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I am in a group discussion with some people, and I told them what I wrote below as a reason why using a star tracker does not make sense for a spacecraft in an asteroid belt. Furthermore, I am curious to know what other sensors could you use.

A star tracker is used to determine the location and attitude of a spacecraft by analyzing the placement of the surrounding stars relative to the spacecraft.

A star is a luminous ball of gas, mostly hydrogen and helium, held together by its own gravity.

Asteroids are small, rocky objects that orbit the Sun. Although asteroids orbit the Sun like planets, they are much smaller than planets.

The asteroid belt is mainly made of rock and stone, but a small portion of them contain iron and nickel metals.

That's why you would not use a star tracker in an asteroid belt, correct?

Which other sensors would you use?

JRE
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Defrans
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    I don't follow the logic behind this question. Yes, stars are luminous distant balls of gas & asteroids are nearby stones orbiting our star (Sun). How does this preclude a star tracker from being used in the asteroid belt? – Fred Feb 17 '23 at 18:33
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    "The asteroid belt is mainly made of rock and stone" - no, it's mainly made of 99.9999...% empty space. – asdfex Feb 17 '23 at 18:55
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    Why do you think you wouldn't? Dawn in fact did use star trackers, and it wasn't considered in any way remarkable or unusual for doing so. – Christopher James Huff Feb 17 '23 at 20:32
  • @Fred: sorry, i edited the post. I am in a group discussion with some people, and i told them what i wrote in the text above why using a Star Tracker does not make sense for a spacecraft in an asteroid belt. – Defrans Feb 17 '23 at 20:46
  • @asdfex: besides the empty space i meant. – Defrans Feb 17 '23 at 20:48
  • @Chris: because of what I wrote, it would not make sense. I guess thats just one case probably for a specific reason, probably a modified star tracker. Does that make sense to you that they used it?? – Defrans Feb 17 '23 at 20:48
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    A star tracker is going to be able to easily pick out stars at a random point in the asteroid belt easier than it would in low Earth orbit, because the sun's less glaring and there isn't a big blue marble obscuring part of your view. The stars you pin on your star tracker are going to be easy to find; there aren't nearly enough asteroids to obscure or confuse them. – notovny Feb 17 '23 at 20:53
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    What about it doesn't make sense? How does the composition of asteroids have anything at all to do with the functioning of a star tracker? And Dawn used two SED 16 trackers, commonly used by low Earth orbit satellites. And yes, it makes perfect sense to me that they did so. – Christopher James Huff Feb 17 '23 at 22:35
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    What is it with all these "am I right" questions lately? If you are writing a question like that, the answer so far has been "no". – Organic Marble Feb 17 '23 at 23:28
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    The asteroid belt doesn't look like what you might expect from scifi movies. It's quite sparse. "The number of objects in the asteroid belt increases steeply with decreasing size, but even at micrometer sizes the Pioneer spacecraft were hit only a few times during their passage." https://space.stackexchange.com/a/49027/38535 – PM 2Ring Feb 18 '23 at 10:51
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    @Defrans Is your argument that a star tracker couldn't distinguish between stars and asteroids? – Dave Gremlin Feb 18 '23 at 15:38
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    @DaveGremlin if anything, the argument seems to be that star trackers wouldn't be able to track asteroids. But there's no apparent reason why you'd want them to do so, so the question is a bit mystifying... – Christopher James Huff Feb 18 '23 at 18:19
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    The OP appears to have a basic misunderstanding of what star trackers do: "A Star Tracker is used to determine the location and attitude of a spacecraft by analyzing the placement of the surrounding stars relative to the spacecraft." Atttitude, yes. Location, no. The star database in a star tracker's software omits planets, asteroids, and even nearby stars. A star tracker is useless at determining the location of spacecraft within the solar system. We have yet to build spacecraft that roam far from the solar system; that's a problem for future generations. – David Hammen Feb 19 '23 at 12:24
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    @OrganicMarble I suspect ChatGPT. This writing feels like AI writing to me. There's been a lot of "ChatGPT said this, is it correct?" on all Stack Exchange sites and if ChatGPT is banned it's natural to pretend you read it somewhere else. (even though the ban is for answering questions with ChatGPT, not asking about things ChatGPT said) – user253751 Feb 23 '23 at 02:52
  • @user253751 you make an excellent point. – Organic Marble Feb 23 '23 at 02:53

2 Answers2

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A Star Tracker is used to determine the location and attitude of a spacecraft

Attitude, yes. For location it would need some help. Stars are too far away to use for positioning information within the solar system.

