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The Milky Way is only as massive as it is because of collisions and mergers with other galaxies. This is a messy process, and we see the same thing happening with other galaxies throughout the Universe. Currently, we see the Milky Way nibbling at its two satellite galaxies, the Large and Small Magellanic Clouds. Their fate is likely sealed, and they’ll be absorbed into our galaxy.

Researchers thought the last major merger occurred in the Milky Way’s distant past, between 8 and 11 billion years ago. But new research amplifies the idea that it was much more recent: less than 3 billion years ago.

This new insight into our galactic history comes from the ESA’s Gaia mission. Launched in 2013, Gaia is busily mapping 1 billion astronomical objects, mostly stars. It measures them repeatedly, establishing accurate measurements of their positions and motions.

A new paper published in the Monthly Notices of the Royal Astronomical Society presents the findings. It’s titled “The Debris of the ‘Last Major Merger’ is Dynamically Young.” The lead author is Thomas Donlon, a post-doctoral researcher in Physics and Astronomy at the University of Alabama, Huntsville. Donlon has been studying mergers in the Milky Way for several years and has published other work on the matter.

Each time another galaxy collides and merges with the Milky Way, it leaves wrinkles. ‘Wrinkles’ obviously isn’t a scientific term. It’s an umbrella term for several types of morphologies, including phase space folds, caustics, chevrons, and shells. These wrinkles move through different groups of stars within the Milky Way, affecting how the stars move through space. By measuring the positions and velocities of these stars with great precision, Gaia can detect the wrinkles, the imprint of the last major merger.

“We get wrinklier as we age, but our work reveals that the opposite is true for the Milky Way. It’s a sort of cosmic Benjamin Button, getting less wrinkly over time,” said lead author Donlon in a press release. “By looking at how these wrinkles dissipate over time, we can trace when the Milky Way experienced its last big crash—and it turns out this happened billions of years later than we thought.”

The effort to understand the Milky Way’s (MW) last major merger involves different pieces of evidence. One of the pieces of evidence, along with wrinkles, is an Fe/H-rich region where stars follow a highly eccentric orbit. A star’s Fe/H ratio is a chemical fingerprint, and when astronomers find a group of stars with the same fingerprint and the same orbits, it’s evidence of a common origin. This group of stars is sometimes called ‘the Splash.’ The stars in the Splash may have originated in a Fe/H-rich progenitor. They have odd orbits that stand out from their surroundings. Astronomers think they were heated and their orbits altered as a by-product of the merger.

There are two competing explanations for all of the merger evidence.

One says that a progenitor dwarf galaxy named Gaia Sausage/Enceladus (GSE) collided with the MW proto-disk between 8 and 11 billion years ago. The other explanation is that an event called the Virgo Radial Merger (VRM) is responsible for the stars in the inner halo. That collision occurred much more recently, less than 3 billion years ago.

This is a Hubble Space Telescope image of the globular cluster NGC 2808. It might be the old core of the Gaia Sausage. Image Credit: By NASA, ESA, A. Sarajedini (University of Florida) and G. Piotto (University of Padua (Padova)) - http://hubblesite.org/newscenter/archive/releases/2007/2007/18/image/a/ (direct link), Public Domain, https://commons.wikimedia.org/w/index.php?curid=2371715
This is a Hubble Space Telescope image of the globular cluster NGC 2808. It might be the old core of the Gaia Sausage. Image Credit: By NASA, ESA, A. Sarajedini (University of Florida) and G. Piotto (University of Padua (Padova)) – http://hubblesite.org/newscenter/archive/releases/2007/2007/18/image/a/ (direct link), Public Domain, https://commons.wikimedia.org/w/index.php?curid=2371715

“These two scenarios make different predictions about observable structure in local phase space because the morphology of debris depends on how long it has had to phase mix,” the authors explain in their paper.

The
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An Essentials-Only Backpacking Gear Checklist

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By Michael Lanza

What do you need to pack for a three-season backpacking trip? While the specific items depend in part on factors like the time of year, your companions and backpacking style, the trip’s length and the weather forecast, this story provides a core checklist of essential gear to help you organize and efficiently pack—and avoid overpacking—for virtually any backpacking trip.

I use the checklist below for just about every three-season backpacking trip I take in the U.S. and around the world. I’ve developed it over more than three decades of multi-day backcountry trips and more than a quarter-century of writing about backpacking trips and testing and reviewing backpacking gear and apparel, including the 10 years I spent as a lead gear reviewer and Northwest Editor of Backpacker magazine and even longer running this blog.

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Hi, I’m Michael Lanza, creator of The Big Outside. Click here to sign up for my FREE email newsletter. Join The Big Outside to get full access to all of my blog’s stories. Click here for my e-books to classic backpacking trips. Click here to learn how I can help you plan your next trip.

