25

Not strictly satellites/moons, but certainly companions are 2010 TK7 with a diameter of ~300 m, an Earth trojan at the L4 point, and the ~5 km 3753 Cruithne in a peculiar orbit locked to the Earth's.


16

It's almost 100% stars. In good conditions, you can see perhaps 2000 stars. (There are about 6000 naked-eye visible stars; of these, 3000 are above the horizon at any time, and about 1000 are hidden because they're too close to the horizon and blocked by the atmosphere.) The number of non-star objects you can see without assistance is tiny in comparison: ...


14

One kilometer, no way! That would've been known since long ago. Most asteroids of that size have already been found, all the way out to the asteroid belt beyond Mars. Earth has no second Moon. But there are always some tiny asteroids around, which are temporarily captured by Earth's gravity. Here's a funny illustration of such an orbit, it is not what we ...


8

Is there any publicly available, NEO-related database out there? Or is there a specific institution a hobby-astronomer can/should turn to to be able to learn more about individual NEOs? Yes, there is NASA's Near Earth Object Program that catalogues all detected NEO's and had advanced reporting and seearch capabilities (a bit overwhelming number of ...


7

That's the Moon alright, and it's definitely real and definitely there. If you go outside and look at the Sun right now, the Moon will be almost but not quite on top of it, though it's impossible to make out due to the Sun's glare. If it were any closer we would have had a solar eclipse around the time of the New Moon that took place 20 minutes ago. Two ...


7

Yes, Near Earth Objects (NEOs) include asteroids (Near Earth Asteroids (NEAs)) and a few percent of comets (Near Earth Comets (NECs)). As shown by the Update to Determine the Feasibility of Enhancing the Search and Characterization of NEOs (NEO SDT Report) in Section 2.3, the risk from Near Earth Comets is about 1% that of asteroids. So "NEO" tends to be ...


6

The cause for the oscillations perpendicular to the galactic plane is the gravity of the non-spherical mass distribution (needed for a plane Kepler ellipse) in the Milky Way. Simplified, there is a dense galactic plane. The density is not exactly known; therefore there is some uncertainty (a few million years) about the precise oscillation period. Details ...


6

If we accept the Giant impact hypothesis, which says that the Moon was once a part of the Earth pushed away by a big impact, the answer is obviously 0.


6

For gravity the force is : $$F=\frac {G M_1 M_2} {r^2}$$ But the acceleration : $$a=\frac F {M_2} =\frac {G M_1 } {r^2}$$ and this is the same for all bodies as it depends only on the primary's mass ($M_1$).


6

Most planets don't have rings. The ring region is inside the Roche limit which is quite close to the planet. A ring system outside the Roche limit needs to be either very faint or, it would over time coalesce into a moon or possibly pair of moons at Trojan points to each other. There's different Roche limits for rocky bodies vs icy, but an icy body ...


6

[rewritten to address the revised question] Maybe, depending on how fussy you want to be about "resolved". This is a study from 1995, using observations of asteroid 4179 Toutatis made in 1992 with HST. They reported marginal resolution of the asteroid, as suggested by this figure comparing a deconvolved image of a star (observed with the same filter and ...


5

No and yes. I'll address the "no" answer first. The answer is no if you truly mean "water ice". Ceres is close to the limit of where water can exist as water, as opposed to in the form of hydrated rock. Those intriguing bright spots on Ceres may well be water ice that has been exposed to vacuum (but they might just be salts). Ceres most likely is inside the ...


5

Unlikely. Plugging numbers into the Stefan–Boltzmann law gives us a temperature near 273°K (0°centigrade) for bodies near earth's orbit. The exact answer for atmospherless bodies depends on albedo. Any water on nearby asteroids will thus boil until it freezes, and then sublimate. That's why the search for nearby ice is focused on lightless, cold regions of ...


5

It depends on your definition of "layman" but Patrick Wallace has a slide presentation on what changed and why with the new reference systems. There is also the USNO Circular 179 which covers all of this and is pretty readable. In a lot of ways it boils down to the realization that the wobbly (in space and rotation) Earth was a lousy thing to base co-...


5

2016 HO3 does not really orbit Earth. It orbits the Sun in such a way that it happens to loop around Earth at distances of 0.1 to 0.25 AU. Earth's gravitational sphere of influence is much smaller, about 0.01 AU at most; outside that radius, the Sun is the dominant attractor. The Minor Planet Center has an orbital diagram which you can examine in 3D. Earth'...


