Tag: Deep Impact

Asteroid 2011 AG5: a football-stadium-sized rock to watch carefully

By Phil Plait | March 6, 2012 7:00 am

Over the weekend, I posted about asteroid 2012 DA14, which is not the threat some people are claiming it is (at least not right away). And now I have to tell you about an asteroid that might be a threat in the year 2040. Most likely it won’t be, but it’s something we need to look at carefully.

There’s some background I have to give you so this all makes sense, but let me sum up here: the odds of an impact from asteroid 2011 AG5 are low, but not easily dismissed. If it passes us at just the right distance in 2023, it’ll swing back again and impact the Earth in 2040. We don’t know the orbit of it well enough to say either way just yet, and it may be late 2013 before we can be sure. An asteroid expert at NASA says waiting until next year for more observations is not a problem, but another asteroid expert is saying that waiting that long is a bad idea: we should start analyzing a possible deflection campaign for this rock now. I’m personally leaning toward the idea that getting moving on the initial analysis now is not such a bad idea. If you prefer, I have a list of bullet points at the conclusion of this post with summarized information.

[In the interest of full disclosure: Below, I will be talking about Rusty Schweickart and Don Yeomans. I've known Rusty for several years, and Don and I will be on a panel together talking about asteroid impacts at SXSW next week. I honestly like both Rusty and Don. They're good men, very intelligent and honest, and I have a lot of respect for both of them.

Also, because of the length and nature of this post, I strongly urge everyone to read the whole thing, carefully, before commenting. Thank you.]

The rock

The asteroid, called 2011 AG5, was discovered in early 2011 by a telescopic survey of the sky designed to look for asteroids that can get near the Earth. Although its exact size is unknown, it’s roughly 140 meters across — the size of a football stadium. As you can see from this diagram, it orbits the Sun on an elliptical path that brings it out past the orbit of Mars and inside the orbit of Earth. It circles the Sun once every 1.7 years.

As it happens, the orbit of AG5 brings it close to Earth every few orbits. In 2023, it will pass us at a distance of about 1.6 million km (1 million miles). That’s a safe distance, with no chance of it hitting us at all. However, you have to appreciate the gravity of this upcoming situation.

The keyhole

When AG5 passes us in February 2023, the Earth’s gravity will bend its orbit a little bit, changing the path the rock takes. If it passes close to the Earth the orbit changes a lot; if it’s too far the orbit changes only a little. But if AG5 passes us at just the right distance, the orbit will change just the right amount to put it on a collision course with Earth. This region of space is called a “keyhole”, and in this case, should AG5 slip through it, it will hit us 17 years later, in 2040. That collision, though not global in scope, would be catastrophic: equal to about a 100 megaton explosion, twice that of the largest nuclear weapon ever detonated.

The problem is, we don’t know the orbit of AG5 well enough to know if it will travel through the keyhole or not. As I pointed out in the article about the asteroid 2012 DA14, it can be tricky to try to predict asteroid orbits too far into the future. The orbit of an asteroid is determined by making measurements of its position over time, and because of various effects (like blurring due to our atmosphere) it is impossible to get exactly precise positions. They can be good enough to get an accurate orbit for the next few years, but the farther into the future you look, the fuzzier that path gets.

In the case of AG5 we know its orbit well enough to know for sure it’ll miss us by a million miles in 2023, but we don’t have the accuracy yet to know if it will thread the eye of this keyhole, which is very roughly 360 km (240 miles) across. It’s like standing by the side of a road and knowing a car driving down it will safely miss you by 10 meters, but you can’t be sure if that exact distance will be 10.004 meters or 9.996 meters. And that’s the sort of accuracy we need for AG5.

The odds

At the moment, given the observations we have, the odds of AG5 passing through the keyhole in 2023 are about 1 in 625. For an asteroid impact, that’s actually pretty high as these things go, but still pretty low in realistic terms. Let me be clear: any professional poker player will tell you never to bet on an inside straight, and the odds of getting the card you need in that case are only 1 in 13 or so. The odds of AG5 hitting us are much lower than that!

Moreover, since the orbit of the asteroid is uncertain, as we get better observations the predicted path is likely to change, to move. In that case — which is almost certainly the way things will play out — the predicted orbit will move away from the keyhole and we’ll be safe from a 2040 impact. This sort of thing has happened several times before with asteroids as their positions are observed over time, and the orbital paths clarified.