Backpackers on the Tonto Trail in the Grand Canyon.
” data-image-caption=”Mark Fenton and Todd Arndt backpacking the Tonto Trail in the Grand Canyon. Click photo to read about “the best backpacking trip in the Grand Canyon.”
” data-medium-file=”https://i0.wp.com/tbo-media.sfo2.digitaloceanspaces.com/wp-content/uploads/2019/10/06231800/Gran8-036-Backpackers-on-the-Tonto-Trail-Grand-Canyon.jpg?fit=300%2C200&ssl=1″ data-large-file=”https://i0.wp.com/tbo-media.sfo2.digitaloceanspaces.com/wp-content/uploads/2019/10/06231800/Gran8-036-Backpackers-on-the-Tonto-Trail-Grand-Canyon.jpg?fit=900%2C600&ssl=1″ src=”https://i0.wp.com/tbo-media.sfo2.digitaloceanspaces.com/wp-content/uploads/2019/10/06231800/Gran8-036-Backpackers-on-the-Tonto-Trail-Grand-Canyon-1024×683.jpg?resize=900%2C600&ssl=1″ alt=”Backpackers on the Tonto Trail in the Grand Canyon.” class=”wp-image-36029″ srcset=”https://tbo-media.sfo2.digitaloceanspaces.com/wp-content/uploads/2019/10/06231800/Gran8-036-Backpackers-on-the-Tonto-Trail-Grand-Canyon.jpg 1024w, https://tbo-media.sfo2.digitaloceanspaces.com/wp-content/uploads/2019/10/06231800/Gran8-036-Backpackers-on-the-Tonto-Trail-Grand-Canyon.jpg 300w, https://tbo-media.sfo2.digitaloceanspaces.com/wp-content/uploads/2019/10/06231800/Gran8-036-Backpackers-on-the-Tonto-Trail-Grand-Canyon.jpg 768w, https://tbo-media.sfo2.digitaloceanspaces.com/wp-content/uploads/2019/10/06231800/Gran8-036-Backpackers-on-the-Tonto-Trail-Grand-Canyon.jpg 1080w, https://tbo-media.sfo2.digitaloceanspaces.com/wp-content/uploads/2019/10/06231800/Gran8-036-Backpackers-on-the-Tonto-Trail-Grand-Canyon.jpg 1200w” sizes=”(max-width: 900px) 100vw, 900px” data-recalc-dims=”1″ />Mark Fenton and Todd Arndt backpacking the Tonto Trail in the Grand Canyon. Click photo to read about “the best backpacking trip in the Grand Canyon.”

The list below is preceded by some insights on how I make gear choices. The links in this story and checklist will take you to menus of product reviews; photos link to stories about those trips.

See my Custom Trip Planning page to learn how I can help you plan your next trip—including answering all of your questions. Please share your thoughts on my list and tips and offer your own suggestions in the comments section at the bottom of this story. I try to respond to all comments.

Plan your next great backpacking trip in Yosemite, Grand Teton,
and other parks using my expert e-books.

A backpacker hiking over Clouds Rest in Yosemite National Park.
” data-image-caption=”Jeff Wilhelm backpacking over Clouds Rest in Yosemite National Park. Click the photo for my e-book “The Best First Backpacking Trip in
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The Fine Art of Stashing a Backpack in the Woods

Tet19 047 Me on Teton Crest Trail copy cropped 38

By Michael Lanza

Stashing a backpack in the woods is just what it sounds like. If you’re on a multi-day backpacking trip and want to take a side hike of any significant distance, like to a summit, and then return to the same spot to resume your backpacking route, it’s a waste of energy (not to mention entirely pointless) to carry your heavy pack with you. But there are ways to do it wrong, and ways to make sure your pack and everything inside it are still there and not torn apart or gone when you return. Here’s how to do it right.

The tips below are based on my experience of many thousands of trail miles and more than three decades of backpacking, dayhiking, climbing, trail running, and taking ultra-hikes and ultra-runs—including more than a quarter-century of doing this professionally and testing and reviewing gear as a past field editor for Backpacker magazine and running this blog.

Tet19 047 Me on Teton Crest Trail copy cropped 39
Hi, I’m Michael Lanza, creator of The Big Outside. Click here to sign up for my FREE email newsletter. Join The Big Outside to get full access to all of my blog’s stories. Click here for my e-books to classic backpacking trips. Click here to learn how I can help you plan your next trip.

