How far is Up?

And other Interesting Questions

I just returned from a trip where I gave five astronomy presentations, followed by Q&A sessions.  It’s always a revelation to hear people’s questions about space and astronomy.  I thought I’d share them with you, plus some answers.

How far is up?

Mucho laughter ensued when that one was asked, because it really sounds like a silly question. But, it’s not. And it’s a question that’s not substantially different (in meaning) from “How far is out?”

In one sense, “up” and “out” end at the observable limits of the universe.  At this point, that’s the time right after the Big Bang when the “cosmic dark ages” ended and the first stars began to shine, maybe 13.3 or so billion years ago.

Perhaps the questioner was wondering how far it was to the edge of space from Earth’s surface?  Well, do you mean the limit of the observable universe? Or, perhaps the place in space where Earth’s atmosphere ends and “space” begins?  If so, then some calculations suggest that it begins at about 100 kilometers above the surface, although the atmosphere really still exists out to just under 1,000 kilometers. That’s where I’d consider the “edge of space” to be.

According to my friend Phil Erickson, at MIT’s Haystack Observatory, the atmosphere itself ends at the level of something called the “geocorona”. This is a region where all that is left is of our atmosphere is a collection of neutral/ionized hydrogen atoms. Here, the  “mean free path” of a particle — that is, the average distance covered by a moving particle, atom, photon, etc. between successive impacts with other particles — is so long that collisions don’t happen very frequently and the particles are on ballistic trajectories through space.  Depending on the season, the part of the solar cycle we’re in, the temperature, and other factors, the geocorona can begin anywhere from 500 to 800 kilometers.

How do supermassive black holes in galaxy cores get started?

That’s a good one. One theory is that perhaps a stellar black hole begins to swallow up more and more material — getting more massive as it does. Eventually, after millions and millions of years, a supermassive black hole results.  Another idea is that perhaps a cluster of stellar black holes merge to become one big supermassive one.  Astronomers don’t have a single formation scenario worked out yet, but they will, especially as they study galaxy evolution at different epochs in cosmic history. Galaxy evolution and supermassive black holes seem to be connected in many ways.

Can comets suck water away from Mars and

could that explain where all the water on Mars went?

Wow. That’s ingenious.  But, as much as we’d like to know exactly WHERE Mars’s water went and how the planet lost its atmosphere, the idea that a comet could suck out tons and tons of water from a planet isn’t correct.  Look at the physics of the situation: comets are chunks of ices, not very large and certainly don’t have the gravitational pull to suck water from a planet.  It’s more likely that Mars’s gravitational pull could have caused a comet or several to swerve into the surface in the past. But, not the other way around.

How much does it cost to be an astronomer?

A lot.  College costs a lot, graduate school costs more.  But, the price depends on the schools one attends.  Mentally and physically, it’s stimulating work, and if you go at it with the right attitude, the intangible costs are far outweighed by the rewards of discovery and knowledge.

To the folks who attended my talks in the past couple of weeks — it was a pleasure to meet you and share astronomy!  Keep looking up!


  1. Awesome questions, LOL… found you in the 2010 Top Space blog award winners at, very happy to see such informative posts. Enjoyed the geocorona definition — very artfully put for what can be a complex concept =)

  2. simon

    Hi, someone already asked “How far is up?” and you gave an answer but i’m going to ask again, simply because you hear NASA say things like “the shuttle is 100 miles above earth” but how far do you have to be to not be “above earth?” surely at some point you are just ‘in space’

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