5

It's not true; this object is not a moon of Earth's. Here's a NASA/JPL announcement of it. 2016 HO3 is the "name" being used for it at the moment. They're calling it a "quasi-moon". It's in an orbit that is in resonance with Earth's but is not permanently bound. (It's not the first object to be found with such properties too.)


4

This site is an asteroid tracking program started up by NASA. It seems to be fairly effective at detecting asteroids. Read up on this article for further information about NASA's plan.


4

This is more of a recent example than a record. On 2019-07-25, asteroid 2019 OK passed about 65000 km from the Earth at a relative speed of 24.5 km/s. The Minor Planet Center lists multiple observations from sites in Italy and Armenia an hour before closest approach. Using pairs of these (streak endpoints?) by the ISON-Castelgrande observatory, I compute ...


4

The Near-Earth close approches website shows close approaches to the Earth by near-Earth objects (NEOs). The table showing all close encounters indicates the absolute magnitude. The data can be exported to a CSV file to estimate the apparent magnitude for each object, using the following equation. $$ m = H + 5 \log_{10} \bigg( \frac{d_{BS}d_{BO}}{d_0^2} \...


4

It was fairly poor. Wikipedia has a list of large, potentially hazardous asteroids, and 9 on that list were known before 1998: 2201 Olijato 1620 Geographos 4183 Cuno 1981 Midas 3122 Florence 3200 Phaethon 4486 Mithra 4197 Toutatis (4953) 1990 MU The largest of these, 3200 Phaethon, is 5.8 km, so considerably smaller than the Yucatan impactor, but big ...


4

You are right. If Mars orbited in exactly the same plane as the Earth, instead of an S or a loop, we would see Mars moving prograde relative for the stars along the ecliptic, then slowing and stopping, moving retrograde for a few months, as Earth overtakes it, still on the ecliptic, then moving prograde again. But Mars doesn't orbit in the same plane, so it ...


3

For the asteroid not to break up it needs to either be big, or tough. You can experiment with the impact effects calculator but you should notice that a rocky asteroid with a diameter of less than about 1km will partially break up on impact with the atmosphere, and there may be multiple craters formed. If the asteroid is less than about 100m in diameter ...


3

I posted an answer for this on Physics SE recently, but have also just had a query on this from another Astronomy SE answer, so I am adding this here for completness. You can approximate the plane of the galaxy as a disk made up of stars and gas, with a density $\rho(|z|)$, that decreases with absolute distance $|z|$ from the plane. If then assume that the ...


3

Hear and smell: Nothing. The comet and its coma are well outside the atmosphere, no sound or scent can travel through space. The coma of the comet could be 30000km across, bigger than the moon, and the comet is half the distance to the moon, so the coma could appear about 20 times larger than the full moon. However, it would be very dim. Although its total ...


3

The first point to make is that categorization isn't an exact science. It's done for convenience and practicality and sometimes, familiarity. It's not going to be perfect. The second point to make is that scientists like categorizing things. Take the Taxonomic ranking of living creatures. Everything from a paramecium to an elephant has a species, a ...


3

This is because there are many more small NEOs than there are large ones and the larger ones are easier to find and so were found first. The numbers of NEOs follow what's know as a power law distribution with an exponent of ~1.75. This means that given the roughly factor 7 difference between 140m and 1000m NEOs, there will be 7^1.75 = ~31 times as many 140m ...


3

The least stable orbits are likely the temporarily captured orbiters of Earth and other planets. These are bodies that have been captured by the Earth/Moon gravity well and move from solar orbits to terrestrial orbits. They normally have complex orbits that take them well beyond the orbit of the Moon and most don't last long. Most such objects are small, ...


2

I think you answered your own question with your second paragraph. According to Newton's law of gravity the Gravitational force is directly proportional to the product of the masses and inversely proportional to the square of the distance between them. So the biggest body in the neighborhood has the most pulling power provided it agrees with the equation....


2

Most likely it's Jupiter. According to the software I use to run my telescope and plan my observations, Jupiter rose at about 6:40 pm and set the following day at about 8:30 am. It would have been about -2.6 magnitude ,making it the brightest natural object in the sky that night. It, of course, rose in the east and would have traveled in an arc across the ...


2

The short answer is "not very, but we're getting better". In the case of particularly large Earth orbit intersecting bodies, kilometers across and bigger, they're fairly well known and tracked. We'd probably get months to years of warning. Technologically, we've probably got the capability of diverting such an object if we have enough warning, but it'd ...


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