A backpacker hiking the Spray Park Trail in Mount Rainier National Park.
” data-image-caption=”Todd Arndt hiking the Spray Park Trail in Mount Rainier National Park.
” data-medium-file=”https://i0.wp.com/tbo-media.sfo2.digitaloceanspaces.com/wp-content/uploads/2020/09/06230643/Todd-Arndt-hiking-the-Spray-Park-Trail-Mount-Rainier-National-Park-.jpg?fit=200%2C300&ssl=1″ data-large-file=”https://i0.wp.com/tbo-media.sfo2.digitaloceanspaces.com/wp-content/uploads/2020/09/06230643/Todd-Arndt-hiking-the-Spray-Park-Trail-Mount-Rainier-National-Park-.jpg?fit=683%2C1024&ssl=1″ src=”https://i0.wp.com/tbo-media.sfo2.digitaloceanspaces.com/wp-content/uploads/2020/09/06230643/Todd-Arndt-hiking-the-Spray-Park-Trail-Mount-Rainier-National-Park–683×1024.jpg?resize=683%2C1024&ssl=1″ alt=”A backpacker hiking the Spray Park Trail in Mount Rainier National Park.” class=”wp-image-41154″ style=”width:602px;height:903px” srcset=”https://tbo-media.sfo2.digitaloceanspaces.com/wp-content/uploads/2020/09/06230643/Todd-Arndt-hiking-the-Spray-Park-Trail-Mount-Rainier-National-Park-.jpg 683w, https://tbo-media.sfo2.digitaloceanspaces.com/wp-content/uploads/2020/09/06230643/Todd-Arndt-hiking-the-Spray-Park-Trail-Mount-Rainier-National-Park-.jpg 200w, https://tbo-media.sfo2.digitaloceanspaces.com/wp-content/uploads/2020/09/06230643/Todd-Arndt-hiking-the-Spray-Park-Trail-Mount-Rainier-National-Park-.jpg 768w, https://tbo-media.sfo2.digitaloceanspaces.com/wp-content/uploads/2020/09/06230643/Todd-Arndt-hiking-the-Spray-Park-Trail-Mount-Rainier-National-Park-.jpg 800w” sizes=”(max-width: 683px) 100vw, 683px” data-recalc-dims=”1″ />Todd Arndt hiking the Spray Park Trail in Mount Rainier National Park. Click photo to read about that trip.

Basically, you want to make sure no animals (including humans) will find it and take or damage the pack or anything inside. Most hikers aren’t dishonest, but some adults might mistakenly think a pack was inadvertently left behind and assume it’s fair game for whomever finds it, or that they should deliver it to whatever agency manages the land so that its owner might reclaim it later (which is not helpful to you for the remainder of your hike); and kids will more readily take something they find.

That’s more of a concern for me on popular hikes that attract a lot of inexperienced hikers. In more remote areas, where you’ll generally only see experienced backpackers who aren’t likely to make that assumption, I worry less about a pack being visible to people.

Wild animals are a concern virtually everywhere. Rodents, squirrels, and larger animals like
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Hubble’s Back, but Only Using One Gyro

Hubble gyros

The Hubble Space Telescope has experienced ongoing problems with one of its three remaining gyroscopes, so NASA has decided to shift the telescope into single gyro mode. While the venerable space telescope has now returned to daily science operations, single gyro mode means Hubble will only use one gyro to maintain a lock on its target. This will slow its slew time and decrease some of its scientific output. But this plan increases the overall lifetime of the 34-year-old telescope, keeping one gyro in reserve. NASA is also troubleshooting the malfunctioning gyro, hoping to return it online.

Last week, NASA said that the telescope and its instruments are stable and functioning normally.

Gyroscopes help the telescope orient itself in space, keeping it stable to precisely point at astronomical targets in the distant Universe. Hubble went into safe mode back in November 2023, and then again in April and May 2024 due to the ongoing issue, where the one gyro had been increasingly returning faulty readings.

Hubble gyros 1
The end of a Hubble gyro reveals the hair-thin wires known as flex leads. They carry data and electricity inside the gyro. Credit: NASA

Going in to safe mode suspends science operations, and in the meantime, engineers tried to troubleshoot to figure out why the gyro experiencing the fault-producing issues and doing work-arounds to get the telescope up and running again. The most recent last safe-mode event in May led the Hubble team to transition from a three-gyro operating mode to observing with only one gyro. This enables more consistent science observations while keeping the other operational gyro available for future use.

Launched in 1990, Hubble has more than doubled its expected design lifetime, providing stunning images and scientific discoveries that have changed our understanding of the Universe and re-written astronomy textbooks.

During its 34-year history, Hubble has had eight out of 22 gyros fail due to a corroded flex lead, which are thin (less than the width of a human hair) metal wires, that carry power in, and data out, of the gyro. The flex leads pass through a thick fluid inside the gyro and over time, the flex leads begin to corrode and can physically bend or break.

mike good hubble sm4
With his feet firmly anchored on the shuttle’s robotic arm, astronaut Mike Good maneuvers to retrieve the tool caddy required to repair the Space Telescope Imaging Spectrograph during the final Hubble servicing mission in May 2009. Periodic upgrades have kept the telescope equipped with state-of-the-art instruments, which have given astronomers increasingly better views of the cosmos. Credits: NASA

Thankfully, for the first 18 years of Hubble’s life in space, the telescope had the advantage of being able to be serviced and upgraded by space shuttle astronauts. For example, in 1999, four out of six gyros had failed, with the last one failing about a month before a servicing mission was scheduled to replace them (and do other upgrades to the telescope). This meant Hubble sat in safe mode waiting for the space shuttle and astronauts to arrive.

When the final planned Hubble servicing mission was (temporarily) canceled following the space shuttle Columbia disaster, engineers developed and inaugurated a two-gyro mode to prolong Hubble’s life. The mission was reinstated after outcry from scientists and the public, and so NASA figured out a way to mitigate the risks of flying the space shuttle. Servicing Mission 4 replaced all six gyros one last time in 2009, but it has been running on three since 2018. The
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