Still, a 1 in 625 chance is high enough that we need to be sure. So how do we do that?

Read More

Followup: Deep Impact crater on Tempel 1

By Phil Plait | February 16, 2011 7:00 am

At the Stardust press conference yesterday, they displayed an image that purported to show the crater where the Deep Impact impactor slammed into the Tempel 1 comet nucleus back in 2005. When I wrote up my previous post that picture was not available, but it’s been a few hours and it’s now online. So here you go:

On the left is the Deep Impact image taken by the impactor not long before the end. On the right is the Stardust image; note the resolution isn’t as good. That’s what you get when you’re 178 km away instead of a dozen or so! Still, the arrows mark the outline of the impact crater rim. On the left, I marked a dark mound of material that existed in 2005… but is gone in the new image. Not too surprising! The impact was like setting off nearly 5 tons of TNT, so that mound is now most likely vaporized water orbiting the Sun on its own.

I’ll note this image is much better than what impact expert Pete Schultz had available at the press conference (they were scrambling to get the images together right up until the conference started). I couldn’t see the crater in the image he showed then, but in this one the rim is actually fairly clear. You can also see the faint central mound inside the crater; that’s probably material that was lifted up by the explosion and then fell back in. I wonder though: In big impacts on rocky bodies (like the Moon) you get a central peak, and that’s due to molten material flowing back from the rim and splashing up (the process is called isostatic rebound, a phrase I love). I don’t know if that would apply here or not, but it’ll be interesting to hear what the scientists say as they have more chance to study the images.

Also in the Deep Impact image on the left is a heart-shaped hole or depression. In the Stardust image it appears to have rounded out a bit. Hard to say with the lower resolution, but things do look different.

In fact, take a look at this image: Read More

CATEGORIZED UNDER: Cool stuff, Pretty pictures

Stardust snaps close-ups of a second-hand comet!

By Phil Plait | February 15, 2011 2:33 pm

A philosopher once asked, "Are we human because we gaze at the stars, or do we gaze at them because we are human?" Pointless, really… "Do the stars gaze back?" Now that’s a question.
-Narrator, "Stardust"

Late last night, the NASA mission Stardust flew within 178 km (110 miles) of the nucleus of the comet Tempel 1, seeing it up close for the first time since July 2005! Here’s one of the better images from closest approach:

[Click to embiggen.]

Wow! The whole flyby sequence has been posted on NASA’s Stardust site (and Emily Lakdawalla at The Planetary Society blog has created a nifty animation of it too).

Ian O’Neill, from Discovery News, posted a nice animation of it as well on YouTube:

To give you an idea of what you’re seeing here, the comet is roughly 7.6 x 4.9 kilometers (4.7 x 3.0 miles) in size.

So, why did NASA fly Stardust past this comet? Ah, set the way-back machine for 5.5 years ago…
Read More

CATEGORIZED UNDER: Astronomy, Cool stuff, Pretty pictures

EPOXI comet flyby timeline

By Phil Plait | November 3, 2010 2:00 pm

wise_hartley2Emily Lakdawalla at The Planetary Society blog has posted a timeline of the events on Thursday, leading up to and after the spacecraft EPOXI (née Deep Impact) flies past the comet Hartley 2, currently gracing the northern skies.

Deep Impact will fly past the nucleus of the comet on November 4, 2010 at about 14:50 UTC. It will pass at the insanely close distance of 700 kilometers (420 miles), where it will see objects as small as just a few meters across on the comet’s surface. This won’t be the highest resolution a spacecraft has seen a comet nucleus, but it’ll still be pretty impressive. We have precious few closeup images of comets, so this is bound to be a terrific event. Stay tuned here, and check with Emily’s blog during the event; she’s bound to have the most up-to-date info and pictures!

Related posts:

- Three views to a comet
- EPOXI sticks like glue to extrasolar planets
- Ten things you don’t know about comets

CATEGORIZED UNDER: Astronomy, Cool stuff

Three views to a comet

By Phil Plait | October 8, 2010 7:00 am

103P/Hartley 2 is the somewhat prosaic name for a pretty nice comet in our skies right now. It’s still a little bit too faint to see without aid of binoculars or a telescope, but it’s getting brighter: on October 20th it’ll pass by the Earth at a distance of a mere 18 million km (11 million miles)!

