The question Did a spacecraft ever use an atmosphere to accelerate away from a planet? was unfortunately given five down votes and answered with:

Entering the atmosphere introduces drag, which could only reduce your energy.


Your premise is incorrect. In no case does "skipping off the atmosphere" leave you going faster than you arrived, engines on or not.

and yet a few days later the New York Times said of a 2017 fireball (quotes are from "Patrick Shober, a graduate student at Curtin University in Western Australia who led a team that studied the event" which was measured by the Desert Fireball Network published in arXiv as Where Did They Come From, Where Did They Go. Grazing Fireballs and will be published by The Astronomical Journal):

By triangulating its trajectory from multiple positions, Mr. Shober traced the fireball back to the asteroid belt between Mars and Jupiter, his team reports in a paper that will be published by The Astronomical Journal. As it reached Earth, the planet gave it an extra kick.

“It gained orbital energy from the close encounter the same way a space mission might use a slingshot maneuver,” he said, referring to the orbital navigations NASA and other space agencies use to speed robotic probes toward their destinations.

That sent it careening toward Jupiter, giving it an elongated, outbound orbit more like a comet’s than an asteroid’s. Its path interests astronomers, who can’t study anything this small through a telescope.

Question: Superficially at least it seems the quotes in the NYTimes seem to contradict those answers, but do they? Or do they perhaps just need some refinement (e.g. gain/lose energy with respect to what or in which frame)?

Further reading from the arXiv preprint.

4.2 Short-term Simulations

... As a result of the grazing encounter with the Earth, the meteoroid was flung into an orbit with a higher energy (Fig. 8). The geometry of the encounter enabled the meteoroid to gain angular momentum around the Sun (Fig. 10). As a result, the semi-major axis and eccentricity both increased due to the increase in energy, and the object was inserted into a JFC (Jupiter family comets) orbit. Hereon, the object’s future is strongly governed by its interactions with the gas-giant. Fig. 9 shows the evolution of the orbital elements for the meteoroid ±100 years relative to the grazing encounter.

  • $\begingroup$ Maybe the asteroid underwent a perturbation that had increased its orbital energy, however, its trajectory happened to intersect the Earth's surface? $\endgroup$ – BMF Mar 31 '20 at 0:27
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    $\begingroup$ For example backscattering a tennis ball off of a moving bowling ball can increase the tennis ball's speed in the bowling alley's frame, even if that tennis ball is old and soft and so loses energy in the bowling ball's frame. $\endgroup$ – uhoh Mar 31 '20 at 0:59
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    $\begingroup$ I don't think "bouncing off the atmosphere" is a thing. But I realized this is not a question for me. $\endgroup$ – Organic Marble Mar 31 '20 at 1:57
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    $\begingroup$ My interpretation of the summary at xArchiv is that the meteor gained far more energy-- or at least speed -- from the slingshot than it lost to atmospheric friction. $\endgroup$ – Carl Witthoft Mar 31 '20 at 14:29
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    $\begingroup$ This might be what uhoh just said. Atmospheric drag would slow the meteor relative to the planet. But if, say in the sun frame, the earth is moving faster in its orbit than the meteor is in its own, that might give it a boost. Like a fast-moving tennis ball hitting a BB, an imperfectly elastic collision. $\endgroup$ – Greg Apr 4 '20 at 14:35

I am Patrick Shober (the lead author of the study). Thanks so much for checking it out! If you check out Figure 10 in the paper, I have plotted the specific angular momentum in the Sun-centered frame.

So this shows how the meteoroid (the rock) gained energy during the close encounter but then lost a fraction of it due to the atmospheric passage. This can be seen in the non-continuos drop in the plotted curve; it is discontinuous because the time the rock spent in the atmosphere is omitted. So while it went through the atmosphere, it did lose energy, but not as much as it gained from the close encounter.

So, to answer your original question, the object we observed did not gain energy from hitting the Earth's atmosphere. It gained energy DESPITE hitting the atmosphere. You could imagine if the Earth didn't have an atmosphere, then the rock would have gained more energy than it did in reality.

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    $\begingroup$ How great to get this authoritative answer! Welcome to space stack exchange! $\endgroup$ – Organic Marble Apr 7 '20 at 16:41
  • $\begingroup$ Thank you for your answer, it's great when someone from scientific community with an authoritative perspective stops by the site to leave an insightful answer! While in this case the object did not gain additional energy in the heliocentric frame due to atmospheric drag's effect on the encounter, here I'm really wondering if it's necessarily axiomatic that something like that could never happen under any circumstance, or if it can't necessarily be ruled out in all cases. $\endgroup$ – uhoh Apr 7 '20 at 17:25
  • $\begingroup$ For example, I mentioned a scenario in this comment where I have a hunch an encounter with the Earth's atmosphere could "pull the object along" and give it a mild boost in energy, raising its perihelion. $\endgroup$ – uhoh Apr 7 '20 at 17:26
  • $\begingroup$ just fyi here's another Desert Fireball Network question: Will they really be able to “see” OSIRIS-REx from Australia? With meteor cameras? and though I don't know if your cameras are covered during the day and protected from the Sun, there's this question in Photography SE: Are there industry standards or specs for image sensor resistance to damage from intense light? $\endgroup$ – uhoh Apr 7 '20 at 17:41
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    $\begingroup$ In the case when the Earth is catching up with an object from behind, like in your comment, the object would lose energy in the heliocentric frame due to the encounter but it may lose less due to the atmosphere, but it certainly wouldn't net gain energy. But this all depends on the geometry during the encounter, either way (energy gained or lost) the atmosphere I believe would only dampen the effect. Also, thanks for the heads up. I just commented an update on that question. Cheers! $\endgroup$ – Patrick Apr 8 '20 at 1:39

well, its called a gravity assist. the asteroid took a small amount of earth's velocity and used it to speed up. this is different than skipping off a atmosphere think about skipping a rock on a lake, does it speed up? No. With a gravity assist it's like stealing a bit of energy from the planet to propel your much smaller ship, asteroid whatever it is. Skipping on a planet can keep you from hitting it, like a rock skipping off of water, it doesn't sink. anyways, no contradiction here. hope i helped!

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    $\begingroup$ But we also need to consider the recoil effect as discussed in these comments 1, 2 $\endgroup$ – uhoh Mar 31 '20 at 1:01
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    $\begingroup$ The problem is that typical slingshot distances are well outside the atmosphere, and no "fireball" effect takes place. $\endgroup$ – Carl Witthoft Mar 31 '20 at 14:26
  • $\begingroup$ yes but what could have happened is it skipped off, after entering and then gained energy from the gravity assist later on. $\endgroup$ – Topcode Mar 31 '20 at 14:44
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    $\begingroup$ They're confusing two different things is what is happening. Theoretically, you could do a gravity assist in a very, very, very thin atmosphere and gain velocity from the maneuver. This isn't in any way thanks to the atmosphere though. All the atmosphere was casue drag on the craft. $\endgroup$ – Ocelotus Apr 1 '20 at 17:54

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