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Sunday, November 20, 2011

Going the distance

Scientific American this month has a very good article on some possibilities for NASA's manned space exploration program, written by two scientists involved in robotic deep-space exploration.

They proposed using a combination of the SLS chemical rockets or Delta IV heavy lift and Hall-effect ion drive spaceships to explore the moon, near-earth asteroids, Mars and Phobos. The argument for exploring near-earth asteroids was as a stepping stone for Mars and Phobos, and they were specifically looking at NEO 2008 EV5.

2008 EV5 follows a slightly oblique orbit very similar to earth's, and oscillates from about 1 AU (the distance from the sun to the earth) and 2 AU, at least in the immediate future. Mars at opposition is only about 0.5 AU from earth. My admittedly limited understanding of Hohmann transfer orbits is that a trip to 2008 EV5 or an earth-Trojan should take longer than a trip to Mars. Given that 2008 EV5 is only about 400 meters across, I must confess that I am at a loss to understand the wisdom of establishing 2008 EV5 as a waypoint to Mars. This is either a failing of my own understanding of how one navigates between two objects in different parts of the same orbit (very likely), or else NASA has other reasons for prioritizing a trip to 2008 EV5. My guess is that both are true; ie, I'm misunderstanding the geometry of a same-orbit Hohmann transfer, and NASA has more need of landing on 2008 EV5 than as a test-run for Phobos. My completely uneducated guess is that either NASA is more freaked out about near-earth asteroids than they let on, or that they want congress to think that they're more freaked out about near-earth asteroids than they let on. I lean toward the latter; perhaps at some point I'll bother to post my very own patented conspiracy theory on the subject. Anyway--

Based on the SciAm article and some of the recent posts here on candidates for outmigration, I created a graphic tonight to illustrate the relative distances of the different candidates being discussed. The planets, asteroids and moons themselves are not drawn even close to scale, but the relative distances between them is mostly right. The only exception to this is the distance between the earth and the moon, which is actually so small on the scale of the drawing that it could not be correctly shown at all. Also, the distances between Jupiter and her moons is completely erroneous, I simply included the Galilean moons to illustrate that the body depicted was in fact Jupiter.

1. The moon, and also the L4 and L5 Lagrangian orbits

2. Near-earth asteroid 2008 EV5, and also the earth-Trojans

3. Mars and Phobos

4. Ceres

5. Jupiter and Europa, Callisto and Ganymede

6. Saturn and Enceladus

It becomes apparent that Enceladus, intriguing though it is, is very, very far away. For a new home for our species, Ceres starts looking better all the time.

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