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Planets aren’t the only celestial objects with moons – asteroids can have them too. They are usually other, smaller asteroids in orbit around a larger central one.  Now, a team of Thai and French astronomers found an asteroid system with three satellites.  The new four-body system makes complex gravitational problems like the three-body problem look simple by comparison.

The asteroid with so many hangers-on is known as (130) Elektra.  First found in 1873 by Christian Peters, it measured about 200 km in diameter and weighs approximately 7 x 10^18 kg.  That’s more than enough mass to capture its three orbiting smaller asteroids, known as S/2003 (130) 1, S/2014 (130) 1, and S/2014 (130) 2 – the most recent discovery was only detailed in a paper published on arXiv on February 10th.  

Projected orbits of Elektra's three moons.
Projected orbits of Elektra’s three moons.
Credit – Berdeu et al.

Their designations reflect the year they were first observed, which shows that S/2014(130) 2 sat unobserved in a dataset for over seven years.  That dataset was collected by the Spectro-Polarimetric High-contrast Exoplanet REsearch facility (SPHERE), an observing platform on the Very Large Telescope (VLT).  The telescope turned its attention to (130) Elektra back in 2014.  Over that time, the dataset captured S/2014 (130) 2 one hundred and twenty different times over 22 days.  So why was it so hard to find?

There are two answers to that question. First, the total amount of data has to be reduced.  Second, even when the data was reduced, the signal for the moon itself was hidden in the larger asteroid’s “halo.”  Dr. Anthony Berdeu and his team developed novel algorithms for both that allowed them to tease out the previously indiscernible moon.

Another potential detection of the moon around Elektra in a different data set - this one was collected in 2019.
Another potential detection of the moon around Elektra in a different data set – this one was collected in 2019.
Credit – Berdeu et al

Reducing the amount of data while analyzing it might seem counter-intuitive, but data reduction pipelines are commonplace in spectrography. This is mainly because data sets have optical artifacts derived from other spectra that aren’t necessarily of interest to certain studies.  To eliminate these artifacts, Dr. Berdeu and his team utilized a novel data reduction technique for the SPHERE instrument called Projection, Interpolation, Convolution (PIC) and published in a separate paper back in 2020.  With this tool, they could eliminate enough spectral artifacts to get a clearer picture of their intended target.

That target was still surrounded by a “halo,” which is another type of spectral artifact that made the asteroid’s surface appear to glow.  It also reached far enough out from the asteroid’s surface to engulf S/2014(130) 2, which has a semi-major axis of only 344 kilometers from Elektra’s surface.

Moons can happen anywhere as discussed in this UT video.

So the researchers developed another signal processing algorithm, which attempted to model the physics of the halo, and then remove those modeled light levels from any captured image of the target.  Using an algorithm known as a point spread function (PSF), the scientists were able to successfully remove the halo effect from the asteroid, revealing Elektra’s smallest and most closely orbiting satellite.

Hearteningly, this data reduction and halo elimination technique can be used on other asteroid observations as well and can be used well after the data was collected, as evidenced by the seven-year time lag in data analysis.  So there might be plenty more multi-body asteroid systems in the asteroid belt that make the orbital mechanics of even a four-body problem look like a piece of cake.

Learn More:
Berdeu et al – First observation of a quadruple asteroid — Detection of a third moon around (130) Elektra with SPHERE/IFS
NY Times – The First Quadruple Asteroid: Astronomers Spot a Space Rock With 3 Moons
cNet – Scientist Spots First Known Quadruple Asteroid System

Lead Image:
Image of (130) Elektra with the asteroid’s “halo” removed.
Credit – Berdeu et al.

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Gravastars are an Alternative Theory to Black Holes. Here’s What They’d Look Like

gravastar jpg

One of the central predictions of general relativity is that in the end, gravity wins. Stars will fuse hydrogen into new elements to fight gravity and can oppose it for a time. Electrons and neutrons exert pressure to counter gravity, but their stability against that constant pull limits the amount of mass a white dwarf or neutron star can have. All of this can be countered by gathering more mass together. Beyond about 3 solar masses, give or take, gravity will overpower all other forces and collapse the mass into a black hole.

While black holes have a great deal of theoretical and observational evidence to prove their existence, the theory of black holes is not without issue. For one, general relativity predicts that the mass compresses to an infinitely dense singularity where the laws of physics break down. This singularity is shrouded by an event horizon, which serves as a point of no return for anything devoured by the black hole. Both of these are problematic, so there has been a long history of trying to find some alternative. Some mechanism that prevents singularities and event horizons from forming.

One alternative is a gravitational vacuum star or gravitational condensate star, commonly called a gravastar. It was first proposed in 2001, and takes advantage of the fact that most of the energy in the universe is not regular matter or even dark matter, but dark energy. Dark energy drives cosmic expansion, so perhaps it could oppose gravitational collapse in high densities.

gravastar 1 jpg
Illustration of a hypothetical gravastar. Credit: Daniel Jampolski and Luciano Rezzolla, Goethe University Frankfurt

The original gravastar model proposed a kind of Bose-Einstein condensate of dark energy surrounded by a thin shell of regular matter. The internal condensate ensures that the gravastar has no singularity, while the dense shell of matter ensures that the gravastar appears similar to a black hole from the outside. Interesting idea, but there are two central problems. One is that the shell is unstable, particularly if the gravastar is rotating. There are ways to tweak things just so to make it stable, but such ideal conditions aren’t likely to occur in nature. The second problem is that gravitational wave observations of large body mergers confirm the standard black hole model. But a new gravastar model might solve some of those problems.

