# Collision of two Earths

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Assume that two Earth-like planets orbited the same star, with elliptic orbits. Eventually, something occurs that caused the two planets to collide.

What would be the outcome of such a catastrophe? I figure that both planets (and any life if it exists) would be destroyed, but exactly how?

Would both planets explode, combine together into a larger planet, or be able to rebound and either reestablish an orbit or be pulled into the sun?

To start with, such two orbits are extremely unlikely. First of all, any object orbiting a star and deviating strongly from a circular orbit has a high risk of crashing into something. As you notice, there are very few planets in our solar system, and all of them except Pluto have a very regular orbit. Planets which have intercrossing orbits would have crashed into each other and fallen apart on the planetesimal stage, much earlier than becoming a planet.

Then, if you suppose that such an event does occur, the consequences will depend on multiple factors. For one, the composition of the planets - if the two planets are terrestrial (rocky), one would expect them to fragmentate and lose a large amount of their mass. Their cores would maybe fuse into one planet, and the debris would form one or several satellites. The exact outcome would be dependent on their speed, their density, and the angle of their collisions. A similar (but not the same) scenario happened during the late stage of Earth formation, when an object close to Earth's size crashed into it to create the Moon. If the sizes are the same and the formation is complete, chances are that most of the matter would fly away and the planet that would be left would be tiny.

It would be a different story if the two planets are gaseous. They have almost no solid matter in them, so chances are that the planets will fuse whilst losing (proportionally) significantly less matter than terrestrial planets. Still, there would be losses.

Lastly, if a terrestrial and a gaseous planet meet, the outcome is quite easy to predict. Gas planets are much larger than rocky planets, so the rocky planet would simply be engulfed. It will maybe cause a disturbance on the surface of the gas giant.

To wrap it up let me just note that such a collision would almost definitely change the orbit of whatever is left, so that it would either get pushed into the sun or away from it, causing it to float eternally in outer space. Have a nice day.

## Big Collision, Beautiful Moon

A demolition expert surveys the building designated for destruction. With one swing of the wrecking ball, he must bring down the building without scattering the debris off the property. Such a precise operation requires the right size wrecking ball hitting at just the right speed. Hitting too high only removes the roof too low and the ground absorbs all the wrecking force. The possibilities for a failed demolition far exceed the ways to succeed. After exacting calculations, the wrecking ball scores a direct hit, transforming the building into an easily cleaned-up pile of debris.

About 50 million years after the formation of the solar system, a similarly fine-tuned collision between Earth and a Mars-sized body occurred. However, instead of destroying Earth, the collision provided raw materials for the formation of Earth’s moon. The collision ejected debris into orbit that eventually coalesced into the Moon. Recent high-resolution simulations of the impact event1 confirm the fine-tuning of the impact to insure the survival of Earth, formation of the Moon, and transformation of Earth’s atmosphere.2

The simulations show that the debris ejected from Earth must have consisted primarily of solid or liquid material-not gas-or else the debris disk would have dissipated too quickly to coalesce into a Moon-sized satellite. A larger impactor would have generated more energy during the collision and, consequently, more vaporized, gaseous material in the debris disk. However, a smaller impactor would not enrich Earth with the necessary heavy elements to drive long-standing plate tectonics nor provide sufficient energy to completely eject Earth’s life-suffocating primordial atmosphere into space. (This gas does not become part of the debris disk, but is completely removed from the Earth-Moon system.) Thus, if the impactor were larger or smaller, the capacity of Earth to support advanced complex life (like humans) or abundant, long-standing microbial life rapidly diminishes. Additionally, the authors note that if a planet is too large, it cannot have a moon formed by a giant impact event. The Moon-forming impact requires a just-right-sized impactor striking Earth at the just-right speed, at the just-right location, with the just-right angle, and at the just-right time.

Just as the demolition expert must carefully prepare his work in order to avoid failure, so the Moon-forming impact required a number of just-right factors in order to succeed. As scientific advances continue to reveal more fine-tuning factors, the idea that the impact happened purely by chance seems less and less feasible. On the other hand, such fine-tuning comports well with RTB’s biblical creation model, in which a supernatural Creator intervenes to ensure Earth’s long-standing habitability in preparation for humankind.

## Two planets colliding: one short, but beautiful simulation

Above, you can see what two planets colliding might look like. It was posted on img.ur the other day, and a lot of people went crazy over it – good crazy. Science crazy. You get it.

