HopDavid

Tyson has pretty low standards for rigor and accuracy. Take everything he says with a grain of salt.

Tyson's an entertaining bull shitter. It's easier to be entertaining when you have no need for rigor and accuracy. See biologist PZ Myers comment on Tyson: https://scienceblogs.com/pharyngula/2016/03/17/some-days-its...

Much of Murphy's math and physics are wrong.

For example to get delta V to go to Mars he adds earth's 11 km/s escape velocity to the 3.6 km/s Mars injection velocity. A freshman aerospace student could tell the speed of a Mars bound ship in a hyperbolic earth orbit is sqrt(11^2 + 3.6^2).

I've done a number of posts calling out Murphy's bad arguments:

https://hopsblog-hop.blogspot.com/2014/02/the-most-common-de...

https://hopsblog-hop.blogspot.com/2014/03/murphys-reply.html

and https://hopsblog-hop.blogspot.com/2018/06/space-meow-boys_30...

I generally agree with him that we live on a fragile, limited planet and need to learn to live within our means.

However I don't see preserving our own planet and opening a new frontier as mutually exclusive. Musk is one of the most passionate space advocates and he's working for a sustainable future (solar panels, electric cars). Some technologies benefit both space development and sustainable comfort here on earth. For example CELSS (Closed Ecological Life Support Systems) technologies needed for space might be used here on earth.

Murphy may be correct that opening a new space frontier is implausible. But he hasn't conclusively demonstrated that's the case.

I would call Tyson a good spokesman for science if he had a high regard for rigor and accuracy. But he does not. http://hopsblog-hop.blogspot.com/2016/01/fact-checking-neil-...

Altitude of EML1 is about 320,000 kilometers. An ordinary 320,000 x 300 km earth orbit would be moving 10.81 km/s at perigee.

As an object falls towards earth it will surge ahead of the moon and the moon will be pulling it backwards. This makes effective apogee more in the range of 300,000 km.

A 300,000 x 300 km orbit has velocity of about 10.8 km/s at perigee.

In other words, dropping from the neighborhood of L1 will still give you speed very close to escape velocity by the time it reaches earth.

In the case of orbital tether, an elevator car would have gripping wheels in contact with the tether. If its's descending the wheels can power a generator to charge the elevator car's battery.

A.K.A. Coriolis force. Something that needs to be looked at in space elevators and orbital tetehrs as ascending (or descending) payloads exert a sideways push (or pull) on the tether and thus induce oscillations.

http://hopsblog-hop.blogspot.com/2016/04/liftport-lunar-teth...

There has actually been a lunar elevator proposed, the most massive part lying at Earth Moon Lagrange 1. In my opinion, not practical. I took a look using Xylon: http://hopsblog-hop.blogspot.com/2016/04/liftport-lunar-teth...

For the numbers I used a safety factor of 1. The slightest nick or scratch would cause a break. Safety factor of 3 is more sensible.

XKCD is usually accurate as well. Tyson on the other hand...

An elevator anchored to the earth and extending past geosynchronous orbit is wildly implausible.

However orbital vertical tethers are doable with Zylon. A series of 3 orbital tethers could move stuff between LEO and the lunar neighborhood. http://hopsblog-hop.blogspot.com/2016/08/tran-cislunar-railr...

Both Musk and Bezos seem well on their way to reusable boosters. But reusable upper stages is another story.

An upper stage re-enters at around 8 km/s. An ~7 km/s delta V budget plus the need to carry a payload means a very slim dry mass fraction, an egg shell space craft.

Recovering and reusing upper stages will be much harder than reusing boosters.

When mass parameter is big, use of WSBs is interesting. But mass parameters for sun-earth or sun-Mars are tiny. I've expressed my skepticism for ITN between earth and Mars: http://hopsblog-hop.blogspot.com/2015/04/potholes-on-interpl...

Tom Murphy has an ax to grind. He fancies himself a modern day Paul Revere shouting the urgent message "Peak Oil is coming! Peak Oil is coming!". Anything that might lead us to complacency he attempts to debunk. So of course he will slam the possibility of space resources. But his "math" is wrong. See http://hopsblog-hop.blogspot.com/2012/02/in-his-blog-strande...