![\"\"](\"https:\/\/cdn.arstechnica.net\/wp-content\/uploads\/2025\/09\/Webb-Image-1_IFU-IR.jpg\")
<\/p>\n
The James Webb Space Telescope observed interstellar comet 3I\/ATLAS on August 6 with its Near-Infrared Spectrograph instrument.<\/p>\n
Credit:
\n NASA\/James Webb Space Telescope<\/p>\n<\/figure>\n
Because of that, the closest any active spacecraft will get to 3I\/ATLAS happened Friday, when it passed less than 20 million miles (30 million kilometers) from Mars. NASA\u2019s Perseverance rover and Mars Reconnaissance Orbiter were expected to make observations of 3I\/ATLAS, along with Europe\u2019s Mars Express and ExoMars Trace Gas Orbiter missions.<\/p>\n
The best views of the object so far have been captured by the James Webb Space Telescope and the Hubble Space Telescope, positioned much closer to Earth. Those observations helped astronomers narrow down the object\u2019s size, but the estimates remain imprecise. Based on Hubble\u2019s images, the icy core of 3I\/ATLAS is somewhere between the size of the Empire State Building to something a little larger than Central Park.<\/p>\n
,<\/p>\n
That may be the most we\u2019ll ever know about the dimensions of 3I\/ATLAS. The spacecraft at Mars lack the exquisite imaging sensitivity of Webb and Hubble, so don\u2019t expect spectacular views from Friday\u2019s observations. But scientists hope to get a better handle on the cloud of gas and dust surrounding the object, giving it the appearance of a comet. Spectroscopic observations have shown the coma around 3I\/ATLAS contains water vapor and an unusually strong signature of carbon dioxide extending out nearly a half-million miles.<\/p>\n
On Tuesday, the European Space Agency released the first grainy images of 3I\/ATLAS captured at Mars. The best views will come from a small telescope called HiRISE on NASA\u2019s Mars Reconnaissance Orbiter. The images from NASA won\u2019t be released until after the end of the ongoing federal government shutdown, according to a member of the HiRISE team.<\/p>\n Europe\u2019s ExoMars Trace Gas Orbiter turned its eyes toward interstellar comet 3I\/ATLAS as it passed close to Mars on Friday, October 3. The comet\u2019s coma is visible as a fuzzy blob surrounding its nucleus, which was not resolved by the spacecraft\u2019s camera.<\/p>\n Credit: Studies of 3I\/ATLAS suggest it was probably kicked out of another star system, perhaps by an encounter with a giant planet. Comets in our Solar System sometimes get ejected into the Milky Way galaxy when they come too close to Jupiter. It roamed the galaxy for billions of years before arriving in the Sun\u2019s galactic neighborhood.<\/p>\n The rogue comet is now gaining speed as gravity pulls it toward perihelion, when it will max out at a relative velocity of 152,000 mph (68 kilometers per second), much too fast to be bound into a closed orbit around the Sun. Instead, the comet will catapult back into the galaxy, never to be seen again.<\/p>\n Anyone who studies planetary formation would relish the opportunity to get a close-up look at an interstellar object. Sending a mission to one would undoubtedly yield a scientific payoff. There\u2019s a good chance that many of these interlopers have been around longer than our own 4.5 billion-year-old Solar System.<\/p>\n ,<\/p>\n One study from the University of Oxford suggests that 3I\/ATLAS came from the \u201cthick disk\u201d of the Milky Way, which is home to a dense population of ancient stars. This origin story would mean the comet is probably more than 7 billion years old, holding clues about cosmic history that are simply inaccessible among the planets, comets, and asteroids that formed with the birth of the Sun.<\/p>\n This is enough reason to mount a mission to explore one of these objects, scientists said. It doesn\u2019t need justification from unfounded theories that 3I\/ATLAS might be an artifact of alien technology, as proposed by Harvard University astrophysicist Avi Loeb. The scientific consensus is that the object is of natural origin.<\/p>\n Loeb shared a similar theory about the first interstellar object found wandering through our Solar System. His statements have sparked questions in popular media about why the world\u2019s space agencies don\u2019t send a probe to actually visit one. Loeb himself proposed redirecting NASA\u2019s Juno spacecraft in orbit around Jupiter on a mission to fly by 3I\/ATLAS, and his writings prompted at least one member of Congress to write a letter to NASA to \u201crejuvenate\u201d the Juno mission by breaking out of Jupiter\u2019s orbit and taking aim at 3I\/ATLAS for a close-up inspection.