Anyway, I couldn’t help noticing how everybody was asking about some background info. So, let’s shed some light. The simulation was made based on a numerical model developed by Dr. Robin M. Canup, a Principal Investigator in NASA’s “Origins of Solar Systems”, “Planetary Geology and Geophysics”, “Outer Planets Fundamental Research”, and LASER programs and NSF’s “Planetary Astronomy” program. In 2003, she published a paper in which 100 hydrodynamic simulations of potential Moon-forming impacts were modeled.

“The most favorable conditions for producing a sufficiently massive and iron-depleted protolunar disk involve collisions with an impact angle near 45 degrees and an impactor velocity at infinity < 4 km/sec. For a total mass
and angular momentum near to that of the current Earth–Moon system, such impacts typically place about a lunar mass of material into orbits exterior to the Roche limit, with the orbiting material composed of 10 to 30% vapor by mass. In all cases, the vast majority of the orbiting material originates from the impactor, consistent with previous findings. By mapping the end fate (escaping, orbiting, or in the planet) of each particle and the peak temperature it experiences during the impact onto the figure of the initial objects, it is shown that in the successful collisions, the impactor material that ends up in orbit is primarily that portion of the object that was heated the least, having avoided direct collision with the Earth,” Canup wrote in the study abstract.

The scope was to find just the right conditions that might explain how the moon was formed, given the leading moon formation theory among scientists is that early-Earth collided with another Mars-sized planet. The lion’s share of the debris coalesced to form the Earth as it is today, while a portion of the Theia (the name of the planet which smashed into Earth) and early Earth mass joined to from the moon.

This theory was first conceived in 1946 by Reginald Aldworth Daly from Harvard University. The impact theory explained many of the challenges about the formation of the Moon. For example, one question was: why do the Earth and Moon have very different-sized cores.

You can find more animated simulations based on Canup’s numerical models here.

A good deal of work has been done on protoplanet-protoplanet collisions, mainly focused on testing the Giant Impact Hypothesis for the formation of the Moon. A number of fluid simulations (many smoothed-particle hydrodynamics) have been performed, for varying angles of attack and initial relative velocities (see e.g. Canup 2012, Eiland et al. 2013).

The takeaway from those simulations is that the planets initially coalesce within half a day to a day. However, the resulting body isn't round it's somewhat elliptical, even a bit pointy at the ends. Some models have tails of matter (typically one or two) attached at the ends, which, though tenuous, may form another body, i.e. the Moon. By the end of about 24 hours, there is a clear central body surrounded by this excess material, but it may take up to a month for it to regain its spherical shape - a key characteristic of a planet.

• It may take time for the interior of the planet to become differentiated, i.e. for it to take on a traditional planet-like structure. Even after coalescence, the cores may still be separated.
• Glancing, indirect collisions tend to produce more ellipsoidal shapes than direct collisions, even if there's a merger.
• There will still be debris orbiting for quite some time after the merger - again, perhaps weeks or months.
• The final body will remain quite hot for some time, with surface temperatures of perhaps up to 6000 K in the day or so immediately following the collision.

Please check out this link in case you find it interesting. It's about how the moon formed from a similar impact.

In the link above, it is assumed that there was an explosive collision (moderate at celestial standards) between Earth and Theia at an oblique angle. Despite such a collision, it is thought that it took surprisingly little time to form the moon, whereas it took around 0.1 billion years for Earth to form. A corresponding collision between your two planets would likely take longer, as the creation of the Earth itself (normally) took hundreds of millions of years.

A point about Roche limits: Roche limit takes effect 2.5 radii away from the larger planet. If these planets are equal in mass, they would merge into a central mass between them. This would basically be the same as forming a brand new planet from scratch.

Edit: I forgot to give you an actual answer - sorry lol. With little-to-no actual science to back this up (we don't know much about the formation of planets) I'm going to say between 0.5-1 billion Earth years IF both planets completely shatter into debris and then coalesce to form another planet. If they merge perfectly the way you described, could take 100,000 years, as the commenters suggested. That's assuming that these two planets don't just turn into an asteroid belt or something, and that nothing else gets in the way. I'm also not accounting for bombardment of debris from the collision of these two planets, or the possibility of smaller moons forming.

I'd also like to point out that the probability of 2 celestial objects 'only' crashing into each other at 1000 km/h wouldn't be much of a collision. Is this being done deliberately? If not, incredible luck.