Why wouldnt you use a Star Tracker in the Asteroid Belt?

No reason that I can think of. If a star tracker is appropriate for a craft, it doesn't stop working if the craft is in the asteroid belt.

As stars are still perfectly visible within the asteroid belt, star trackers work fine there. (You don't point it at the asteroids, you still point it at known bright stars).

Which other sensors would you use?

That depends on your mission. Many probes don't have onboard sensors to know their position in space. They rely on the ground to figure that out and then command the craft. So radio communication from earth may be sufficient.

BowlOfRed
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I might be entirely off-base...

Because your question isn't entirely clear. You might be asking why you can't use a star tracker to track asteroids in an asteroid belt. I wonder this because if your discussion is literally asking why one couldn't use a star tracker to track stars in an asteroid belt, then it only makes sense if you're thinking asteroid belts are like what are shown in Star Wars, which in our solar system isn't true. (It might not be true in any solar system. That many asteroids would be pulled together to form planets....)

From this question over at Worldbuilding we learn that our own asteroid belt is so empty that the New Horizons space probe sailed right through it and nobody shed a drop of sweat over it. I believe I once read that if all the mass in our asteroid belt were brought together it wouldn't add up to the mass of our moon.

But if you're asking about tracking asteroids...

You're absolutely correct — it won't work. Star Tracker tracks emitted light. Bright emitted light. Asteroids aren't known for emitting bright light or even reflecting bright light, so the tool's value for tracking them is limited. Another issue you probably didn't think of (and, frankly, I don't know enough about) is whether or not the asteroids are moving too fast for a tracking system that's designed to track what, for all intent and purposes, are stationary objects.

But what really becomes a hard no is how difficult it would be to detect your location based solely on asteroids. You'd need to know exactly which asteroids you detected and where they were expected to be at the moment you performed the analysis. Yuck. Remember that such a tracker would have trouble due to the speed of the asteroids? Even if you could track them... there's still the effort of extracting useful information from the data. There are a lot of asteroids in our belt the size of tennis balls and smaller.

What other sensors could be used?

You're stuck with just one at our current level of technology: radar. So if we assume (via magic) that our radar is infinitely precise (could track a tennis ball light years away), then we must ask what's the point of tracking an asteroid that's a light-hour away? The value of such a radar system is based on how quickly your ship can react to the detection of an imminent threat. And that's a function of your velocity, the asteroid's velocity, and how far away you can detect the asteroid. The odds are in my favor that an asteroid a light-hour away will take a year and a half to hit you.

But let's ignore all that. The value of radar is directly related to the power of the pulse and the time between pulses. Check out this quick tutorial about calculating maximum useful radar range. You can easily detect imminent threats — so long as you carry a big enough array and a big enough power plant. This, because you really don't need to care about objects a year and a half away.

But we're back to your question — could you use radar to track your location in an asteroid belt? Theoretically it can be done, if you have an accurate enough radar pushing far enough out into space with an accurate enough map of all the flotsam and jetsam in the belt. But I believe it stretches credulity.

Unless I misunderstood your question. If I did, let me know and I'll delete this answer.

JBH
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  • FWIW, the total current mass of the asteroid belt is roughly 4% that of our Moon. – PM 2Ring Feb 21 '23 at 23:03
  • @PM2Ring Oh, that's even worse! That makes the odds of being significantly damaged passing through the belt what, one in trillions or worse? It might as well not be there but for the handful of bigger asteroids. – JBH Feb 22 '23 at 01:19
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    Exactly. As I said here https://space.stackexchange.com/a/49027/38535 "the Pioneer spacecraft were hit only a few times during their passage." And that's by micrometer sized particles. Also see https://astronomy.stackexchange.com/a/49425/16685 – PM 2Ring Feb 22 '23 at 01:41