<\/p>\n The problem is that Juno simply doesn\u2019t have enough fuel to reach the comet, and its main engine is broken. In fact, the total boost required to send Juno from Jupiter to 3I\/ATLAS (roughly 5,800 mph or 2.6 kilometers per second) would surpass the fuel capacity of most interplanetary probes.<\/p>\n Ars asked Scott Bolton, lead scientist on the Juno mission, and he confirmed that the spacecraft lacks the oomph required for the kind of maneuvers proposed by Loeb. \u201cWe had no role in that paper,\u201d Bolton told Ars. \u201cHe assumed propellant that we don\u2019t really have.\u201d<\/p>\n ,<\/p>\n Avi Loeb, a Harvard University astrophysicist.<\/p>\n Credit: So Loeb\u2019s exercise was moot, but his talk of aliens has garnered public attention. Loeb appeared on the conservative network Newsmax last week to discuss his theory of 3I\/ATLAS alongside Rep. Tim Burchett (R-Tenn.). Predictably, conspiracy theories abounded. But as of Tuesday, the segment has 1.2 million views on YouTube. Maybe it\u2019s a good thing that people who approve government budgets, especially those without a preexisting interest in NASA, are eager to learn more about the Universe. We will leave it to the reader to draw their own conclusions on that matter.<\/p>\n Loeb\u2019s calculations also help illustrate the difficulty of pulling off a mission to an interstellar object. So far, we\u2019ve only known about an incoming interstellar intruder a few months before it comes closest to Earth. That\u2019s not to mention the enormous speeds at which these objects move through the Solar System. It\u2019s just not feasible to build a spacecraft and launch it on such short notice.<\/p>\n Now, some scientists are working on ways to overcome these limitations.<\/p>\n One of these people is Colin Snodgrass, an astronomer and planetary scientist at the University of Edinburgh. A few years ago, he helped propose to the European Space Agency a mission concept that would have very likely been laughed out of the room a generation ago. Snodgrass and his team wanted a commitment from ESA of up to $175 million (150 million euros) to launch a mission with no idea of where it would go.<\/p>\n ESA officials called Snodgrass in 2019 to say the agency would fund his mission, named Comet Interceptor, for launch in the late 2020s. The goal of the mission is to perform the first detailed observations of a long-period comet. So far, spacecraft have only visited short-period comets that routinely dip into the inner part of the Solar System.<\/p>\n ,<\/p>\n A long-period comet is an icy visitor from the farthest reaches of the Solar System that has spent little time getting blasted by the Sun\u2019s heat and radiation, freezing its physical and chemical properties much as they were billions of years ago.<\/p>\n Long-period comets are typically discovered a year or two before coming near the Sun, still not enough time to develop a mission from scratch. With Comet Interceptor, ESA will launch a probe to loiter in space a million miles from Earth, wait for the right comet to come along, then fire its engines to pursue it.<\/p>\n Odds are good that the right comet will come from within the Solar System. \u201cThat is the point of the mission,\u201d Snodgrass told Ars.<\/p>\n ESA\u2019s Comet Interceptor will be the first mission to visit a comet coming directly from the outer reaches of the Sun\u2019s realm, carrying material untouched since the dawn of the Solar System.<\/p>\n Credit: But if astronomers detect an interstellar object coming toward us on the right trajectory, there\u2019s a chance Comet Interceptor could reach it.<\/p>\n \u201cI think that the entire science team would agree, if we get really lucky and there\u2019s an interstellar object that we could reach, then to hell with the normal plan, let\u2019s go and do this,\u201d Snodgrass said. \u201cIt\u2019s an opportunity you couldn\u2019t just leave sitting there.\u201d<\/p>\n But, he added, it\u2019s \u201cvery unlikely\u201d that an interstellar object will be in the right place at the right time. \u201cAlthough everyone\u2019s always very excited about the possibility, and we\u2019re excited about the possibility, we kind of try and keep the expectations to a realistic level.\u201d<\/p>\n For example, if Comet Interceptor were in space today, there\u2019s no way it could reach 3I\/ATLAS. \u201cIt\u2019s an unfortunate one,\u201d Snodgrass said. \u201cIts closest point to the Sun, it reaches that on the other side of the Sun from where the Earth is. Just bad timing.\u201d If an interceptor were parked somewhere else in the Solar System, it might be able to get itself in position for an encounter with 3I\/ATLAS. \u201cThere\u2019s only so much fuel aboard,\u201d Snodgrass said. \u201cThere\u2019s only so fast we can go.