Throw a Probe into Orbit


We send astronauts into space for many reasons. Science, for instance. Listen to what Steve Squyres, who heads up the science on the Mars Exploration Rovers thinks about human exploration. In 2009 he famously said, “What Spirit and Opportunity have done in 5 1/2 years on Mars, you and I could have done in a good week. Humans have a way to deal with surprises, to improvise, to change their plans on the spot. All you’ve got to do is look at the latest Hubble mission to see that.” I agree. I would go farther to say that we’ve barely scratched the surface of what humans can do in space. When people interact with the space environment, they find ways to adapt to and exploit the surprising physics of microgravity. Remember how the Apollo astronauts learned to hop on the surface of the moon, as a quick way to travel in that one-sixth gravity? Let’s consider an idea that may seem a little frivolous at first but, I think, makes this point even more clearly. How about an asteroid rover that an astronaut can launch simply by throwing it into orbit?

First, a few comments about asteroids. They come in many sizes, but in fact nature makes no clear distinction among minor planets, planetoids, dwarf planets, and many other names that keep students busily memorizing taxonomies in astronomy classes. To make this discussion simple, let’s agree that we’re considering celestial bodies small enough that their gravitational field would allow someone to throw a space probe into orbit around it.

How small?  Well, remember that the more mass, the greater the gravitational field. And the farther from the celestial body, the weaker the gravitational pull. So, the gravitational field of two spherical asteroids of the same mass would be the same at a given distance from the center, but the surface of one asteroid might be closer to its center if that asteroid is denser than the other. That’s because the same mass takes up less space, and the volume (therefore the radius) of the asteroid would be lower. The density of an asteroid composed of mostly rocky material could be less than 2000 kg/m3. A nickel-iron asteroid may be triple that density, and it would have less than 70% the radius of the more rarefied one and about double the gravitational pull at its surface.

A spherical asteroid of about average density (just over 3000 kg/m3) and 25 km in diameter would do it. At about that size, an astronaut standing on the surface would have to toss this probe at 25 mph for it to enter a circular orbit just above the surface. The period of this orbit is about 111 minutes. She should set an alarm on her watch so that she remembers to duck an hour and 51 minutes later when it comes around again. Throwing it a little harder—about 37 mph—allows it to escape the gravity of the asteroid entirely. I normally use metric units of measurement, but I think of baseball pitches in mph. I found out recently at the Ithaca Sciencenter that my fastball is now down below 70 mph. But even I could toss a spacecraft into orbit this way.

Let’s take a look at 433 Eros, which happens to be about the right size. Here’s a picture of it, taken in 2000 by NASA’s NEAR spacecraft. It’s also the asteroid referenced in the novel Ender’s Game.


Image courtesy of NASA

Yes, it’s lumpy. It’s about 11 km in the narrow direction and about 34 in the other. That makes the orbit mechanics more subtle. The rover wouldn’t really execute a nice circular orbit. It would wander around, probably smacking into the surface at some point. It might also escape at even lower speed. But consider the opportunities: an astronaut could toss equipment, such as science sensors, prospecting hardware, communications-network nodes, transmitters, and many other useful components to virtually anywhere on the asteroid’s surface. An astronaut looking for valuable materials, such as water, might never even have to leave her landing site.

This idea suggests that human exploration of the cosmos will likely be conducted in a way that the Apollo astronauts would find very unfamiliar. These days we carefully script the activities of astronauts, safely planning their extravehicular activities and scientific investigations aboard the International Space Station. And that’s perfectly appropriate—for now. In the decades to come, we should expect our natural creativity, resourcefulness, and adventurousness to once again determine how we make the cosmos our own, like our ancestors did when they left Africa hundreds of thousands of years ago.


One thought on “Throw a Probe into Orbit

  1. Pingback: Sometimes Even a Low Ballistic Coefficient Needs a Little Help - Spacecraft Lab

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