## Help fund Monitor journalism for $11/ month Already a subscriber? Login Monitor journalism changes lives because we open that too-small box that most people think they live in. We believe news can and should expand a sense of identity and possibility beyond narrow conventional expectations. Our work isn't possible without your support. ## Fault Zones and Mountain Building Figure 5. San Andreas Fault: We see part of a very active region in California where one crustal plate is sliding sideways with respect to the other. The fault is marked by the valley running up the right side of the photo. Major slippages along this fault can produce extremely destructive earthquakes. (credit: John Wiley) Along much of their length, the crustal plates slide parallel to each other. These plate boundaries are marked by cracks or faults. Along active fault zones, the motion of one plate with respect to the other is several centimeters per year, about the same as the spreading rates along rifts. One of the most famous faults is the San Andreas Fault in California, which lies at the boundary between the Pacific plate and the North American plate (Figure 5). This fault runs from the Gulf of California to the Pacific Ocean northwest of San Francisco. The Pacific plate, to the west, is moving northward, carrying Los Angeles, San Diego, and parts of the southern California coast with it. In several million years, Los Angeles may be an island off the coast of San Francisco. Unfortunately for us, the motion along fault zones does not take place smoothly. The creeping motion of the plates against each other builds up stresses in the crust that are released in sudden, violent slippages that generate earthquakes. Because the average motion of the plates is constant, the longer the interval between earthquakes, the greater the stress and the more energy released when the surface finally moves. For example, the part of the San Andreas Fault near the central California town of Parkfield has slipped every 25 years or so during the past century, moving an average of about 1 meter each time. In contrast, the average interval between major earthquakes in the Los Angeles region is about 150 years, and the average motion is about 7 meters. The last time the San Andreas fault slipped in this area was in 1857 tension has been building ever since, and sometime soon it is bound to be released. Sensitive instruments placed within the Los Angeles basin show that the basin is distorting and contracting in size as these tremendous pressures build up beneath the surface. ### Example 1: full zones and plate motion After scientists mapped the boundaries between tectonic plates in Earth’s crust and measured the annual rate at which the plates move (which is about 5 cm/year), we could estimate quite a lot about the rate at which the geology of Earth is changing. As an example, let’s suppose that the next slippage along the San Andreas Fault in southern California takes place in the year 2017 and that it completely relieves the accumulated strain in this region. How much slippage is required for this to occur? #### Check Your Learning If the next major southern California earthquake occurs in 2047 and only relieves one-half of the accumulated strain, how much slippage will occur? Figure 6. Mountains on Earth: The Torres del Paine are a young region of Earth’s crust where sharp mountain peaks are being sculpted by glaciers. We owe the beauty of our young, steep mountains to the erosion by ice and water. (credit: David Morrison) When two continental masses are moving on a collision course, they push against each other under great pressure. Earth buckles and folds, dragging some rock deep below the surface and raising other folds to heights of many kilometers. This is the way many, but not all, of the mountain ranges on Earth were formed. The Alps, for example, are a result of the African plate bumping into the Eurasian plate. As we will see, however, quite different processes produced the mountains on other planets. Once a mountain range is formed by upthrusting of the crust, its rocks are subject to erosion by water and ice. The sharp peaks and serrated edges have little to do with the forces that make the mountains initially. Instead, they result from the processes that tear down mountains. Ice is an especially effective sculptor of rock (Figure 6). In a world without moving ice or running water (such as the Moon or Mercury), mountains remain smooth and dull. ## Contents Rhoda Williams (Brit Marling), a brilliant 17-year-old girl who has spent her young life fascinated by astronomy, is delighted to learn that she has been accepted into MIT. She celebrates, drinking with friends, and in a reckless moment, drives home intoxicated. Listening to a story on the radio about a recently discovered Earth-like planet, she gazes out her car window at the stars and inadvertently hits a stopped car at an intersection, putting John Burroughs (William Mapother) in a coma and killing his pregnant wife and young son. After serving her four-year prison sentence, Rhoda chooses to work with her hands and to have minimal contact with other people, becoming a janitor at her former high school. Hearing more news stories about the mirror Earth, Rhoda enters an essay contest sponsored by a millionaire entrepreneur who is offering a civilian space flight to the mirror Earth. One day Rhoda sees John laying a toy at the accident site. She visits his house, intending to apologize. He answers the door and she loses her nerve. Instead, she pretends to be a maid offering a free day of cleaning as a marketing tool for a cleaning service. John, who has dropped out of his Yale music faculty position, has been letting his home and himself go, and accepts Rhoda's offer. He has no idea who she is, and when she finishes asks her to come back the next week. In time, a caring relationship develops and they have sex. Rhoda wins the essay contest and is chosen to be one of the first to travel to the other Earth. John asks her not to go, believing they might have a future together. She finally decides to tell him the truth about who she is. He is upset and throws her out of the house. Rhoda hears an astrophysicist talking on television, describing a "broken mirror" hypothesis which states that upon the sighting of the twin-Earth the synchronicity of events happening in both the earths was broken. Rhoda rushes back to John's house, but he refuses to let her in. She breaks into his house, and he begins to strangle her. He stops, and when she recovers she tells him about the theory and that there might be a possibility for his family to still be alive on the other Earth. She leaves him the ticket. In time, she learns that John accepted the gift and becomes one of the first civilian space travelers to the other Earth. Four months later, on a foggy day, Rhoda approaches her house, discovering her other self from Earth 2 standing in front of her. as Rhoda Williams as John Burroughs • Jordan Baker as Kim Williams as Jeff Williams • Flint Beverage as Robert Williams as Purdeep as Dr. Joan Tallis as Keith Harding as himself (narrator) [3][4] The idea behind Another Earth first developed out of director Mike Cahill and actress Brit Marling speculating as to what it would be like were one to encounter one's own self. In order to explore the possibility on a large scale, they devised the concept of a duplicate Earth. The visual representation of the duplicate planet was deliberately made to evoke the Moon, as Cahill was deeply inspired by the 1969 Apollo 11 lunar landing. [5] This movie shares some of its plot details with the 1969 British sci-fi movie Doppelganger. Another Earth was filmed in and around New Haven, Connecticut, Mike Cahill's hometown – with some scenes taking place along the West Haven shoreline and at West Haven High School and Union Station – so that he could avail himself of the services of local friends and family and thus reduce expenses. His childhood home was used as Rhoda's home and his bedroom as Rhoda's room. The scene of the car collision was made possible through the help of a local police officer with whom Cahill was acquainted, who cordoned off part of a highway late one night. The scene in which Rhoda leaves the prison facility was filmed by having Marling walk into an actual prison posing as a yoga instructor and then exiting. [6] According to Brit Marling, she approached William Mapother for the role of John after "being haunted" by his performance in In the Bedroom (2001). Mapother consented to work on Another Earth for$100 a day. [7] When asked why he agreed to join the cast, considering the "notoriously hit or miss" nature of independent films, Mapother replied that he was drawn by the film's subject and by the names involved in the project. At Mapother's insistence, he and the production team worked extensively on the scenes of John and Rhoda in order to develop John's character in the film. [8]

