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.
SelfAwarePatterns 
Exactly what I was wondering… 🙂
Interesting and informative. And now I’m thinking of Weird Al’s happy birthday song. =p
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Even Arthur Clarke got it wrong! I like how the article says how NASA may have created that confusion by trying to provide an explanation understandable to the public.
It’s funny how urban myths evolve. The FORTRAN version often has the spacecraft lost and never heard from again as it approached Mars.
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It’s also interesting that some of the story variations have a kernel of truth. The Mars Climate Orbiter was reportedly lost in 1999 because of ground based computing software using English rather than metric units, causing the craft to accidentally enter the Martian atmosphere and burn up.
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Yeah, I’ve heard about that one. I wish I could find it again, but I once saw a great “infographic” that pictured most (all?) of our planetary missions and what happened to them as fat arrows from Earth to the planets. Or maybe it was just various Mars missions. It’s been so long I can’t recall!
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I wanted to read that NASA series and look up some numbers before I said anything, but I think there may be a degree of misunderstanding here. There is an important difference between orbital trajectories and the orbits themselves.
The smaller the average radius of an orbit around a body, the faster that orbit is.
The article you linked to has a couple of very important statements: “A spacecraft’s apoapsis altitude can be raised by increasing the spacecraft’s energy at periapsis.” and “A spacecraft’s periapsis altitude can be lowered by decreasing the spacecraft’s energy at apoapsis.” [I added the bolding for emphasis.]
A transfer orbit is a highly eccentric orbit with its periapsis at one end and its apoapsis at the other. With a trip to Mars, Earth is at the periapsis and Mars is at the apoapsis, hence the need to increase speed at Earth to reach Mars. Conversely, to reach Venus, it’s the other way around, hence the need to shed speed. If either of those orbits missed their destinations, the spacecraft would end up back at Earth orbit.
For reference, here are some planetary orbital velocities:
Mercury: 47.362 km/s
Venus: 35.02 km/s
Earth: 29.78 km/s
Mars: 24.077 km/s
Jupiter: 13.07 km/s
Pluto: 4.7 km/s
And here are some Earthly ones:
ISS: 7.66 km/s
GPS: 3.88 km/s
Geosynch: 3.1 km/s
Moon: 1.022 km/s
As you can see, the closer the orbit, the faster the speed. That seems really counter-intuitive, but it’s the way it works. You drop to a lower (stable) orbit by speeding up and rise to a higher (stable) orbit by slowing down (weird, right?).
None which changes that, as you said, an orbit with apoapsis at Earth and periapsis at the Sun requires a lot of negative delta-v. Transfer orbits are the most efficient. We could also get there with a spacecraft that increased velocity to achieve stable orbit closer to the sun. But to even reach an orbit matching Mercury’s, we’d need a delta-v over 17.5 km/s.
Either way, as you say, it’s a bit of a trick!
One other thing: it’s not always clear from your post that “delta-v” means change in velocity (“delta” being a physics-geek’s way of saying “difference”). I’m sure you know that, but it’s not always clear in the text. For example, “with enough velocity, or delta-v, to escape Earth’s gravity.”
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You covered a lot of detail and terminology that I omitted for simplicity, but I’m not following where you see the misunderstanding.
“You drop to a lower (stable) orbit by speeding up and rise to a higher (stable) orbit by slowing down (weird, right?).”
Not sure whether you mean the final orbital speed here or what you have to do to get to the new orbit. To drop to a lower circular orbit, you first have to slow down which creates a new eccentric orbit with your current position as apoapsis and a reduced periapsis. To make the lower orbit circular, when you reach periapsis, you have to slow down again. If you don’t do the second deceleration, you stay in an eccentric orbit with your speed at periapsis faster than an object at that altitude in a circular orbit. As you said, once made circular, the final speed in the new lower orbit is faster than in the higher circular orbit.
To go into a higher orbit, you have to speed up, creating a new orbit with you at periapsis and a raised apoapsis. When you reach the new apoapsis, you have to speed up again to make the orbit circular. But, again, as you noted, the final speed in the new higher circular orbit is slower than it was in the lower circular orbit.
Of course, slowing down takes just as much energy in space as speeding up, so either way you’re forced to burn energy. Fortunately, if you’re trying to intercept a planet, the planet’s gravity, with clever maneuvering, can save some of the energy requirements.
You’re right. I should have explicitly explained delta-v or avoided the term. In retrospect, I probably should have pulled it when I cut out aphelion, perihelion, and other jargon.
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“I’m not following where you see the misunderstanding.”
It could well be mine (and don’t take it too strongly — I said “may be a degree of misunderstanding”). You didn’t really get into that closer orbits are faster orbits, so it wasn’t clear to me you were distinguishing between circular orbits and transfer orbits.
The part: “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.” It’s important to take that in the context of transfer orbits and not circular ones and I thought it worth mentioning.
“To drop to a lower circular orbit, you first have to slow down which creates a new eccentric orbit with your current position as apoapsis and a reduced periapsis. To make the lower orbit circular, when you reach periapsis, you have to slow down again.”
Yes. As a real example, let’s say we’re in orbit at GPS distance. That’s roughly 20,000 km with a (circular) orbit speed of 3.88 km/s. We want to drop to ISS orbit, a distance of 412 km with an orbital speed of 7.66 km/s.
So we slow down to some speed that gives us a periapsis of 412 km. Our speed is 3.88 km/s, so we slow down to less than that (at apoapsis). And we slow down again at periapsis to enter a circular orbit.
How do we end up with an orbital speed of 7.66 km/s if we’ve slowed down twice? That’s due to the speed up at periapsis from an elliptical orbit, a point you did touch on but briefly (and which is the key to the whole thing).