\u201d<\/p>\n ,<\/p>\n It\u2019s even harder to send a spacecraft to encounter an interstellar object than it is to visit one of the Solar System\u2019s homegrown long-period comets. The calculation of whether Comet Interceptor could reach one of these galactic visitors boils down to where it\u2019s heading and when astronomers discover it.<\/p>\n Snodgrass is part of a team using big telescopes to observe 3I\/ATLAS from a distance. \u201cAs it\u2019s getting closer to the Sun, it is getting brighter,\u201d he said in an interview.<\/p>\n \u201cYou don\u2019t have to claim that they\u2019re aliens to make these exciting,\u201d Snodgrass said. \u201cThey\u2019re interesting because they are a bit of another solar system that you can actually feasibly get an up-close view of, even the sort of telescopic views we\u2019re getting now.\u201d<\/p>\n Colin Snodgrass, a professor at the University of Edinburgh, leads the Comet Interceptor science team.<\/p>\n Credit: Comets and asteroids are the linchpins for understanding the formation of the Solar System. These modest worlds are the leftover building blocks from the debris that coalesced into the planets. Today, direct observations have only allowed scientists to study the history of one planetary system. An interstellar comet would grow the sample size to two.<\/p>\n Still, Snodgrass said his team prefers to keep their energy focused on reaching a comet originating from the frontier of our own Solar System. \u201cWe\u2019re not going to let a very lovely Solar System comet go by, waiting to see \u2018what if there\u2019s an interstellar thing?\u2019\u201d he said.<\/p>\n Snodgrass sees Comet Interceptor as a proof of concept for scientists to propose a future mission specially designed to travel to an interstellar object. \u201cYou need to figure out how do you build the souped-up version that could really get to an interstellar object? I think that\u2019s five or 10 years away, but [it\u2019s] entirely realistic.\u201d<\/p>\n Scientists in the United States are working on just such a proposal. A team from the Southwest Research Institute completed a concept study showing how a mission could fly by one of these interstellar visitors. What\u2019s more, the US scientists say their proposed mission could have actually reached 3I\/ATLAS had it already been in space.<\/p>\n ,<\/p>\n The American concept is similar to Europe\u2019s Comet Interceptor in that it will park a spacecraft somewhere in deep space and wait for the right target to come along. The study was led by Alan Stern, the chief scientist on NASA\u2019s New Horizons mission that flew by Pluto a decade ago. \u201cThese new kinds of objects offer humankind the first feasible opportunity to closely explore bodies formed in other star systems,\u201d he said.<\/p>\n An animation of the trajectory of 3I\/ATLAS through the inner Solar System.<\/p>\n Credit: It\u2019s impossible with current technology to send a spacecraft to match orbits and rendezvous with a high-speed interstellar comet. \u201cWe don\u2019t have to catch it,\u201d Stern recently told Ars. \u201cWe just have to cross its orbit. So it does carry a fair amount of fuel in order to get out of Earth\u2019s orbit and onto the comet\u2019s path to cross that path.\u201d<\/p>\n Stern said his team developed a cost estimate for such a mission, and while he didn\u2019t disclose the exact number, he said it would fall under NASA\u2019s cost cap for a Discovery-class mission. The Discovery program is a line of planetary science missions that NASA selects through periodic competitions within the science community. The cost cap for NASA\u2019s next Discovery competition is expected to be $800 million, not including the launch vehicle.<\/p>\n A mission to encounter an interstellar comet requires no new technologies, Stern said. Hopes for such a mission are bolstered by the activation of the US-funded Vera Rubin Observatory, a state-of-the-art facility high in the mountains of Chile set to begin deep surveys of the entire southern sky later this year. Stern predicts Rubin will discover \u201cone or two\u201d interstellar objects per year. The new observatory should be able to detect the faint light from incoming interstellar bodies sooner, providing missions with more advance warning.<\/p>\n ,<\/p>\n \u201cIf we put a spacecraft like this in space for a few years, while it\u2019s waiting, there should be five or 10 to choose from,\u201d he said.<\/p>\n Alan Stern speaks onstage during Day 1 of TechCrunch Disrupt SF 2018 in San Francisco.