The film ignores the physical consequences of having a similar-sized planet and moon appear nearby (e.g. effect on tides, gravity and atmosphere) other than depicting night time as brighter due to the reflection of the Sun's light off the other planet. The DVD / Blu-ray deleted scenes feature reveals that the film makers did intend to illustrate some of the consequences by filming a scene in which Rhoda encounters flowers floating in mid-air, but the scene was cut from the final film.

The musical score was composed by Fall on Your Sword, with the exception of the song played in the musical saw scene, composed by Scott Munson and performed by Natalia Paruz. [9] Mike Cahill came upon Paruz, known also as the "Saw Lady", while riding the subway in New York. Mesmerized by her playing, he obtained her contact information and arranged for her to coach William Mapother on how to hold and act as if playing the saw for the scene in the film. [5] [10]

Another Earth had its world premiere at the 27th Sundance Film Festival in January 2011. It was released in dramatic competition. Variety reported: "[It] has been deemed one of the more highly praised pics of the fest as it received a standing ovation after the screening and strong word of mouth from buyers and festgoers." The distributor Fox Searchlight Pictures won distribution rights to the film in a deal worth $1.5 million to$2 million , beating out other distributors including Focus Features and the Weinstein Company. [11]

Fox Searchlight is the distributor of Another Earth in the United States, Canada, and other English-speaking territories. [11] The film had a limited release in the United States and Canada on July 22 , 2011, expanding to a wide release in ensuing months. [12]

In its first week in theaters, it grossed $112,266. [13] Eventually, the film grossed$1.9 million worldwide. [2]

Rotten Tomatoes gives Another Earth a rating of 66% based on 128 reviews and an average score of 6.29/10. The critical consensus reads: "Another Earth is often weighed down by placid pacing and ponderousness, but this soulful sci-fi nevertheless offers plenty of profound concepts to ponder." [14]

