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The Dawn spacecraft continues to close in on Vesta, one of the last unexplored objects of appreciable size between here and Pluto. Dawn is expected to go into orbit around the asteroid on July 16.

NASA/JPL-Caltech/UCLA/MPS/DLR/PSI

This is how Vesta looked in the navigation camera view as of June 20, when Dawn was 117,000 miles away. And here’s a time-lapse view of the approach so far.

Read more about the Dawn mission in our June/July issue, or listen to this recent talk by project co-investigator Carle Pieters (below), who describes what scientists hope to learn at Vesta.

Today, two asteroids pass the Earth at a fairly snug distance. One, 2010 RX30, went by just before 6:00 a.m. EDT about 154,000 miles above the northern Pacific Ocean. That’s equivalent to three-fifths the distance to the moon. The other one, 2010 RF12, goes by just after 5:00 p.m. EDT, above Antarctica, and offers a slightly closer shave: 49,000 miles, a bit bit more than twice the altitude of a geostationary communications satellite.

Two asteroids do a drive by today, September 8, 2010. Image: NASA/JPL

The first rock was a maximum of 65 feet in diameter, and the second a max of 46 feet. So even if they made a direct hit on the home planet—even though they’d each unleash the force of multiple Hiroshima-size atomic bombs—they’d do so tens of thousands of feet up in the atmosphere. If it happened over land, which covers thirty percent of the planet, it probably wouldn’t come close to the destructive power of the Tunguska Event of 1908. That object might have been a stony asteroid a couple hundred feet in diameter, or a more loosely packed comet a few thousand feet in diameter. Either way, not even that object made it near the ground, and probably exploded three-to-six miles high. Still, Tunguska unleashed a few thousand times the force of a Hiroshima explosion, and flattened 830 square miles of Siberian forest.

Here's a top-down view. Image: NASA/JPL

Astronomers guess that asteroids up to 35 feet in diameter shoot by within the Earth-moon distance of 240,000 miles about once a day, and enter the planet’s atmosphere perhaps once every ten years. Asteroids ten times that size arrive on average every 10,000 years. Both of today’s asteroids are close enough to be visible through amateur telescopes.

Seeing as how this subject has a direct bearing on human survival, it’s a topic we’re interested in at Air & Space. Read our article about meteors.

On the Mayflower, at least you could go out on the deck. (Image: Lockheed Martin)

At first I was excited to read press reports of a Lockheed-Martin concept for a bare-bones human asteroid mission, using a pair of Orion capsules yoked together. Finally, a near-term plan! Because the Orion is mostly built, the first “Plymouth Rock” mission could fly as early as 2016, nine years earlier than NASA’s (extremely) loose target for an asteroid trip.

After skimming a 41-page Lockheed white paper on Plymouth Rock, I’m less excited.

I don’t doubt that motivated astronauts would forgo a certain amount of comfort to make history’s first trip into deep space. And the three-month trip to the asteroid wouldn’t be so bad. Two Orions provide about 10 cubic meters of volume for each of two astronauts—not nearly as roomy as the International Space Station, but bearable. On the way back, though, one of the vehicles would be jettisoned, leaving the pair to squeeze into a single capsule. Now they’d each get four cubic meters. For three months. That’s equivalent, says the white paper, to the volume of a modern American minivan with its seats removed. It’s twice the room the three-man Apollo 7 crew had in 1968, but their mission only lasted 11 days—and they were miserable.

Recognizing that this may be too cramped for even the most gung-ho space adventurer, the authors suggest adding another module, something like the logistics modules attached to the space station. But now the simple plan starts to get more complicated, and more expensive.

Like I said: Less excited.

Scientists are keeping tabs on an asteroid called Apophis, an 820-foot chunk of rock moseying toward Earth at about 22 miles per second. Apophis—named after an ancient Egyptian god of evil, naturally—will pass near our planet in 2029. How near is near? Closer than our own communication satellites.

But don’t despair just yet. Apophis is far smaller than the asteroid that took out the dinosaurs. That bad boy was about 6 miles across, creating a 93-mile-wide impact crater when it hit.

In “Asteroid Trackers,” a Smithsonian Channel special, Jay Melosh, an astrophysicist at the University of Arizona, describes what would happen if a similar sized asteroid hit the Earth today. “The sky would turn bright red, we’d begin to feel oppressive heat as if there were six tropical suns in the sky. Clothing would ignite, you would suffer third-degree burns, newspaper ignites, plywood burns, deciduous trees spontaneously ignite, and grass ignites.”

If that doesn’t have you cowering under the bed, watch “Asteroid Trackers” to discover how scientists track the hazards of incoming asteroids. “Asteroid Trackers” will be shown on July 25. See local listings for more details.

See a sneak peek, below.

Photo: JAXA

We had hoped that Japan’s IKAROS solar sail would work as advertised, and it did. Here’s an animated image of the fully deployed sail, taken by a “separation camera” from a short distance away.

In other happenings:

• The Hayabusa asteroid sample return capsule came home in spectacular style last week. Video here.
• Scientists on NASA’s Kepler planet-hunting mission have released a new batch of data, and now have 400 “objects of interest” that could turn out to be new planets. There’s some disagreement over whether the team should be able to hold on to the data until they’re sure, though.