<\/p>\n Credit: Winning NASA funding for a mission like Stern\u2019s concept will not be easy. It must compete with dozens of other proposals, and NASA\u2019s next Discovery competition is probably at least two or three years away. The timing of the competition is more uncertain than usual due to swirling questions about NASA\u2019s budget after the Trump administration announced it wants to cut the agency\u2019s science funding in half.<\/p>\n Comet Interceptor, on the other hand, is already funded in Europe. ESA has become a pioneer in comet exploration. The Giotto probe flew by Halley\u2019s Comet in 1986, becoming the first spacecraft to make close-up observations of a comet. ESA\u2019s Rosetta mission became the first spacecraft to orbit a comet in 2014, and later that year, it deployed a German-built lander to return the first data from the surface of a comet. Both of those missions explored short-period comets.<\/p>\n \u201cEach time that ESA has done a comet mission, it\u2019s done something very ambitious and very new,\u201d Snodgrass said. \u201cThe Giotto mission was the first time ESA really tried to do anything interplanetary\u2026 And then, Rosetta, putting this thing in orbit and landing on a comet was a crazy difficult thing to attempt to do.\u201d<\/p>\n \u201cThey really do push the envelope a bit, which is good because ESA can be quite risk averse, I think it\u2019s fair to say, with what they do with missions,\u201d he said. \u201cBut the comet missions, they are things where they\u2019ve really gone for that next step, and Comet Interceptor is the same. The whole idea of trying to design a space mission before you know where you\u2019re going is a slightly crazy way of doing things. But it\u2019s the only way to do this mission. And it\u2019s great that we\u2019re trying it.\u201d<\/p>\n","protected":false},"excerpt":{"rendered":" A few days ago, an inscrutable interstellar interloper made its closest approach to Mars, where a fleet of international spacecraft seek to unravel the red planet\u2019s ancient mysteries. Several of the probes encircling Mars took a break from their usual activities and turned their cameras toward space to catch a glimpse of an object named […]<\/p>\n","protected":false},"author":1,"featured_media":44113,"comment_status":"","ping_status":"","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"inline_featured_image":false,"footnotes":"","_links_to":"","_links_to_target":""},"categories":[2],"tags":[2884,3097,2616,831,2424,1929,190,2612],"class_list":["post-44112","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-news","tag-alan-stern","tag-comet","tag-comet-interceptor","tag-european-space-agency","tag-interstellar-object","tag-juno","tag-nasa","tag-swri"],"acf":[],"_links":{"self":[{"href":"https:\/\/starpath.global\/blog\/wp-json\/wp\/v2\/posts\/44112"}],"collection":[{"href":"https:\/\/starpath.global\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/starpath.global\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/starpath.global\/blog\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/starpath.global\/blog\/wp-json\/wp\/v2\/comments?post=44112"}],"version-history":[{"count":0,"href":"https:\/\/starpath.global\/blog\/wp-json\/wp\/v2\/posts\/44112\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/starpath.global\/blog\/wp-json\/wp\/v2\/media\/44113"}],"wp:attachment":[{"href":"https:\/\/starpath.global\/blog\/wp-json\/wp\/v2\/media?parent=44112"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/starpath.global\/blog\/wp-json\/wp\/v2\/categories?post=44112"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/starpath.global\/blog\/wp-json\/wp\/v2\/tags?post=44112"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}![\"\"](\"https:\/\/cdn.arstechnica.net\/wp-content\/uploads\/2025\/09\/ExoMars_Trace_Gas_Orbiter_observes_comet_3I_ATLAS_GIF_article.gif\")
<\/p>\n
\n ESA\/TGO\/CaSSIS<\/p>\n<\/figure>\nWe need to talk about aliens<\/h2>\n
![\"\"](\"https:\/\/cdn.arstechnica.net\/wp-content\/uploads\/2025\/09\/GettyImages-1616035191.jpg\")
<\/p>\n
\n Anibal Martel\/Anadolu Agency via Getty Images<\/p>\n<\/figure>\nSo you\u2019re saying there\u2019s a chance?<\/h2>\n
![\"\"](\"https:\/\/cdn.arstechnica.net\/wp-content\/uploads\/2025\/09\/Comet_Interceptor_s_journey.jpg\")
<\/p>\n
\n European Space Agency<\/p>\n<\/figure>\n![\"\"](\"https:\/\/cdn.arstechnica.net\/wp-content\/uploads\/2025\/09\/Profile-Colin-Snodgrass_ce283eb5e10920543226ef4c39e048dc.jpg\")
<\/p>\n
\n University of Edinburgh<\/p>\n<\/figure>\nAn American answer<\/h2>\n
![\"\"](\"https:\/\/cdn.arstechnica.net\/wp-content\/uploads\/2025\/09\/3I-ATLAS-in-NASA-Eyes-demo.gif\")
<\/p>\n
\n NASA\/JPL<\/p>\n<\/figure>\n![\"\"](\"https:\/\/cdn.arstechnica.net\/wp-content\/uploads\/2025\/09\/GettyImages-1027207758.jpg\")
<\/p>\n
\n Photo by Kimberly White\/Getty Images for TechCrunch<\/p>\n<\/figure>\n