The new model essentially nests multiple gravastars together, somewhat like those nested Matryoshka dolls. Rather than a single shell enclosing exotic dark energy, the model has a layers of nested shells with dark energy between the layers. The authors refer to this model as a nestar, or nested gravastar. This alternative model makes the gravastar more stable, since the tension of dark energy is better balanced by the weight of the shells. The interior structure of the nestar also means that the gravitational waves of a nestar and black hole are more similar, meaning that technically their existence can’t be ruled out.

That said, even the authors note that there is no likely scenario that could produce nestars. They likely don’t exist, and it’s almost certain that what we observe as black holes are true black holes. But studies such as this one are great for testing the limits of general relativity. They help us understand what is possible within the framework of the theory, which in turn helps us better understand gravitational physics.

Reference: Jampolski, Daniel and Rezzolla, Luciano. “Nested solutions of gravitational condensate stars.” Classical and Quantum Gravity 41 (2024): 065014.

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Japan’s New H3 Rocket Successfully Blasts Off

240125 slim moon3 1024x513 1 jpg

Japan successfully tested its new flagship H3 rocket after an earlier version failed last year. The rocket lifted off from the Tanegashima Space Center on Saturday, February 17, reaching an orbital altitude of about 670 kilometers (420 miles). It deployed a set of micro-satellites and a dummy satellite designed to simulate a realistic payload.

With the successful launch of the H3, Japan will begin transitioning away from the previous H-2A rocket which has been in service since 2001 and is set to be retired after two more launches. Several upcoming missions depend on the H3, so this successful test was vital.

The launch came after two days of delays because of bad weather. The H3 rocket, built by Mitsubishi Heavy Industries, is now set to become the main launch vehicle of Japan’s space program. The rocket’s first flight in March 2023 failed to reach orbit, which resulted in the loss of an Earth imaging satellite.

The successful launch and deployment of the satellites was a relief for JAXA and members of the project. A livestream of the launch and subsequent successful orbit insertion showed those in the JAXA command cheering and hugging each other.

“I now feel a heavy load taken off my shoulders,” said JAXA H3 project manager Masashi Okada, speaking at a press briefing after the launch. “But now is the real start for H3, and we will work to steadily improve it.”

H3 stands about 57-meter (187-feet) tall and is designed to carry larger payloads. The two microsatellites were deployed approximately 16 minutes and 43 seconds after liftoff. They included an Earth observation satellite named CE-SAT-IE, developed by Canon Electronics, and TIRSAT, an infrared Earth observation instrument that will observe the temperature of the Earth’s surface and seawater.

“We feel so relieved to be able to announce the good results,” JAXA President Hiroshi Yamakawa said at the briefing. Yamakawa added that the main goals of H3 are to secure independent access to space and allow Japan to be competitive as international demand for satellite launches continues to grow. “We made a big first step today toward achieving that goal,” he said.

Image of SLIM lander on moon
An image sent back by a mini-probe shows Japan’s SLIM lander on its side on the lunar surface. (JAXA / Takara Tomy / Sony Group / Doshisha Univ.)

The successful launch comes after two other recent successes for JAXA last month where the H-2A rocket successfully placed a spy satellite into orbit, and just days later JAXA’s robotic SLIM (Smart Lander for Investigating Moon) made the first-ever precise “pinpoint” Moon landing – although unfortunately the lander came down on its side. However, during the final stages of the descent two autonomous rovers were successfully deployed: a tiny hopping robot and the other designed to roll about the surface. Both have sent back pictures and can continue exploring and sending back information even if SLIM cannot be operated.

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European Satellite ERS-2 to Reenter Earth’s Atmosphere This Week

ERS 2 reentry how and why is it happening pillars 1024x576 1 jpg

One of the largest reentries in recent years, ESA’s ERS-2 satellite is coming down this week.

After almost three decades in orbit, an early Earth-observation satellite is finally coming down this week. The European Space Agency’s (ESA) European Remote Sensing satellite ERS-2 is set to reenter the Earth’s atmosphere on or around Wednesday, February 21st.

Trail Blazing Mission

Launched atop an Ariane-4 rocket from the Kourou Space Center in French Guiana on April 21st, 1995, ERS-2 was one of ESA’s first Earth observation satellites. ERS-2 monitored land masses, oceans, rivers, vegetation and the polar regions of the Earth using visible light and ultraviolet sensors. The mission was on hand for several natural disasters, including the flood of the Elbe River across Germany in 2006. ERS-2 ceased operations in September 2011.

Reentry
Anatomy of the reentry of ERS-2. ESA

ERS-2 was placed in a retrograde, Sun-synchronous low Earth orbit, inclined 98.5 degrees relative to the equator. This orbit is typical for Earth-observing and clandestine spy satellites, as it allows the mission to image key target sites at the same relative Sun angle, an attribute handy for image interpretation.

Ice
ERS-2 tracks and ice floe. ESA

The Last Days of ERS-2

Reentry predictions for the satellite are centered on February 21st at 00:19 Universal Time (UT)+/- 25 hours. As we get closer, expect that time to get refined. The mass of ERS-2 at launch (including fuel) was 2,516 kilograms. Expect most of the satellite to burn up on reentry.

Orbit
The orbital path of ERS-2. Orbitron

For context, recent high profile reentries include the UARS satellite (6.5 tons, in 2011), and the massive Long March-5B booster that launched the core module for China’s Tiangong Space Station in late 2022 (weighing in at 23 tons).

ERS2
ERS-2 in the clean room on Earth prior to launch. ESA

ESA passed its first space debris mitigation policy in 2008, 13 years after ERS-2 was launched. In 2011, ESA decided to passively reenter the satellite, and began a series of 66 deorbiting maneuvers to bring its orbit down from 785 kilometers to 573 kilometers. Its fuel drained and batteries exhausted, ERS-2 is now succumbing to the increased drag of the Earth’s atmosphere as we near the peak of the current solar cycle.

North Prague Floods ERS

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