Linkdump: February 2018

San Jose and San Francisco and the Bay photographed from a U-2

4 thoughts on “Linkdump: February 2018

  1. Did anyone read the solar gravitational lensing paper?

    I only read it in snippets but although they look at likely technologies for getting to the SGL focal line (which is something like 15 times as far from the sun as Pluto), they don’t seem to propose any way to stop once the craft gets there! They talk about needing gradual movement to keep up with solar precession etc but they don’t address the fact that your craft reaches the focal line travelling at 150km/s and needs to take integrations for several seconds per pixel! Stopping from that speed takes a crazy amount of energy & you can’t just slam on the brakes in the depths of interstellar space. Or maybe I missed something.

    Anyone get something I missed from this?


    1. PS back of the envelope calculation:

      0.5 x m x v^2 for a 100kg craft at 150km/s = 10^12 J ish. That’s 1 Terrajoule of keinetic energy!

      The 15 kiloton nuclear bomb that was detonated on Hiroshima released 63 TJ – that means that we are looking at the energy in around 250 tons of TNT to stop a 100kg craft.

      That’s orders around 3 orders of magnitude out. No way this could be done chemically. Some sort of small nuclear devices is the only way we could possibly stop something at that speed.

    2. You are indeed missing something: “…reaching these distances in ~20 years implies moving at a radial velocity of >20 AU/year, which makes stopping impractical. Instead, the spacecraft will proceed outwards while it continues the imaging campaign.”

      Even if it could get there, there are good reasons to question how useful of a telescope it would be — many of which are discussed in that (and other) papers on the subject.

      1. Ah – that makes more sense, assuming that you can continue imaging as you move at that speed.

        I must have missed that as I dipped in and out of the paper. It’s a fascinating idea. And that’s probably the most understated use of the word “impractical” I’ve yet met!



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