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This is similar to other questions here (like this one or this one), but I couldn't find one that was identical to it.

The scenario: a small spaceship (10-20 meters in size), or possibly a small fleet of half a dozen of them, is approaching present-day Earth from the outer solar system. We have no particular reason to be expecting it, so we're not actively trying to overcome any "stealth" technology it might be using. Assuming the people who built the ship wanted it to go unnoticed and had a slightly better tech level as our own, how close could it get to Earth before it was spotted by the public? If the answer is "not very close at all", then how advanced would their technology have to be to change that?

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    $\begingroup$ What kind of approach are we talking here? Are they stopping at some distance to observe, or is their intent to land? "10-20 meters" is about the size of the Chelyabinsk meteor, which came in undetected from roughly the direction of the Sun and made it all the way into the atmosphere before detection, so if they just want to get to Earth, that'd be their model to emulate. $\endgroup$
    – parasoup
    Commented Oct 21, 2020 at 17:10
  • $\begingroup$ @parasoup The intent is to land safely, so there would need to be some deceleration involved so they didn't explode on entry, if that affects things. $\endgroup$
    – Admiral Jota
    Commented Oct 21, 2020 at 17:17
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    $\begingroup$ I think he is saying that a deliberately stealthed approach direction could easily fool us $\endgroup$
    – Innovine
    Commented Oct 21, 2020 at 18:47
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    $\begingroup$ What kind of propulsion are they using? While 10-20 meters in size maybe small enough not to notice, any realistic engine burn would shine quite brightly. $\endgroup$
    – Alexander
    Commented Oct 21, 2020 at 18:49
  • $\begingroup$ Simply fly in past the sun, and you will remain virtually undetectable. $\endgroup$
    – Karl
    Commented Oct 21, 2020 at 20:34

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Minor frame challenge

Unless they have some sort of quasi-magic FTL drive, they have to be much higher up the technology ladder than we are to be able to make an interstellar trip in something 10-20m in diameter. Now, if you're using the Road Not Taken route, that's reasonable enough, but otherwise there are enormous questions of fuel, energy, and life support.

Ignoring that...

It Depends on How Long They're Willing to Take

If the sneaky spaceships are willing to take a few years to get here, they could use an orbital transfer from the gas giants to Earth orbit, and (I am not an astrophysicist) could probably position the burns so that a planet or the sun lie between the ship and the Earth when they occur. These transfer orbits are popular with people like NASA because they take very little propellant and minimize the energy required to bring a probe from location A to location B, but limiting energy expenditure also makes your spacecraft less easy to detect.

The problem is that on a mostly ballistic orbital course, getting to Earth would take years from even Jupiter's orbit.

So if your space travellers are very patient, they can arrive virtually undetected until they pass geostationary orbit. If they're not, the deceleration burns will be visible from light-minutes away, and earth-based telescopes will spot them even if we're not looking for them.

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  • $\begingroup$ If they are able to make all delta v when hidden by jupiter, it takes a year in free fall from there if they take the New Horizons trajectory backwards. $\endgroup$
    – Karl
    Commented Oct 21, 2020 at 20:20
  • $\begingroup$ @Karl the problem is that an orbital transfer requires burns at two orbital "heights". So the New Horizons trajectory would include a burn aimed right at Earth - not great for stealth. A more roundabout trajectory might allow them to keep objects between them and Earth for the duration, but would doubtless take longer. $\endgroup$
    – jdunlop
    Commented Oct 21, 2020 at 22:33
  • $\begingroup$ FTL won't be an issue, since this isn't an interstellar trip. They're just coming from the outer solar system (Jupiter's moons, specifically). $\endgroup$
    – Admiral Jota
    Commented Oct 22, 2020 at 3:38
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Very Close

We are not always scanning the sky for threats. According to this resource, space agencies perform this task intermittently, often a few years between scans, when funding and resources are available.

When an object is detected, it's orbital parameters are determined and it is added to a catalog of Near Earth Objects. The present-day positions are estimated using this information until the next scan.

Missile defense radar systems look at objects rising into to horizon. They are not looking into the deep sky.

In 2013, the 20 meter Chelyabinsk asteroid entered Earth's atmosphere completely undetected and airburst over the Russian sky.

If your alien vehicles can perform a powered descent, and avoid big, bright re-entry heating, then the alien craft could be well on their way to landing before being detected in the sky by missile early warning systems.

What about if the fleet had the poor luck to be arriving during a scan?

According to this article, the state-of-the-art technology for detection of an 11 square foot object is 900 miles. Converting, this is about 1 foot / 0.33 meter radius, or 2 feet / 0.66 m diameter.

Scaling up to your ships of 10 or 20 meters diameter, if it's a linear relationship, they'd be detected between 13,636 and 27,272 miles. If we are scanning at the time. This altitude is the lower-end of geostationary orbit.

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With our current technology, we can create materials that bend light around them, only leaving the background visible.

Example video

There is even a building started in 2020 that will have this tech applied to the whole thing. So with technology more advanced than us, it wouldn't even be a stretch to say they could apply this principle to moving ships the size of asteroids.

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  • $\begingroup$ What building is currently under construction that uses (horrendously expensive) metamaterials over the entire building? (Also, the metamaterials that bend light do so in very specific wavelengths. Doing so for all detection (using metamaterials) may be impossible. $\endgroup$
    – jdunlop
    Commented Oct 21, 2020 at 18:06

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