Film critic Roger Ebert of the Chicago Sun-Times gave the film three and a half stars out of four. Ebert commented that, "Another Earth is as thought-provoking, in a less profound way, as Tarkovsky's Solaris, another film about a sort of parallel Earth". [15]

Another Earth won the Alfred P. Sloan Prize at the 2011 Sundance Film Festival, for "focusing on science or technology as a theme, or depicting a scientist, engineer or mathematician as a major character." [16] It went on to earn the Audience Award in the category of Narrative Feature at the 2011 Maui Film Festival. [17] [18]

Another Earth was named one of the top 10 independent films of the year at the 2011 National Board of Review Awards and was nominated for a Georgia Film Critics Association Award for Best Picture.

## Double Earths/ Why Magick Works

In the first half, electrical engineer and physicist Brooks Agnew shared updates on a variety of topics, including his work on North Pole Inner-Earth expeditions, his theories of Earth being made of two planets (one very high frequency, and one low frequency), and how the global elite and a "shadow government" conspire behind-the-scenes. "What I kept discovering. was that there was another government inside our government basically taxing us and fining us, calling for all kinds of fees and regulations-- they're really the ones that are writing our laws-- not Congress," he said. The problem is, he continued, "we don't have any representation inside that government" and good people are losing their lives as part of the conspiracy.

There is still hope, he reported, for an expedition in an icebreaker ship (that plows through frozen waters) to explore a possible Inner Earth entry near the North Pole, though the cost would be around $3.5 million. Many ancient civilizations have talked about a cataclysmic cosmic event, and Agnew has concluded that Earth is made up of two planets, one about 4.5 billion years old, and the other around 6,000 years old. He postulated that Earth was just 1/3 the size it once was, and the current planet was formed during a collision between a mostly water object and a rocky planet. This has happened elsewhere in the solar system, he added, suggesting that Uranus collided with a world twice the size of Earth. PEN Award-winning author and occult historian Mitch Horowitz explored a variety of magick, spellwork, and fortunetelling and why they work. He discussed far-flung oracular methods—ceremonial, Hermetic, astrology, positive mind, chaos magick, hoodoo, witchcraft, etc.—and presented a theoretical working model of why such practices are not just "all in our heads." We live, he explained, "in a universe of infinite possibilities," and we're selecting from different outcomes all the time, based on where we place our attention and focus. He detailed how "anarchic magick" (a kind of DIY blend of a person's own spiritual ceremonies and rituals) is particularly potent, and worked to change the creative direction of his own life. Horowitz has just completed an experiment, providing more than two hundred tarot readings over a whirlwind July Fourth weekend to people at a distance who follow him on social media. The results were sometimes uncanny, he said, noting that the cards, through their meaningful archetypal images, offer a snapshot of the present moment as though taken by a sort of "transcendental camera." Mitch offered 3-card tarot readings to callers in the last hour, finding a narrative in the cards to intuit answers to their questions. ## Most Read "Theia was thoroughly mixed into both the Earth and the moon, and evenly dispersed between them," Edward Young, lead author of the new study and a UCLA professor of geochemistry and cosmochemistry, said in a statement. "This explains why we don't see a different signature of Theia in the moon versus the Earth." Theia — which scientists believe was similar in size to Mars or the Earth — did not survive the collision, but its impact lives on with Earth and the moon. ## SKY WATCH : A Cosmic Collision : Beginning July 16, pieces of Comet Shoemaker-Levy 9 will slam into Jupiter. Astronomers are calling it the most cataclysmic cosmic event of this century--more unusual even than the appearance of Halley’s Comet. And you don’t have to leave the San Fernando Valley to catch it. For about a week beginning July 16, the scattered pieces of Comet Shoemaker-Levy 9 will slam into Jupiter with more force than dozens of atomic bombs. Although the collisions will occur on the dark side of the solar system’s largest planet, some scientists expect the impacts to briefly affect Jupiter’s appearance, as seen from Earth. Comets are essentially dirty icebergs traveling through space, heating up as they pass close to the sun--an effect that releases comet dust into tails that are stretched out by the solar wind. Most comets are believed to originate from a cloud of comets at the edge of the solar system and to somehow get knocked into orbit around the sun. Jupiter is a ball of hydrogen and helium gas about 1,000 times more massive than Earth. It rotates about every 10 hours, making it the fastest spinning planet in the solar system. Jupiter’s Great Red Spot, which could engulf two Earths, is a cyclonic storm that has been raging for centuries. On July 8, 1992, Comet Shoemaker-Levy 9 passed close enough to Jupiter for gravity to break it into a string of 21 fragments, most with comet tails of their own. By July 16 of this year, the distance between these fragments could span more than 4 million miles, with particles ranging in size from that of the Sherman Oaks Fashion Square to the city of San Fernando. The comet was discovered by observers at Palomar Observatory near San Diego on March 25, 1993. Like all comets, it was named after those who spotted it first--professional astronomers Carolyn and Eugene Shoemaker and amateur astronomer David Levy. The “9" signifies that this is the ninth comet discovered by this team. Analysis of the comet’s orbit revealed two startling facts: that it was the first known comet to orbit a planet (Jupiter) rather than the sun, and that the comet’s icy fragments will collide with Jupiter during the week of July 16 to 22. Never before have earthbound stargazers had the opportunity to witness the effects of a comet smashing into a planet. And although the spacecraft Galileo will be positioned perfectly to send us photos of the comet shelling the planet’s atmosphere, scientists have no clear idea of what we will see from Earth. In the most dramatic scenario, the exploding fragments could illuminate the Jovian moons caught in the shadow of Jupiter. The most likely candidate is Europa, which will be behind Jupiter when fragment K--the particles are given letter names--slams into the planet on July 19. Other scientists predict that the impacts will briefly illuminate Jupiter’s faint ring. A more likely effect will be atmospheric turbulence on Jupiter. Some scientists expect the impacts to briefly disrupt the planet’s cloud belt, perhaps creating miniature spots south of its Equator. Another prediction is that shock waves could ripple across the entire planet. Theoretically, the event could also be a dud: Some scientists predict that Jupiter’s atmosphere will quietly swallow up the comets like a whale scooping up shrimp. Although scientists may not agree on the outcome, there are three things on which they can agree: First, earthbound observers will have to wait at least 30 minutes after each collision before the impact site rotates into view. Next, observers shouldn’t expect to see a dramatic explosion, but rather subtle changes in the appearance of the planet. And finally, it will take knowledge of how Jupiter looks now to appreciate the changes. The cheapest way is to buy the July issue of “Sky & Telescope” magazine at a newsstand, or a$5 guide to the comet / Jupiter impact published by the nonprofit Planetary Society in Pasadena. Familiarize yourself with Jupiter’s cloud belts in the southern hemisphere (about 40 to 50 degrees south latitude), where the impacts will occur.

