here.
So, how can I say that NASA accidentally sending a probe into the Sun is implausible? After all, the Sun is a giant ball of fusion heated plasma, over a million kilometers in diameter, sitting in the center of the solar system. Why isn’t something like that a major navigation hazard? And why is the idea of accidentally sending anything into it unlikely?
The answer is that orbital mechanics actually make the Sun the most difficult location in the solar system to reach, even on purpose. It’s more difficult to send something to the Sun than it is to send it completely out of the solar system, as we’ve done with the Voyager probes.
To understand why, let’s start by remembering that everything in the solar system orbits the Sun: Earth, the other planets, asteroids, comets, etc. Note that almost all of it is orbiting in the same direction. And that an orbit is essentially an object moving fast enough to avoid falling into a gravity source, but not fast enough to break away from it. Slow the orbiting object down, and gravity brings it closer to the gravitational source; speed it up, and the additional speed brings it further from the gravitational source. Slow it down enough, and it falls into the gravity well; speed it up enough, and it breaks free.
When NASA, or anyone else, sends a probe to another planet, such as Mars, what they’re actually doing is putting the probe into a transfer orbit that intersects the orbit of Mars, hopefully when Mars is in that position.
The way this works is that when an interplanetary probe is launched, it first is launch with enough velocity, or delta-v, to escape Earth’s gravity. This is a little over 11 kilometers per second. If the probe is being sent to Mars, it’s launched in the direction that Earth moves in its orbit around the Sun, with enough extra velocity (the exact amount varies) to put it in its own orbit that will take it further away from the Sun and intercept the Martian orbit. If launched at the correct time, it will meet Mars at that intersection.
When a probe is sent to Venus, it is actually launched in the direction opposite Earth’s orbital direction, with enough delta-v to put it in orbit around the Sun at a slower velocity than Earth, which will bring it in closer to the Sun. Again, hopefully if all the calculations are correct, this new orbit will intersect Venus’s orbit at the right time to arrive at Venus.
Mars and Venus are the easiest planets to reach, mainly because they are the ones with the closest orbits and with the smallest differences in orbital speed from Earth’s. The delta-v to get into a transfer orbit to them isn’t too severe, and neither is the delta-v to match speeds with the planet at the intersection.
Getting out to the outer planets requires considerably more energy, although when sending probes into the outer solar system, Jupiter’s gravity can be used to slingshot the probe to higher speeds. The Voyager probes actually used multiple gravity assists via the gas giants to build up enough velocity to escape the solar system.
Voyager 1 in particular used all of the gas giants (Jupiter, Saturn, Uranus, Neptune) for successive gravity assists, which is why it’s currently the furthest man made object. (Oops, it was Voyager 2 who used all those planet gravity assists. Voyager 1 is moving slower, but is currently further out due to its trajectory.)
Mercury is actually a pretty difficult planet to reach because of its orbit. Considerable delta-v is required to slow a probe’s orbital velocity around the Sun enough to put it in a transfer orbit to Mercury. In addition, objects move faster at the lowest point in their orbit. An object in an elongated elliptical orbit, when it is at its low point, such as a probe, will be moving much faster than another object in a more circular orbit at the same distance from the Sun, such as Mercury. This speed difference made getting a probe into orbit around Mercury difficult.
In the early 70s, NASA had sent Mariner 10 to both Venus and Mercury, using Venus’s gravity to slow the probe enough to approach Mercury. But all it was able to do was a periodic near pass as its solar orbit swung near Mercury’s. It wasn’t until MESSENGER, which took an elaborate multi-orbit path around the Sun, using Earth and Venus’s gravity to repeatedly slow it down sufficiently, that we were able to get a probe into orbit around Mercury.
Okay, so what does this all mean for sending a probe to the Sun? Well, it means you can’t get there by just naively pointing a rocket in the Sun’s direction. Without enough delta-v, you’ll just end up putting the spacecraft into a different orbit around the Sun.
Earth’s orbital velocity around the Sun is about 30 kilometers per second. The most straightforward way to send a probe to the Sun would be to launch with enough velocity to escape Earth’s gravity (11 km/s) plus enough in the direction opposite of Earth’s orbital direction to kill all the probe’s solar orbital velocity (30 km/s), allowing it to fall into the Sun. That would require a total delta-v of over 41 kilometers per second. Currently there is no rocket that can provide this much velocity. (Although it may be doable with help from long running electric propulsion systems such as VASIMIR, or with solar sails.)
Of course, similar to the MESSENGER probe, we could probably use various gravity assists to lessen the delta-v requirement. But the point is that doing so is very complicated. Huge delta-v requirements plus complexity means that this is not something anyone is going to do accidentally. At least not until our propulsion technologies get a lot better than they currently are. Which is why you really don’t have to hit the NASA archives to know that stories like this are myth.
Incidentally, this urban legend demonstrates something about the way that oral myths evolve, even over a few decades. It likely began from the Mariner I launch failure in the early 60s, which reportedly involved a software bug with a misplaced hyphen. (Although even that isn’t definite.) Somehow, by the early 80s (which is when I can first recall hearing or reading it), it had mutated through various embellishments into the Sun version.
If you’re interested in more details on transfer orbits and the like, I highly recommend NASA’s write up on it.