Ask Ethan: Could an atmosphere slow down a runaway spaceship? (Synopsis)

"Yes, now there is this technological path. But it's just starting." -Mae Jemison

Earlier this month, Yuri Milner and Stephen Hawking teamed up to announce the Breakthrough Starshot, a $100 million investment in technology that would build a laser array to propel a thin, light "laser sail" spacecraft to approximately 20% the speed of light. If we can achieve these speeds and sufficiently aim these sails at the nearest star systems, we'll arrive at our destinations within a single human lifetime.

Illustration of an exoplanetary system. Image credit: NASA/David Hardy, via Illustration of an exoplanetary system. Image credit: NASA/David Hardy, via

But we'll still be going at 20% the speed of light when we get there, or about 1,000 times as fast as the meteors that burn up in our own planet's atmosphere. Is there any chance for slowing these spacecraft down once they arrive, or are they doomed to burn up (or miss completely and leave the galaxy) upon arrival?

Depiction of the Earth-grazing meteor of 1860, by Frederic Edwin Church. Image courtesy of Judith Filenbaum Hernstadt. Depiction of the Earth-grazing meteor of 1860, by Frederic Edwin Church. Image courtesy of Judith Filenbaum Hernstadt.

Find out what physics has to say about it on this week's Ask Ethan!

More like this

"Fundamental physics is like an art more or less. It's completely non-practical, and you can't use it for anything. But it's about the universe and how the world came into being. It's very remote from your daily life and mine, and yet it defines us as human beings." -Yuri Milner In one of the…
"Our existence in this place, this microscopic corner of the cosmos, is fleeting. With utter disregard for our wants and needs, nature plays out its grand acts on scales of space and time that are truly hard to grasp. Perhaps all we can look to for real solace is our endless capacity to ask…
“Not going to the Moon and banging on it with my own hammer has been the biggest disappointment in life.” -Gene Shoemaker, co-discoverer of Comet Shoemaker-Levy 9 Don't fret about your taxes here at Starts With A Bang; I'll continue to bring you the Universe for free! (Although I do take donations…
"Science is the only self-correcting human institution, but it also is a process that progresses only by showing itself to be wrong." -Allan Sandage As April leaves us and May commences here at Starts With A Bang, I'm so pleased to inform you that amazing things are happening! Thanks to the support…

Sounds like a great use of $100 million.

By See Noevo (not verified) on 23 Apr 2016 #permalink

It certainly does sn. The human urge for exploration is strong and a good part of the reason we've achieved so much. It's good to see you getting on board with the support of science and away from your typical dark-ages inspired endorsement of ignorance.

That is what you meant, right?

To Ragtag Media #3:

Well, if it was between those two bets – coal gasification or Ethan’s subject – I think I, like you, would favor the former.

But they better hurry up!
Because Hillary said
“We’re gonna put a lot of coal miners and coal companies out of business.”

By See Noevo (not verified) on 23 Apr 2016 #permalink

Makes an interesting enigma - get to the target planet first, set up a power grid and laser site to slow the probes down on their arrival so that we can explore the local environment to decide where to set up our base.
The chicken and the egg again?

Sounds like a great use of $100 million.

Color me unsurprised that S.N.'s desperate quest for attention would lead him to whine about the expenditure of private monies. Perhaps he could slip Yuri Milner a note of correction, as he was "planning" to do with the pontiff in Philadelphia.

Yeah, it's not his money, but he damn well wants to own it and tell people how to spend it.

Fiddling in other people's business is what the rightwinnutjob desires, just as long as they're the ones investigating, NEVER the ones being investigated.

Hypocrisy being the major feature of the RWNJ. Stupidity being a strong #2.

Another way of looking at how hard breaking is: at 60,000 km/s it will take about 0.2 seconds to pass an Earth sized object. To stop in that time you will face a 30 million g acceleration. So even if you manage to create some magic heat shield that dissipates all the kinetic energy, your probe will still be crushed.

I assume the idea is instead to just pass by and transmit information home from the very brief passage, but I haven't seen any calculation if such a small probe can really carry a sufficiently strong transmitter and antenna to reach Earth. Anyone done calculations on that?

Speaking of the probe suffering from being heating by colliding with gar molecules. If I did my back of the envelope right, hitting a proton, it would have a relative energy of 20MEV, so its not just energy for heating, its heating by hard core ionizing radiation. The material of the probe is going to be severely tested. And hitting enough gas to lose a substantial fraction of your speed, means most probe atoms will suffer more than one collision.

By Omega Centauri (not verified) on 24 Apr 2016 #permalink

@Omega Centauri #9: Yes, you did it right: 19.34 MeV per proton :-) Gamma-1 = 0.0206, but m = 0.938 MeV.

With an average interstellar density of a few atoms per cubic centimeter (1e-4 in hot, ionized regions to 1e+6 in molecular clouds), a solar sail with an area of 1 km2 at 0.2 c will see a flux of something like 6e+16 impacts per second, or a heat load of 192 kW.

The sail is obviously "designed" for that kind of load (the assumption that it will be illuminated by a multi GW laser means it must be :-), but that's still going to be non-trivial drag slowing it down, not to mention the physical damage (atomic displacements and just plain punctures) from those impacts over ~20 years.

Hmmm...another interesting calculation. Average of 6e16 impacts/second for 20 years is just under 4e25 total impacts. A graphene sail with 100 mg mass has just 0.008 moles of carbon, or 5e21 atoms. So over the course of the journey, EVERY SINGLE ATOM in the sail is likely to experience 80 direct hits, invoving either ionization or a nuclear interaction.

Look up "neutron spallation effects" for details on how the sail is likely to end up :-)

By Michael Kelsey (not verified) on 24 Apr 2016 #permalink

Omega Centauri, would you rather reserve that $100 million for something more promising?

You'd make at least three here who would.

By See Noevo (not verified) on 24 Apr 2016 #permalink

See Noevo, if it were my $100 million that's not how I'd spend it. But, its not my money, I am content to allow its owners to use it how they see fit.

If they hadn't committed yet, and asked my opinion, I would have suggested other things, but gain I think its too late for that. Plus I don't expect them to ask.

By Omega Centauri (not verified) on 24 Apr 2016 #permalink

Let's face it, as soon as the craft starts to slow, your payload just went through what remains of the sail.

It's almost as if they plan on burning the money to fire a rocket, the way some people talk about it.

I'm pretty sure the 100 M$ are gonne stay right here on Earth an go back into the economy.


Of course aerobraking is impossible at that speed, but I was wondering if a combination of solar/magnetic braking could appreciably slow the probe as it approaches the target star.

By Michael Hutson (not verified) on 01 May 2016 #permalink

100 mils could be used better, of course. The first problem is to accelerate the probe. Laser beam has not the same diameter- therefore, acceleration must be performed not far from the Earth and it must be really huge. Huge acceleration means large pressure and therefore hi beam's intensity. Sorry, people- sail will be evaporated, even if it gulps 0.01% of beam's energy. Forget about interstellar travels- this task is not for this century. There are many interesting objects in Solar system.

By Andrey Matseevskiy (not verified) on 09 May 2016 #permalink