“Not sure whether you mean the final orbital speed here or what you have to do to get to the new orbit.”
Sorry, that was poorly worded. Final speed — the ultimate effect.
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No worries Wyrd. I didn’t mean to be combative on the misunderstanding thing. (If I did misunderstand something, I’d appreciate having it pointed out to me.)
One of the problems with doing a post like this is deciding how far into the weeds you want to go. My goal was to give a light exposure to solar system navigation, but link to the more hard core stuff for those interested. Of course, anytime you do that, your more knowledgeable readers are going to see omissions and simplifications.
On Earth orbits, you reminded me of a point I had considered making in the article (but ended up omitting). Satellites that are in higher orbits are generally not de-orbited when they’re decommissioned. The reason is the energy required to get them out of orbit. That’s why they’re most often bumped into “graveyard” orbits. There is reportedly a lot of junk orbiting just above the geosynchronous orbit. (Of course, a lot of junk remains in geosynchronous orbit from before the graveyard move became considered mandatory.)
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“…deciding how far into the weeds you want to go”
Boy, is it ever! We touched on this talking about my SR series. There I was trying to write for readers at your level as well as those at, say, Tina’s level. Looking back at it, I’m not sure I hit either target as well as I hoped to.
I keep swearing off the idea of “educational writing” because I’m not sure I’m as good at it as I need to be and with resources like Wikipedia and such I don’t feel like I’m contributing much. But I so love trying to share the things that fascinate and enthrall me that I keep falling back into it.
I’m sure you can relate to wanting to share the wonders of the real world with others! There’s just so much cool stuff! 😀
” There is reportedly a lot of junk orbiting just above the geosynchronous orbit.”
One of my regular wallpapers was that image someone did that showed all the junk in orbit. We manage to pollute no matter where we do, don’t we. (Lately, people have been starting to complain about what a garbage heap the top of Mt. Everest has become.)
I’ve always thought that some day there’s going to be a profession of “Orbital Garbage Collector” and wouldn’t it be an interesting idea of an SF story. There is some degree of risk in the space travel part alone, plus some involving matching orbits with the junk. You could have all the beauty and thrill of Gravity without the BS. 🙂
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I found your SR series enlightening, particularly the Twin Paradox part. I knew about the relativity of simultaneity, but hadn’t grasped how central it was to that issue. And that’s the kind of series that will sit out there for years to come with people continually discovering it.
As bloggers, we definitely do write for our audiences, but I think many of us also write just to express thoughts we feel an urge to express, to get our thoughts out there. I know trying to explain a subject sharpens my understanding of it. And even if I already thoroughly understand it, there’s a lot to be said for finding a good way to get the ideas across.
I actually enjoyed Gravity, but I probably would have enjoyed it more if its grasp of orbital mechanics hadn’t been so ludicrous. I’ve heard some proposals to take care of the orbital junk problem with magnetic nets.
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I somehow never got around to replying to this…
Very much agree about the reasons we blog, and if I write something that doesn’t quite work as well as I’d hoped, I can still fall back on, as you say, just writing to express (or even just record) a thought.
Also, one of the reasons I’ve loved doing the teaching I’ve done (and it is another reason I try about technical topics) is exactly that you never learn something so well as when you try to explain it to others.
And practice makes perfect, right? (Or at least, hopefully, better. 🙂 )
I liked Gravity okay. The space and character scenery were both, as they say, easy on the eyes, so it was a nice film to look at. It’s kind of interesting to me in how it seems right on the edge of being science fiction. I’m tempted to classify it as a modern day thriller rather than SF. It seemed a lot closer to Apollo 13 than it did, say, Europa Report.
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I think I read where the director of Gravity strongly feels that it is not science fiction, because all of the technology is current. I can see his point, but most of the public seems to disagree with him, labeling it as a science fiction thriller. Still, if it had been a submarine movie about a sailor surviving an underwater disaster, it would probably have been labeled a techno-thriller (although techno-thrillers themselves are often borderline, if not outright, science fiction).
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Given that Alfonso Cuarón wrote it, directed it, and is the first name listed for producing it and editing it,… I’m inclined to agree with him! (And, not to re-open that discussion, but is a good example of why what the public thinks doesn’t impress me much.)
Even Marooned from back in 1969 really wasn’t all that much of an SF film so much as a thriller (IN SPACE). Considering the source novel, it was very much grounded in real, or at least well-anticipated, technology. Even at the time we didn’t look at it as being in the same class of things as 2001 let alone Star Trek. After all, we’d seen men walk on the moon earlier that year!
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Interestingly, Marooned might be considered slightly more science fiction than Gravity. If I recall correctly, the rescue was made with an experimental spacecraft, a sort of lightweight proto-space shuttle like lifting body thing, which didn’t exist when the movie was made. Gravity was straight up existing (actually some obsolete) technology; the only speculation was the debris storm.
Of course, the existence of advanced technology isn’t the dividing line. If it were, we’d have to exclude the Mad Max movies from science fiction, although you could argue that post-apocalyptic sci-fi has sociological speculation.
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Yeah, agreed on all points!
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Very good. I had never before considered how hard it would be to launch a probe into the sun.
On the subject of urban myths, observing their evolution is intriguing. The ideal myth must be dramatic (probe dives into sun), yet plausible (to the general public). Central to the myth must be a dim, generalized understanding of the phenomenon (NASA sends probes into space; computer bugs are relatively common in commercial software), but too subtle to be properly understood (relative orbital velocities).
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Thanks. Good point about the drama enhancing the myth. It was probably inevitable that it would get more lurid over time since a lurid meme will have a higher chance of propagation than a (relatively) boring one about a launch failure. It also reminds me of the quote, “A lie gets halfway around the world before truth has a chance to get its boots on.”
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