Supposedly there are between 1.1 to 1.9 millions asteroids larger than 1 km in the asteroid belt. I assume there are many more , a lot of smaller ones, which cant be detected from here.

According to Wikipedia Asteroid Belt

The first spacecraft to traverse the asteroid belt was Pioneer 10, which entered the region on 16 July 1972. At the time there was some concern that the debris in the belt would pose a hazard to the spacecraft, but it has since been safely traversed by 12 spacecraft without incident

If SpaceX succesfully lands a human mission on Mars in 2024, and if they actually start building a self sustained permanent base there by 2029, space exploration could speed up really fast and the closer (on the cold side of the solar system) solar system objects are in the asteroid belt, with a dwarf planet and some large asteroids which were confused with planets in the past due to their large sizes. And some of them get closer to Mars in distance than the distance of Earth-Mars. So one of the biggest problems could be traveling among all those asteroids and debris without getting your spaceship destroyed. That's what I would like to know,

Can small asteroids in the asteroid belt be detected on the fly by a spaceship and how much of a threat do they represent for a human manned space mission there, is there any technology developed for this?

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    $\begingroup$ Read wikipedia the chapter about Exploration. "Due to the low density of materials within the belt, the odds of a probe running into an asteroid are now estimated at less than 1 in 1 billion." $\endgroup$
    – Uwe
    Commented Feb 22, 2019 at 16:09

1 Answer 1


The orbital speed of Mars is about 24 km/sec or ~ 2 million km/day, and so relative velocity to an asteroid could be fairly slow for one in a similar orbit, or easily 1 million km/day or more for one in a weird orbit.

Optical (Visual, thermal IR)

You could set up several optical (visible or thermal IR) survey telescopes around Mars, and try to vigilantly scan the most celestial sphere (except for directions near the Sun) for unexpected, very dim objects moving unpredictably, but that's quite a challenge, and there's still the big blind spot anywhere in the general direction of the Sun.

A better way is to start looking now! For potential NMO's (near-Mars-objects) you would start looking now, or at least several years before you started to need the information. And you would look using a space telescope with an orbit closer to the Sun than Mars for two reasons:

  1. With a different period than Mars, the object will approach close to you within a few orbits. The synodic period $\frac{1}{T_S} = \left(\frac{1}{T_1} - \frac{1}{T_2} \right) $ is fairly short when the two periods are significantly different.

  2. With the target objects farther from the Sun than you are, their close approaches happen when they are in the night sky, so you don't have the Sun to worry about.

Number 2. is important because it means you can shield your telescope from the Sun's heating effects as well as from the light. You want to keep an asteroid-hunter's telescope's cryogenic optics and sensors very cold.

It turns out that it's easier to search for these trouble-makers in the thermal infrared than it is in visible light. They are fairly black (albedo's often from 0.05 to 0.1) and reflect direct sunlight poorly. But that means that they absorb most of the Sun's light and effectively radiate it in thermal infrared. But the only way to have a telescope sensitive to thermal IR is to have the entire telescope (mirrors and optics and sensors) very cold so that the thermal radiation of the distant asteroid isn't swamped by thermal IR from the telescope itself.

Most stars (but not all objects) are much dimmer in thermal IR than they are in the visible, so that could potentially help when sorting out asteroids versus stars in preprocessing, compared to visual surveys.

Source: NEOCam: Finding Asteroids Before They Find Us (Orbit)

Source: NEOCam: Finding Asteroids Before They Find Us (Orbit)

For more about WISE, NeoWISE, NEOCam (now NEO Surveyor), and B612, see:

Radar, Lidar

You might consider searching using Radar (similarly to the way space debris is monitored) but you'd dismiss it quickly. The Sun paints objects between here and Mars with 0.5 to 1.3 kilowatts per square meter, and at a million kilometers you won't be able to match that with radar or even Lidar, at least not for the foreseeable future.

Another way this is often expressed is that the strength of signals using optical detection varies as $1/r^2$ whereas for radar (where you have a round-trip) it varies as $1/r^4$.

  • $\begingroup$ "The big blind spot anywhere in the general direction of the Sun" is mostly covered by asteroid survey missions on Earth/in Earth orbit. $\endgroup$
    – Hobbes
    Commented Feb 23, 2019 at 8:42
  • $\begingroup$ @Hobbes I think I've already said something like that: "For potential NMO's (near-Mars-objects) you would start looking now... And you would look using a space telescope with an orbit closer to the Sun than Mars for two reasons..." except I don't guarantee that it's "already covered" because the way those surveys are scheduled it may cause them to miss some NMOs. They might catch many/most, but I'm not certain. $\endgroup$
    – uhoh
    Commented Feb 23, 2019 at 8:57
  • $\begingroup$ If you look at this asteroid discovery video, you see that the asteroid surveys are not aimed at NEOs but find lots of asteroids between Mars and Jupiter. $\endgroup$
    – Hobbes
    Commented Feb 23, 2019 at 8:58
  • $\begingroup$ @Hobbes I have! ;-) Anyway I adjusted my comment and replaced "NEO surveys" with "those surveys". I'm not so familliar with the specifics, so if you think current surveys have got Mars fairly well "covered", that would be an excellent answer to add. This is as far as I can go. $\endgroup$
    – uhoh
    Commented Feb 23, 2019 at 9:04

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