Then head to Griffith Observatory in Griffith Park to view the action.

A more expensive option--but without the long lines expected at the observatory--is to use your own telescope to study Jupiter in the weeks before and during the impact. Fortunately, Jupiter is bright enough to be seen from your back yard. Turn off indoor lights and take about 30 minutes to let your eyes adjust. The darker the site, the better the view through the telescope.

If you are going to purchase a telescope, get the right one. It should have at least a four-inch aperture. The bigger the aperture, the more you will see. Consult a local telescope store and avoid toy-store varieties. Do not, however, buy a telescope just for the comet / Jupiter impact: If it’s a dud, you’ll feel ripped off. Although this is a once-in-a-lifetime event, a telescope is a purchase best made for a lifetime of stargazing.

What: Griffith Observatory’s comet collision activities. Free to the public, the observatory’s 12-inch telescope will be focused on Jupiter throughout the week of July 16-22, with a live video feed of Jupiter from the telescope available for viewing on a screen inside the museum. Additional telescopes will be on the lawn July 16. During this week, the observatory’s 7:30 p.m. planetarium show will feature a 10-minute segment on the collision.

Location: 2800 E. Observatory Road, Los Angeles.

Hours: Observatory museum is open 12:30 p.m. to 10 p.m. daily. Telescope viewing dusk to 9:45 p.m., except for July 16, 20 and 21, when it will be open until midnight. Planetarium show times are 1:30, 3 and 7:30 p.m. Monday through Friday 1:30, 3, 4:30 and 7:30 p.m. Saturday and Sunday.

Price: Observatory entry is free. Planetarium show is $4 for adults,$3 seniors 65 and older, $2 children 5-12. Children under 5 are admitted only to the 1:30 p.m. show on weekends, for free with a paid adult admission. Call: (213) 664-1181 for general information (213) 663-8171 for weekly sky report. Also: To purchase the Planetary Society’s guide to the collision, “Once In A Thousand Lifetimes,” call (818) 793-5100. The price is$5.