Astronomers all over the planet are jumping at a chance to see a comet up close… and the views they’re getting are really, really cool. Behold!


That’s Hartley 2 as seen by WISE, an orbiting NASA mission designed to survey the sky in the infrared, so it’s very sensitive to objects that astronomers consider warm — that is, a few degrees above absolute zero! It’s seen comets before, since they fit the warm-to-astronomers-but-freezing-to-anyone-else category.

Mind you, the actual solid part of the comet is only a few kilometers across, far smaller than a single pixel in this picture. But it’s loaded with frozen ices which turn into gas as the comet nears the Sun, enshrouding the nucleus with fuzz, and streaming behind it as that long, long tail. In this picture, the tail is nearly 2 million kilometers in extent — well over a million miles!

Read More

CATEGORIZED UNDER: Astronomy, Pretty pictures

Two upcoming spacecraft encounters

By Phil Plait | June 2, 2010 12:08 pm

We humans have been busy lately… there are a lot of spacecraft buzzing around the solar system. Sure, you’ve heard of Cassini, and the Mars probes, but there are two very interesting spacecraft making two very interesting encounters in the next few weeks.

epoxi1) On June 27, NASA’s Deep Impact spacecraft — which sent a chunk of copper smashing into a comet back in 2005, and which has now been repurposed for planetary science — will swing by the Earth, using our planet’s gravity to change its direction and speed. DI will pass at a distance of just 37,000 km (23,000 miles)! That’s around the same height above the surface as geosynchronous (i.e. weather and communication) satellites. This maneuver will send the little spacecraft on its way to an encounter with the comet Hartley 2 in November.

rosetta2) The European Space Agency’s amazing Rosetta spacecraft will fly by the asteroid 21 Lutetia on July 10. The asteroid is about 95 km across (60 miles), and the flyby distance will be about 3200 km (2000 miles). That’s pretty close, certainly near enough to provide some nice images of the rock. In 2008, Rosetta passed the smaller asteroid 2867 Steins and returned nice images, and in 2009 swung by the Earth, sending back an image so heart-achingly beautiful I chose it as one of my Top Ten images of the year.

Rosetta’s primary mission is taking it to the comet 67P/Churyumov-Gerasimenko, where it will drop an actual honest-to-FSM lander on the comet’s surface! This is a tremendously exciting mission, and I can’t wait to see what new wonders it will send us.

Tip o’ the Whipple Shield to Emily Lakdawalla for the Rosetta news.


A glint from Earth

By Phil Plait | January 7, 2010 7:34 am

Back in 2005, NASA’s Deep Impact probe slammed a hunk of copper into the comet Tempel 1 to determine what was under its surface, as well as to see what happens in a hypervelocity collision.

The copper block vaporized in the high-energy impact, but the spacecraft lived on. It’s now on an extended mission called EPOXI, and one part of that is EPOCh: Extrasolar Planet Observations and Characterization, designed to look back at the Earth and see what a habitable and inhabited planet looks like from a distance. The idea is to see what we can observe about our own world that can be used to look at worlds orbiting other stars.


One hope was that the spacecraft would see sun glints; flashes of light from standing water on Earth (much like Cassini did with Titan). And see them it did! Check out the video below. It’s short, so you can watch it a few times; there are several glints, and I labeled the region of Earth where they occur.

That video was put together by Don Lindler (my old boss back in the STIS/Hubble days) using images from Deep Impact. It shows a full rotation of the Earth as seen above the north pole, taken when Deep Impact was still 18 million kilometers (11 million miles) from home (that’s 75 times the distance to the Moon!). Another video, from a different part of Deep Impact’s travels, shows the view as seen from the south. Due to the geometry of the Sun, Earth, and spacecraft, the glints all appear on the same place on the Earth’s face, though the location on the Earth’s surface changes as it rotates.

These images were taken in the infrared, where the contrast between land and water is highest. There may come a day when our spacecraft observe other planets orbiting other stars, and glints like these may be the tell-tale signs of liquid on their surface. In those cases, the planet may not be more than an unresolved dot, but the sudden increase in brightness may be the giveaway we’re looking for.

There’s been speculation lately that some extrasolar planets may be water worlds. We can’t know for sure just yet, but EPOXI may be showing us one way we might be able to find out.

Related post: HOLY FRAK! Moon transits Earth!

CATEGORIZED UNDER: Astronomy, Cool stuff, Pretty pictures

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